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20-03-88/T\OSB18
This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.
Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.
| Tape/disk: | Home » CEEFAX disks » telesoftware5.adl |
| Filename: | 20-03-88/T\OSB18 |
| Read OK: | ✔ |
| File size: | 5531 bytes |
| Load address: | 0000 |
| Exec address: | FFFFFFFF |
File contents
OSBITS - An Exploration of the BBC Micro at Machine Level
By Programmer
..........................................................
Part 18: Interrupts I
Bear with me while I repeat the definition of foreground
versus background I originally gave in the last module.
Here you are reading a module of OSbits when the telephone
rings. You stop reading, answer the 'phone and talk to
whoever is ringing. When you have finished you put the
'phone down and start reading again.
Reading was your main, or foreground task, but it was
interrupted by the telephone. The ringing of the 'phone was
an interrupt and by answering the 'phone you handled (or
serviced) that interrupt. When you had finished you
returned to your foreground task.
As I said last time one of the strengths of the 6502
microprocessor is its response to interrupts. Essentially
pieces of hardware interrupt the 6502 and the OS has a
built-in pecking order for these devices so it knows in
which order to respond.
In the last module we looked at Events. These are a
packaged form of interrupt and some of the difficult aspects
of handling interrupts are taken care of by the OS. This
time I plan to get down to a more basic level so that you
will know how to deal with an interrupt directly yourself.
Although an Event can be serviced in a second processor
(with limits on calling OS routines) interrupts can only be
serviced in the I/O processor. So this module's program
will not work in a second processor and if it thinks you
have one it will ask you to turn it off.
Firstly we must mention that there are two different kinds
of interrupt recognised by the 6502. They are the maskable
and non-maskable interrupts and in both cases are generated
by hardware changing the voltage on one of two separate pins
of the 6502. Non-maskable interrupts (NMIs) are usually
generated by devices that need very fast response from the
micro. The standard unexpanded BBC B has no such devices
and disc and Econet interfaces are usually the only
additional modules that use NMIs. Page D of memory is used
for NMI workspace which is why the user can make use of that
page on an unexpanded B but not on any other machine.
The masking referred to in the name is the ability to make
the 6502 ignore the requests for interrupt handling by
using the interrupt flag. Simply put, setting the interrupt
flag (with an SEI) will not stop the machine responding to
NMIs. There is no way to stop an non-maskable interrupt.
There are three distinct aspects to interrupt handling:
intercepting the vector, checking that you are going to
handle the right interrupt, and the interrupt handling code
itself.
Intercepting the vector is carried out exactly as you would
when intercepting any other vector, although you should
understand the distinction between the two interrupt vectors
IRQ1 and IRQ2 (IRQ stands for Interrupt ReQuest). IRQ1 and
IRQ2 have different priorities. An interrupt request first
passes through IRQ1 to the standard interrupt handling
routines in the OS which do system housekeeping. Then the
interrupt is passed through IRQ2. Diagrammatically it's
like this:
Interrupt Detected
Vectored through IRQ1
System interrupt handling
Vectored through IRQ2
User interrupt handling
This reflects what I called the pecking order. The system
housekeeping has to be done urgently. If your interrupt
routine is not urgent then it should intercept IRQ2. Only
intercept IRQ1 if it is important for there to be a fast
response to your code.
The OS sets up various parameters in the micro in a standard
way so that you know the state of the machine when you
intercept IRQ1 or IRQ2. It happens like this.
When a maskable interrupt is detected by the 6502 it
promptly checks to see if it has been caused by a BRK in the
code it was executing (by looking at the state of the BRK
flag). If it was then the processor executes the current
error routine having found it by going through the break
vector.
If the break flag was not set then a hardware interrupt was
the cause of the interrupt. This is the condition we are
studying in this module.
Having determined that an IRQ has indeed occurred the 6502
jumps to the contents of the IRQ1V vector. Until now this
has happened automatically and the user does not need to
worry about it. Now though the OS is preparing to hand over
to interrupt handling routines and the following is known
about the state of the micro because the OS has set it up
this way:
The value of the accumulator at the time of the interrupt is
stored in memory location &FC in the I/O processor. The
status register and the return address (the address of the
instruction about to be actioned when the interrupt
occurred) are stored on the stack ready for an RTI (ReTurn
from Interrupt) at the end of the interrupt code. The
original X and Y register values are still in place.
Any interrupt handling code you write should leave the micro
and registers in this state afterwards.
So, on entry to your code you must:
Save the contents of &FC on the stack
Save the X and Y registers on the stack
Clear the decimal flag
and on exit you must
Reload the Y and X registers from the stack
Transfer the original accumulator value from the
stack to &FC
Clearing the decimal flag is a safety measure in case the
routine being executed at the time of the interrupt was
working in decimal mode. This is unlikely. As the status
register is re-set by the RTI instruction at the very end of
the interrupt chain then you need not worry about the status
of the original flags yourself during interrupt handling
except for the effect decimal mode might have on your code.
So clear it!
[There is a very strong argument for NEVER using decimal
mode on the BBC Micro. The OS always assumes binary mode
and so, probably, do other people's interrupt routines. You
should therefore always disable interrupts when using
decimal mode to avoid trashing somebody else's interrupts.
I know I didn't do this back in module 6 but several people
have since pointed out the dangers.]
The code will look like this:
LDA &FC
PHA
TXA
PHA
TYA
PHA
CLD
and on exit
PLA
TAY
PLA
TAX
PLA
STA &FC
RTI
or you might JMP (old_IRQ) to continue down the IRQ chain.
Checking to see if you have the right interrupt is not as
simple as the Event system where you check the number in the
accumulator. With an interrupt you have to look in the
registers of the piece of hardware to which your interrupt handling
routine refers.
With a single routine this is relatively simple, just be
thankful you don't have the job of the OS which has many
devices to service. The OS has to look at the system VIA
(versatile interface adapter), the serial system (for the
cassette filing system and the RS423) and one part of the
user VIA which deals with parallel printers. These
interrupt possibilities are all handled by the time program
execution has reached IRQ2.
The best place to look for details of these devices is in
your advanced guide. It is a little beyond the scope of
this series to get into too much reference detail, that is
best done on paper. However I will give an example which I
hope will serve for all system interrupts.
The BBC Micro has two VIAs. These are very versatile chips
(as the name Versatile Interface Adapter suggests) and
contain timers, counters and other bits and pieces (!). One
of these is set aside for the parallel printer and for the
user (you) and I will be dealing with two aspects of this in
detail later. The other one is used by the OS for system
housekeeping.
The system VIA will generate an interrupt when ....
a key is pressed
the TV picture reaches the vertical interval and flies
back (50 times a second)
the lightpen detects the scan spot from the screen
the A to D converter completes a conversion
either of the two VIA timers reaches zero
It can also generate an interrupt when its shift register
(which counts pulses) times out, but this is not used by the
OS so should never happen.
The OS, when it detects an interrupt, needs to check its
pieces of hardware to see where the interrupt came from in
order to correctly handle it. It can quickly check if an
interrupt came from a particular device since each device
has a flag that is set when any interrupt comes from it.
This is besides a flag to identify the actual part of the
device that caused the interrupt.
So the OS only has to look at the flag on each device and
then check the flag register on the device that says it
caused the interrupt. It doesn't have to check all the
flags on all the devices. This is important because
interrupts, like events, have to be handled very quickly.
If it takes a long time to handle an interrupt then the OS
will stop some of its functions for a while, which could
cause problems.
After an IRQ has occurred the OS will check the interrupt
flag on the system VIA, the serial ULA and the user VIA
until it finds the device that caused the IRQ. It will then
check the device itself to identify the particular cause.
If this is one which the OS expects to service, it will do
so and then return. If it is one which the OS doesn't
recognise it passes it on through IRQ2.
It is possible to stop the OS servicing an interrupt it
would normally do by using OSBYTE 231 to 233 which can force
the OS to pass a particular interrupt through IRQ2 for the
user to deal with. This is known as masking the IRQ and you
should consult your advanced guide for details. [There are
example programs which use IRQ masking on pages 306 and 308
of Bray, Dickens and Holmes.]
I hope you will see why interrupts are a 'risky' area for
experimentation, since it is all too easy to make the
computer hang up. However, it is unlikely to explode so the
worst that can happen is that you will have to press BREAK
and start again.
I think it is best to proceed from here with examples. I
plan to show you more of the VIA timers in the next module
but for now I will initiate you into the little known sport
of mouse driving. A mouse driver is a piece of code that
keeps track of the movement of a mouse (i.e. a pointing
device) attached to the micro and enables you to implement a
pointer system on the screen. I shall use the AMX mouse as
my example, although the Megamouse uses the same code. Even
if you do not posses a mouse I hope you will find the
example useful.
This mouse is connected to the micro's user port and so to
the user VIA behind it. As the mouse is moved a small
voltage (also taken from the user port) is applied to four
wires in the connecting cable in such a way that you can
detect the direction of motion at any instant. The mouse
doesn't tell you where it is, it tells you in which
direction it is moving.
The waveform of the voltage is rectangular, like a square
wave, and if we call the connections X1 Y1 X2 and Y2 the
following rules apply.
If the mouse moves left to right (or vice versa) a waveform
if applied to the X1 and X2 wires such that if it moves one
way the signals are high at the same time and low at the
same time (i.e. in phase) and if it moves the other the
signals are different at any instant (i.e. out of phase).
Similarly for the Y wires when the mouse moves backwards and
forwards.
X1 and Y1 are connected to the two interrupt lines on the
user port so they can be used to generate an interrupt from
the user VIA. We will set the VIA to generate this
interrupt whenever the VIA line detects a positive going
voltage (i.e. the front porch of the rectangular waveform)
from either line.
When the interrupt occurs we look at the corresponding
second line (X2 or Y2) to see if it is high or low at that
moment and hence deduce which way the mouse is moving.
As I have said, the lines on the user port are connected to
the user VIA. In fact the user port connects to the 'B'
side of this device and the parallel printer connects to the
other. The VIA is a device called a 6522 and information on
how to use it can be gleaned (with a little difficulty) from
the specification sheets for the device. Bray, Dickens and
Holmes (Advanced User Guide) also reprints the information
from the spec sheet.
In brief the device is set up by poking bits in its
registers. On the BBC Micro those registers live in the
area of memory known as Sheila and the user VIA lives from
&FE60 to &FE6F. There are 16 registers as follows:
&FE60 I/O for the 'B' side of the VIA
1 I/O for the 'A' side of the VIA
2 Control of the direction of data on the 'B' side
3 Control of the direction of data on the 'A' side
4 Timer 1 control
5 Timer 1 control
6 Timer 1 control
7 Timer 1 control
8 Timer 2 control
9 Timer 2 control
A Shift register
B Auxiliary control
C Peripheral control
D Interrupt flags
E Interrupt enabling
F Same as register 1 but without handshaking
There's a lot in the VIAs. I'll show you an application
using a timer in the next module. The shift register is
used for counting pulses. The auxiliary control sets
parameters for the timers and shift register and the
peripheral control sets up the way the interrupt lines
respond to the voltages applied to them.
To access these sections of memory you would normally use
OSBYTE with A between 146 and 151. These calls read from
and write to the memory-mapped I/O (Fred, Jim and Sheila) as
follows.
OSBYTE 146 - Read Fred (&FC00 to &FCFF)
OSBYTE 147 - Write Fred
OSBYTE 148 - Read Jim (&FD00 to &FDFF)
OSBYTE 149 - Write Jim
OSBYTE 150 - Read Sheila (&FE00 to &FEFF)
OSBYTE 151 - Write Sheila
In each case, on entry X contains the offset within the page
and, if a write, Y contains the byte. On exit, if a read, Y
contains the byte read. This call will presumably work
wherever Fred Jim and Sheila are located in future machines.
Alternatively you could use OSWORDs 5 and 6 to read or write
to I/O processor memory. The OSWORD parameter block, at the
location pointed to by the X and Y registers, contains the
address in the I/O processor in the first 4 bytes and in the
5th byte is put, or is read, the byte to be transferred
across. The OSBYTE calls and the OSWORD calls work across
the tube, i.e. from a second processor.
In the case of interrupts your intercepting code has to be
in the I/O processor, so cross-tube requirements do not come
into it. If we take the (small) risk that Sheila will move
in future the best thing to do is write and read the memory
locations directly. Any interrupt code must be short and to
the point, so we should avoid unnecessary OS calls. In any
case there is a restriction on which OS calls can be used
during interrupts. I will give you more detail in the next
module.
In this module we shall deal directly with the registers of
the VIA. To set up the VIA to detect the mouse we must do
three things. We must enable the interrupts we are going to
use, set up the behaviour of the interrupt lines and set up
the relevant bits of the user port to act as inputs.
The first section of the machine code in B/osb18 resets the
IRQ vector. I have used IRQ1 because we need a fast
response to the mouse interrupt. When you have seen this
code in action you should change the value of 'irq1' in line
130 to &206 and compare the results. [Note that if you have
an AMX mouse chip in your machine you must remove or disable
it before you can get ANY result from using IRQ2 at &206.]
We must enable interrupts from the two 'B' side interrupt
lines, known as CB1 and CB2. To do this we make sure that
bits 3 and 4 are set by ORing the current contents of this
register with 24 (8+16) and re-saving.
To make CB1 and CB2 operate as we wish, by interrupting on
the positive-going edge of the waveform, we have to set bits
4 and 6 of the peripheral control register. This is also
best done by ORing the current contents with 80 (16+64). As
you can see from the listing at lines 390 and 430 the BBC
assembler allows you to use an addition to define the bits
being used. You could even change line 430 to ORA
#(2^4+2^6) if that makes it clearer to you. This will make
no difference to the final code, it just makes the source
code more readable.
As we are going to use the user port as an input we write
zero into the data direction register which, as it clears
all the bits, makes all the lines function as inputs. The
buttons of the mouse will be read from these lines if you
want them, so it is worth clearing all the bits.
You will notice that I disabled interrupts while modifying
the VIA registers. This makes sure that another routine
can't get in and modify the registers while you are doing
it. Of course it may well be that another routine that
uses this function of the VIA would cause a malfunction, but
if there is any chance of the two routines coexisting then
let's take it. That's why this routine avoids affecting
other register bits unnecessarily.
The variables appear next. You can change 'scale' if you
want finer or coarser pointer control but a value of 2 is a
good place to start. The X and Y coordinates of the pointer
controlled by the mouse are stored next. I have allocated
four bytes for each even though screen coordinates only have
two bytes. This is to enable us to read the coordinate with
the ! operator from BASIC. More on this later. The final
variable space holds the old value of IRQ1 to allow
chaining.
When any interrupt (other than an NMI) is detected execution
jumps down the IRQ1 vector. The first thing in that vector
is our code here. The standard routine, to reclaim the
acumulator value from &FC and store the registers, is
carried out in lines 610 to 660. In fact there is no need
to save the X and Y registers because this program does not
change them but, belt and braces, it's best to get into the
habit of saving all registers. Similarly we make sure the
processor is in binary rather than decimal mode.
Next we have to make sure that the cause of the interrupt is
the one for our code. Since interrupts are happening
continuously in the BBC Micro we must be sure we only
service the right ones. If the user VIA caused an interrupt
than bit 7 of its interrupt flag register (IFR) will be set.
Lines 730 and 740 check this.
If the user VIA did indeed cause the interrupt we must check
to see that it was CB1 or CB2, our two interrupt lines, that
were responsible. If it was CB2 (caused by the Y1 line)
then bit 3 of the IFR will be set, if it was CB1 (caused by
the X1 line) then bit 4 of the IFR will be set. Lines 760
to 820 check for this and route execution accordingly using
the BIT opcode. This performs an AND but does not store the
result; indeed an AND would work equally well here. If at
the end of this we know that the interrupt was not caused by
CB1 or CB2 then we must chain on down IRQ1 so someone else
can service the interrupt.
You must clear an interrupt when you service it, otherwise
the 6502 will still signal it. In this case with interrupts
from CB1 and CB2 you do not have to clear explicitly.
Reading from the data lines on the user port will do the job
automatically.
The code operates in a similar way whether an X or Y pulse
caused the interrupt. The relevant second line is tested to
see if it is high or low at the moment of the interrupt.
[Note that if you use IRQ2 the delay before servicing the
interrupt is sufficient to cause false readings at this
point and the pointer can go the wrong way. Using IRQ1
avoids this problem.] If we move positively then the
relevant coordinate location is increased by 'scale' and
decreased of we move negatively. After this has been done
the routine exits by restoring the registers and then
performing an RTI.
The BASIC shell around this program prints a pointer (a
cross) in the centre of the screen and smears it around as
you move the mouse. The X and Y coordinates are read using
the ! operator to minimise errors due to the coordinates
changing while they are being read. If you were to read the
two bytes of the coordinates separately using ? operators
you would get some incorrect results, try it and see.
This code will work with a Marconi Trackerball, although the
pin connections are different to the AMX mouse. The data
sheet with the trackerball will advise you on which pins are
which. CB1 and CB2 are used but the second lines are
different to those used in this program. Lines 960 and 1240
would need to be changed to load 16 and 8 respectively
because bits 4 and 3 carry Y2 and X2.
Of course this code does not implement a proper mouse
pointer in the way Acorn's VFS or AMX's Super-Art ROM does,
but it should give you a basic idea of how an interrupt is
handled.
You should also notice that I have put the code in the
cassette/RS423 input buffer. If you use a cassette (if you
do you should be saving for a disc drive) or have a modem or
something that uses RS423 inputs then running this code will
crash your computer. Perhaps the safest thing to do would
be to raise PAGE by &100 (256) bytes and put the code in
there: but on most Bs and B+s PAGE is too high anyway. I
leave it up to you.
Remember that the vectors are re-set on pressing BREAK.
Also you will notice that this time I haven't allowed for
the code to be called twice without a BREAK. Doing this
will lock up the computer.
Next time we will set up a VIA timer to palette switch on
the screen to get several colours in mode 3, in a limited
way.
00000000 4f 53 42 49 54 53 20 2d 20 41 6e 20 45 78 70 6c |OSBITS - An Expl| 00000010 6f 72 61 74 69 6f 6e 20 6f 66 20 74 68 65 20 42 |oration of the B| 00000020 42 43 20 4d 69 63 72 6f 20 61 74 20 4d 61 63 68 |BC Micro at Mach| 00000030 69 6e 65 20 4c 65 76 65 6c 0d 0d 42 79 20 50 72 |ine Level..By Pr| 00000040 6f 67 72 61 6d 6d 65 72 0d 0d 2e 2e 2e 2e 2e 2e |ogrammer........| 00000050 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e |................| * 00000080 2e 2e 2e 2e 0d 0d 0d 50 61 72 74 20 31 38 3a 20 |.......Part 18: | 00000090 49 6e 74 65 72 72 75 70 74 73 20 49 0d 0d 0d 42 |Interrupts I...B| 000000a0 65 61 72 20 77 69 74 68 20 6d 65 20 77 68 69 6c |ear with me whil| 000000b0 65 20 49 20 72 65 70 65 61 74 20 74 68 65 20 64 |e I repeat the d| 000000c0 65 66 69 6e 69 74 69 6f 6e 20 6f 66 20 66 6f 72 |efinition of for| 000000d0 65 67 72 6f 75 6e 64 0d 76 65 72 73 75 73 20 62 |eground.versus b| 000000e0 61 63 6b 67 72 6f 75 6e 64 20 49 20 6f 72 69 67 |ackground I orig| 000000f0 69 6e 61 6c 6c 79 20 67 61 76 65 20 69 6e 20 74 |inally gave in t| 00000100 68 65 20 6c 61 73 74 20 6d 6f 64 75 6c 65 2e 0d |he last module..| 00000110 0d 48 65 72 65 20 79 6f 75 20 61 72 65 20 72 65 |.Here you are re| 00000120 61 64 69 6e 67 20 61 20 6d 6f 64 75 6c 65 20 6f |ading a module o| 00000130 66 20 4f 53 62 69 74 73 20 77 68 65 6e 20 74 68 |f OSbits when th| 00000140 65 20 74 65 6c 65 70 68 6f 6e 65 0d 72 69 6e 67 |e telephone.ring| 00000150 73 2e 20 20 59 6f 75 20 73 74 6f 70 20 72 65 61 |s. You stop rea| 00000160 64 69 6e 67 2c 20 61 6e 73 77 65 72 20 74 68 65 |ding, answer the| 00000170 20 27 70 68 6f 6e 65 20 61 6e 64 20 74 61 6c 6b | 'phone and talk| 00000180 20 74 6f 0d 77 68 6f 65 76 65 72 20 69 73 20 72 | to.whoever is r| 00000190 69 6e 67 69 6e 67 2e 20 20 57 68 65 6e 20 79 6f |inging. When yo| 000001a0 75 20 68 61 76 65 20 66 69 6e 69 73 68 65 64 20 |u have finished | 000001b0 79 6f 75 20 70 75 74 20 74 68 65 0d 27 70 68 6f |you put the.'pho| 000001c0 6e 65 20 64 6f 77 6e 20 61 6e 64 20 73 74 61 72 |ne down and star| 000001d0 74 20 72 65 61 64 69 6e 67 20 61 67 61 69 6e 2e |t reading again.| 000001e0 0d 0d 52 65 61 64 69 6e 67 20 77 61 73 20 79 6f |..Reading was yo| 000001f0 75 72 20 6d 61 69 6e 2c 20 6f 72 20 66 6f 72 65 |ur main, or fore| 00000200 67 72 6f 75 6e 64 20 74 61 73 6b 2c 20 62 75 74 |ground task, but| 00000210 20 69 74 20 77 61 73 0d 69 6e 74 65 72 72 75 70 | it was.interrup| 00000220 74 65 64 20 62 79 20 74 68 65 20 74 65 6c 65 70 |ted by the telep| 00000230 68 6f 6e 65 2e 20 20 54 68 65 20 72 69 6e 67 69 |hone. The ringi| 00000240 6e 67 20 6f 66 20 74 68 65 20 27 70 68 6f 6e 65 |ng of the 'phone| 00000250 20 77 61 73 0d 61 6e 20 69 6e 74 65 72 72 75 70 | was.an interrup| 00000260 74 20 61 6e 64 20 62 79 20 61 6e 73 77 65 72 69 |t and by answeri| 00000270 6e 67 20 74 68 65 20 27 70 68 6f 6e 65 20 79 6f |ng the 'phone yo| 00000280 75 20 68 61 6e 64 6c 65 64 20 28 6f 72 0d 73 65 |u handled (or.se| 00000290 72 76 69 63 65 64 29 20 74 68 61 74 20 69 6e 74 |rviced) that int| 000002a0 65 72 72 75 70 74 2e 20 20 57 68 65 6e 20 79 6f |errupt. When yo| 000002b0 75 20 68 61 64 20 66 69 6e 69 73 68 65 64 20 79 |u had finished y| 000002c0 6f 75 0d 72 65 74 75 72 6e 65 64 20 74 6f 20 79 |ou.returned to y| 000002d0 6f 75 72 20 66 6f 72 65 67 72 6f 75 6e 64 20 74 |our foreground t| 000002e0 61 73 6b 2e 0d 0d 41 73 20 49 20 73 61 69 64 20 |ask...As I said | 000002f0 6c 61 73 74 20 74 69 6d 65 20 6f 6e 65 20 6f 66 |last time one of| 00000300 20 74 68 65 20 73 74 72 65 6e 67 74 68 73 20 6f | the strengths o| 00000310 66 20 74 68 65 20 36 35 30 32 0d 6d 69 63 72 6f |f the 6502.micro| 00000320 70 72 6f 63 65 73 73 6f 72 20 69 73 20 69 74 73 |processor is its| 00000330 20 72 65 73 70 6f 6e 73 65 20 74 6f 20 69 6e 74 | response to int| 00000340 65 72 72 75 70 74 73 2e 20 20 45 73 73 65 6e 74 |errupts. Essent| 00000350 69 61 6c 6c 79 0d 70 69 65 63 65 73 20 6f 66 20 |ially.pieces of | 00000360 68 61 72 64 77 61 72 65 20 69 6e 74 65 72 72 75 |hardware interru| 00000370 70 74 20 74 68 65 20 36 35 30 32 20 61 6e 64 20 |pt the 6502 and | 00000380 74 68 65 20 4f 53 20 68 61 73 20 61 0d 62 75 69 |the OS has a.bui| 00000390 6c 74 2d 69 6e 20 70 65 63 6b 69 6e 67 20 6f 72 |lt-in pecking or| 000003a0 64 65 72 20 66 6f 72 20 74 68 65 73 65 20 64 65 |der for these de| 000003b0 76 69 63 65 73 20 73 6f 20 69 74 20 6b 6e 6f 77 |vices so it know| 000003c0 73 20 69 6e 0d 77 68 69 63 68 20 6f 72 64 65 72 |s in.which order| 000003d0 20 74 6f 20 72 65 73 70 6f 6e 64 2e 0d 0d 49 6e | to respond...In| 000003e0 20 74 68 65 20 6c 61 73 74 20 6d 6f 64 75 6c 65 | the last module| 000003f0 20 77 65 20 6c 6f 6f 6b 65 64 20 61 74 20 45 76 | we looked at Ev| 00000400 65 6e 74 73 2e 20 20 54 68 65 73 65 20 61 72 65 |ents. These are| 00000410 20 61 0d 70 61 63 6b 61 67 65 64 20 66 6f 72 6d | a.packaged form| 00000420 20 6f 66 20 69 6e 74 65 72 72 75 70 74 20 61 6e | of interrupt an| 00000430 64 20 73 6f 6d 65 20 6f 66 20 74 68 65 20 64 69 |d some of the di| 00000440 66 66 69 63 75 6c 74 20 61 73 70 65 63 74 73 0d |fficult aspects.| 00000450 6f 66 20 68 61 6e 64 6c 69 6e 67 20 69 6e 74 65 |of handling inte| 00000460 72 72 75 70 74 73 20 61 72 65 20 74 61 6b 65 6e |rrupts are taken| 00000470 20 63 61 72 65 20 6f 66 20 62 79 20 74 68 65 20 | care of by the | 00000480 4f 53 2e 20 54 68 69 73 0d 74 69 6d 65 20 49 20 |OS. This.time I | 00000490 70 6c 61 6e 20 74 6f 20 67 65 74 20 64 6f 77 6e |plan to get down| 000004a0 20 74 6f 20 61 20 6d 6f 72 65 20 62 61 73 69 63 | to a more basic| 000004b0 20 6c 65 76 65 6c 20 73 6f 20 74 68 61 74 20 79 | level so that y| 000004c0 6f 75 0d 77 69 6c 6c 20 6b 6e 6f 77 20 68 6f 77 |ou.will know how| 000004d0 20 74 6f 20 64 65 61 6c 20 77 69 74 68 20 61 6e | to deal with an| 000004e0 20 69 6e 74 65 72 72 75 70 74 20 64 69 72 65 63 | interrupt direc| 000004f0 74 6c 79 20 79 6f 75 72 73 65 6c 66 2e 0d 0d 41 |tly yourself...A| 00000500 6c 74 68 6f 75 67 68 20 61 6e 20 45 76 65 6e 74 |lthough an Event| 00000510 20 63 61 6e 20 62 65 20 73 65 72 76 69 63 65 64 | can be serviced| 00000520 20 69 6e 20 61 20 73 65 63 6f 6e 64 20 70 72 6f | in a second pro| 00000530 63 65 73 73 6f 72 0d 28 77 69 74 68 20 6c 69 6d |cessor.(with lim| 00000540 69 74 73 20 6f 6e 20 63 61 6c 6c 69 6e 67 20 4f |its on calling O| 00000550 53 20 72 6f 75 74 69 6e 65 73 29 20 69 6e 74 65 |S routines) inte| 00000560 72 72 75 70 74 73 20 63 61 6e 20 6f 6e 6c 79 20 |rrupts can only | 00000570 62 65 0d 73 65 72 76 69 63 65 64 20 69 6e 20 74 |be.serviced in t| 00000580 68 65 20 49 2f 4f 20 70 72 6f 63 65 73 73 6f 72 |he I/O processor| 00000590 2e 20 20 53 6f 20 74 68 69 73 20 6d 6f 64 75 6c |. So this modul| 000005a0 65 27 73 20 70 72 6f 67 72 61 6d 0d 77 69 6c 6c |e's program.will| 000005b0 20 6e 6f 74 20 77 6f 72 6b 20 69 6e 20 61 20 73 | not work in a s| 000005c0 65 63 6f 6e 64 20 70 72 6f 63 65 73 73 6f 72 20 |econd processor | 000005d0 61 6e 64 20 69 66 20 69 74 20 74 68 69 6e 6b 73 |and if it thinks| 000005e0 20 79 6f 75 0d 68 61 76 65 20 6f 6e 65 20 69 74 | you.have one it| 000005f0 20 77 69 6c 6c 20 61 73 6b 20 79 6f 75 20 74 6f | will ask you to| 00000600 20 74 75 72 6e 20 69 74 20 6f 66 66 2e 0d 0d 46 | turn it off...F| 00000610 69 72 73 74 6c 79 20 77 65 20 6d 75 73 74 20 6d |irstly we must m| 00000620 65 6e 74 69 6f 6e 20 74 68 61 74 20 74 68 65 72 |ention that ther| 00000630 65 20 61 72 65 20 74 77 6f 20 64 69 66 66 65 72 |e are two differ| 00000640 65 6e 74 20 6b 69 6e 64 73 0d 6f 66 20 69 6e 74 |ent kinds.of int| 00000650 65 72 72 75 70 74 20 72 65 63 6f 67 6e 69 73 65 |errupt recognise| 00000660 64 20 62 79 20 74 68 65 20 36 35 30 32 2e 20 20 |d by the 6502. | 00000670 54 68 65 79 20 61 72 65 20 74 68 65 20 6d 61 73 |They are the mas| 00000680 6b 61 62 6c 65 0d 61 6e 64 20 6e 6f 6e 2d 6d 61 |kable.and non-ma| 00000690 73 6b 61 62 6c 65 20 69 6e 74 65 72 72 75 70 74 |skable interrupt| 000006a0 73 20 61 6e 64 20 69 6e 20 62 6f 74 68 20 63 61 |s and in both ca| 000006b0 73 65 73 20 61 72 65 20 67 65 6e 65 72 61 74 65 |ses are generate| 000006c0 64 0d 62 79 20 68 61 72 64 77 61 72 65 20 63 68 |d.by hardware ch| 000006d0 61 6e 67 69 6e 67 20 74 68 65 20 76 6f 6c 74 61 |anging the volta| 000006e0 67 65 20 6f 6e 20 6f 6e 65 20 6f 66 20 74 77 6f |ge on one of two| 000006f0 20 73 65 70 61 72 61 74 65 20 70 69 6e 73 0d 6f | separate pins.o| 00000700 66 20 74 68 65 20 36 35 30 32 2e 20 20 4e 6f 6e |f the 6502. Non| 00000710 2d 6d 61 73 6b 61 62 6c 65 20 69 6e 74 65 72 72 |-maskable interr| 00000720 75 70 74 73 20 28 4e 4d 49 73 29 20 61 72 65 20 |upts (NMIs) are | 00000730 75 73 75 61 6c 6c 79 0d 67 65 6e 65 72 61 74 65 |usually.generate| 00000740 64 20 62 79 20 64 65 76 69 63 65 73 20 74 68 61 |d by devices tha| 00000750 74 20 6e 65 65 64 20 76 65 72 79 20 66 61 73 74 |t need very fast| 00000760 20 72 65 73 70 6f 6e 73 65 20 66 72 6f 6d 20 74 | response from t| 00000770 68 65 0d 6d 69 63 72 6f 2e 20 20 54 68 65 20 73 |he.micro. The s| 00000780 74 61 6e 64 61 72 64 20 75 6e 65 78 70 61 6e 64 |tandard unexpand| 00000790 65 64 20 42 42 43 20 42 20 68 61 73 20 6e 6f 20 |ed BBC B has no | 000007a0 73 75 63 68 20 64 65 76 69 63 65 73 0d 61 6e 64 |such devices.and| 000007b0 20 64 69 73 63 20 61 6e 64 20 45 63 6f 6e 65 74 | disc and Econet| 000007c0 20 69 6e 74 65 72 66 61 63 65 73 20 61 72 65 20 | interfaces are | 000007d0 75 73 75 61 6c 6c 79 20 74 68 65 20 6f 6e 6c 79 |usually the only| 000007e0 0d 61 64 64 69 74 69 6f 6e 61 6c 20 6d 6f 64 75 |.additional modu| 000007f0 6c 65 73 20 74 68 61 74 20 75 73 65 20 4e 4d 49 |les that use NMI| 00000800 73 2e 20 20 50 61 67 65 20 44 20 6f 66 20 6d 65 |s. Page D of me| 00000810 6d 6f 72 79 20 69 73 20 75 73 65 64 0d 66 6f 72 |mory is used.for| 00000820 20 4e 4d 49 20 77 6f 72 6b 73 70 61 63 65 20 77 | NMI workspace w| 00000830 68 69 63 68 20 69 73 20 77 68 79 20 74 68 65 20 |hich is why the | 00000840 75 73 65 72 20 63 61 6e 20 6d 61 6b 65 20 75 73 |user can make us| 00000850 65 20 6f 66 20 74 68 61 74 0d 70 61 67 65 20 6f |e of that.page o| 00000860 6e 20 61 6e 20 75 6e 65 78 70 61 6e 64 65 64 20 |n an unexpanded | 00000870 42 20 62 75 74 20 6e 6f 74 20 6f 6e 20 61 6e 79 |B but not on any| 00000880 20 6f 74 68 65 72 20 6d 61 63 68 69 6e 65 2e 0d | other machine..| 00000890 0d 54 68 65 20 6d 61 73 6b 69 6e 67 20 72 65 66 |.The masking ref| 000008a0 65 72 72 65 64 20 74 6f 20 69 6e 20 74 68 65 20 |erred to in the | 000008b0 6e 61 6d 65 20 69 73 20 74 68 65 20 61 62 69 6c |name is the abil| 000008c0 69 74 79 20 74 6f 20 6d 61 6b 65 0d 74 68 65 20 |ity to make.the | 000008d0 36 35 30 32 20 69 67 6e 6f 72 65 20 74 68 65 20 |6502 ignore the | 000008e0 72 65 71 75 65 73 74 73 20 66 6f 72 20 69 6e 74 |requests for int| 000008f0 65 72 72 75 70 74 20 68 61 6e 64 6c 69 6e 67 20 |errupt handling | 00000900 62 79 0d 75 73 69 6e 67 20 74 68 65 20 69 6e 74 |by.using the int| 00000910 65 72 72 75 70 74 20 66 6c 61 67 2e 20 20 53 69 |errupt flag. Si| 00000920 6d 70 6c 79 20 70 75 74 2c 20 73 65 74 74 69 6e |mply put, settin| 00000930 67 20 74 68 65 20 69 6e 74 65 72 72 75 70 74 0d |g the interrupt.| 00000940 66 6c 61 67 20 28 77 69 74 68 20 61 6e 20 53 45 |flag (with an SE| 00000950 49 29 20 77 69 6c 6c 20 6e 6f 74 20 73 74 6f 70 |I) will not stop| 00000960 20 74 68 65 20 6d 61 63 68 69 6e 65 20 72 65 73 | the machine res| 00000970 70 6f 6e 64 69 6e 67 20 74 6f 0d 4e 4d 49 73 2e |ponding to.NMIs.| 00000980 20 20 54 68 65 72 65 20 69 73 20 6e 6f 20 77 61 | There is no wa| 00000990 79 20 74 6f 20 73 74 6f 70 20 61 6e 20 6e 6f 6e |y to stop an non| 000009a0 2d 6d 61 73 6b 61 62 6c 65 20 69 6e 74 65 72 72 |-maskable interr| 000009b0 75 70 74 2e 0d 0d 54 68 65 72 65 20 61 72 65 20 |upt...There are | 000009c0 74 68 72 65 65 20 64 69 73 74 69 6e 63 74 20 61 |three distinct a| 000009d0 73 70 65 63 74 73 20 74 6f 20 69 6e 74 65 72 72 |spects to interr| 000009e0 75 70 74 20 68 61 6e 64 6c 69 6e 67 3a 0d 69 6e |upt handling:.in| 000009f0 74 65 72 63 65 70 74 69 6e 67 20 74 68 65 20 76 |tercepting the v| 00000a00 65 63 74 6f 72 2c 20 63 68 65 63 6b 69 6e 67 20 |ector, checking | 00000a10 74 68 61 74 20 79 6f 75 20 61 72 65 20 67 6f 69 |that you are goi| 00000a20 6e 67 20 74 6f 0d 68 61 6e 64 6c 65 20 74 68 65 |ng to.handle the| 00000a30 20 72 69 67 68 74 20 69 6e 74 65 72 72 75 70 74 | right interrupt| 00000a40 2c 20 61 6e 64 20 74 68 65 20 69 6e 74 65 72 72 |, and the interr| 00000a50 75 70 74 20 68 61 6e 64 6c 69 6e 67 20 63 6f 64 |upt handling cod| 00000a60 65 0d 69 74 73 65 6c 66 2e 0d 0d 49 6e 74 65 72 |e.itself...Inter| 00000a70 63 65 70 74 69 6e 67 20 74 68 65 20 76 65 63 74 |cepting the vect| 00000a80 6f 72 20 69 73 20 63 61 72 72 69 65 64 20 6f 75 |or is carried ou| 00000a90 74 20 65 78 61 63 74 6c 79 20 61 73 20 79 6f 75 |t exactly as you| 00000aa0 20 77 6f 75 6c 64 0d 77 68 65 6e 20 69 6e 74 65 | would.when inte| 00000ab0 72 63 65 70 74 69 6e 67 20 61 6e 79 20 6f 74 68 |rcepting any oth| 00000ac0 65 72 20 76 65 63 74 6f 72 2c 20 61 6c 74 68 6f |er vector, altho| 00000ad0 75 67 68 20 79 6f 75 20 73 68 6f 75 6c 64 0d 75 |ugh you should.u| 00000ae0 6e 64 65 72 73 74 61 6e 64 20 74 68 65 20 64 69 |nderstand the di| 00000af0 73 74 69 6e 63 74 69 6f 6e 20 62 65 74 77 65 65 |stinction betwee| 00000b00 6e 20 74 68 65 20 74 77 6f 20 69 6e 74 65 72 72 |n the two interr| 00000b10 75 70 74 20 76 65 63 74 6f 72 73 0d 49 52 51 31 |upt vectors.IRQ1| 00000b20 20 61 6e 64 20 49 52 51 32 20 28 49 52 51 20 73 | and IRQ2 (IRQ s| 00000b30 74 61 6e 64 73 20 66 6f 72 20 49 6e 74 65 72 72 |tands for Interr| 00000b40 75 70 74 20 52 65 51 75 65 73 74 29 2e 20 20 49 |upt ReQuest). I| 00000b50 52 51 31 20 61 6e 64 0d 49 52 51 32 20 68 61 76 |RQ1 and.IRQ2 hav| 00000b60 65 20 64 69 66 66 65 72 65 6e 74 20 70 72 69 6f |e different prio| 00000b70 72 69 74 69 65 73 2e 20 20 41 6e 20 69 6e 74 65 |rities. An inte| 00000b80 72 72 75 70 74 20 72 65 71 75 65 73 74 20 66 69 |rrupt request fi| 00000b90 72 73 74 0d 70 61 73 73 65 73 20 74 68 72 6f 75 |rst.passes throu| 00000ba0 67 68 20 49 52 51 31 20 74 6f 20 74 68 65 20 73 |gh IRQ1 to the s| 00000bb0 74 61 6e 64 61 72 64 20 69 6e 74 65 72 72 75 70 |tandard interrup| 00000bc0 74 20 68 61 6e 64 6c 69 6e 67 0d 72 6f 75 74 69 |t handling.routi| 00000bd0 6e 65 73 20 69 6e 20 74 68 65 20 4f 53 20 77 68 |nes in the OS wh| 00000be0 69 63 68 20 64 6f 20 73 79 73 74 65 6d 20 68 6f |ich do system ho| 00000bf0 75 73 65 6b 65 65 70 69 6e 67 2e 20 20 54 68 65 |usekeeping. The| 00000c00 6e 20 74 68 65 0d 69 6e 74 65 72 72 75 70 74 20 |n the.interrupt | 00000c10 69 73 20 70 61 73 73 65 64 20 74 68 72 6f 75 67 |is passed throug| 00000c20 68 20 49 52 51 32 2e 20 20 44 69 61 67 72 61 6d |h IRQ2. Diagram| 00000c30 6d 61 74 69 63 61 6c 6c 79 20 69 74 27 73 0d 6c |matically it's.l| 00000c40 69 6b 65 20 74 68 69 73 3a 0d 0d 20 20 20 20 20 |ike this:.. | 00000c50 20 20 20 20 20 20 20 49 6e 74 65 72 72 75 70 74 | Interrupt| 00000c60 20 44 65 74 65 63 74 65 64 0d 20 20 20 20 20 20 | Detected. | 00000c70 20 20 20 20 20 20 56 65 63 74 6f 72 65 64 20 74 | Vectored t| 00000c80 68 72 6f 75 67 68 20 49 52 51 31 0d 20 20 20 20 |hrough IRQ1. | 00000c90 20 20 20 20 20 20 20 20 53 79 73 74 65 6d 20 69 | System i| 00000ca0 6e 74 65 72 72 75 70 74 20 68 61 6e 64 6c 69 6e |nterrupt handlin| 00000cb0 67 0d 20 20 20 20 20 20 20 20 20 20 20 20 56 65 |g. Ve| 00000cc0 63 74 6f 72 65 64 20 74 68 72 6f 75 67 68 20 49 |ctored through I| 00000cd0 52 51 32 0d 20 20 20 20 20 20 20 20 20 20 20 20 |RQ2. | 00000ce0 55 73 65 72 20 69 6e 74 65 72 72 75 70 74 20 68 |User interrupt h| 00000cf0 61 6e 64 6c 69 6e 67 0d 0d 54 68 69 73 20 72 65 |andling..This re| 00000d00 66 6c 65 63 74 73 20 77 68 61 74 20 49 20 63 61 |flects what I ca| 00000d10 6c 6c 65 64 20 74 68 65 20 70 65 63 6b 69 6e 67 |lled the pecking| 00000d20 20 6f 72 64 65 72 2e 20 20 54 68 65 20 73 79 73 | order. The sys| 00000d30 74 65 6d 0d 68 6f 75 73 65 6b 65 65 70 69 6e 67 |tem.housekeeping| 00000d40 20 68 61 73 20 74 6f 20 62 65 20 64 6f 6e 65 20 | has to be done | 00000d50 75 72 67 65 6e 74 6c 79 2e 20 20 49 66 20 79 6f |urgently. If yo| 00000d60 75 72 20 69 6e 74 65 72 72 75 70 74 0d 72 6f 75 |ur interrupt.rou| 00000d70 74 69 6e 65 20 69 73 20 6e 6f 74 20 75 72 67 65 |tine is not urge| 00000d80 6e 74 20 74 68 65 6e 20 69 74 20 73 68 6f 75 6c |nt then it shoul| 00000d90 64 20 69 6e 74 65 72 63 65 70 74 20 49 52 51 32 |d intercept IRQ2| 00000da0 2e 20 20 4f 6e 6c 79 0d 69 6e 74 65 72 63 65 70 |. Only.intercep| 00000db0 74 20 49 52 51 31 20 69 66 20 69 74 20 69 73 20 |t IRQ1 if it is | 00000dc0 69 6d 70 6f 72 74 61 6e 74 20 66 6f 72 20 74 68 |important for th| 00000dd0 65 72 65 20 74 6f 20 62 65 20 61 20 66 61 73 74 |ere to be a fast| 00000de0 0d 72 65 73 70 6f 6e 73 65 20 74 6f 20 79 6f 75 |.response to you| 00000df0 72 20 63 6f 64 65 2e 0d 0d 54 68 65 20 4f 53 20 |r code...The OS | 00000e00 73 65 74 73 20 75 70 20 76 61 72 69 6f 75 73 20 |sets up various | 00000e10 70 61 72 61 6d 65 74 65 72 73 20 69 6e 20 74 68 |parameters in th| 00000e20 65 20 6d 69 63 72 6f 20 69 6e 20 61 20 73 74 61 |e micro in a sta| 00000e30 6e 64 61 72 64 0d 77 61 79 20 73 6f 20 74 68 61 |ndard.way so tha| 00000e40 74 20 79 6f 75 20 6b 6e 6f 77 20 74 68 65 20 73 |t you know the s| 00000e50 74 61 74 65 20 6f 66 20 74 68 65 20 6d 61 63 68 |tate of the mach| 00000e60 69 6e 65 20 77 68 65 6e 20 79 6f 75 0d 69 6e 74 |ine when you.int| 00000e70 65 72 63 65 70 74 20 49 52 51 31 20 6f 72 20 49 |ercept IRQ1 or I| 00000e80 52 51 32 2e 20 20 49 74 20 68 61 70 70 65 6e 73 |RQ2. It happens| 00000e90 20 6c 69 6b 65 20 74 68 69 73 2e 0d 0d 57 68 65 | like this...Whe| 00000ea0 6e 20 61 20 6d 61 73 6b 61 62 6c 65 20 69 6e 74 |n a maskable int| 00000eb0 65 72 72 75 70 74 20 69 73 20 64 65 74 65 63 74 |errupt is detect| 00000ec0 65 64 20 62 79 20 74 68 65 20 36 35 30 32 20 69 |ed by the 6502 i| 00000ed0 74 0d 70 72 6f 6d 70 74 6c 79 20 63 68 65 63 6b |t.promptly check| 00000ee0 73 20 74 6f 20 73 65 65 20 69 66 20 69 74 20 68 |s to see if it h| 00000ef0 61 73 20 62 65 65 6e 20 63 61 75 73 65 64 20 62 |as been caused b| 00000f00 79 20 61 20 42 52 4b 20 69 6e 20 74 68 65 0d 63 |y a BRK in the.c| 00000f10 6f 64 65 20 69 74 20 77 61 73 20 65 78 65 63 75 |ode it was execu| 00000f20 74 69 6e 67 20 28 62 79 20 6c 6f 6f 6b 69 6e 67 |ting (by looking| 00000f30 20 61 74 20 74 68 65 20 73 74 61 74 65 20 6f 66 | at the state of| 00000f40 20 74 68 65 20 42 52 4b 0d 66 6c 61 67 29 2e 20 | the BRK.flag). | 00000f50 20 49 66 20 69 74 20 77 61 73 20 74 68 65 6e 20 | If it was then | 00000f60 74 68 65 20 70 72 6f 63 65 73 73 6f 72 20 65 78 |the processor ex| 00000f70 65 63 75 74 65 73 20 74 68 65 20 63 75 72 72 65 |ecutes the curre| 00000f80 6e 74 0d 65 72 72 6f 72 20 72 6f 75 74 69 6e 65 |nt.error routine| 00000f90 20 68 61 76 69 6e 67 20 66 6f 75 6e 64 20 69 74 | having found it| 00000fa0 20 62 79 20 67 6f 69 6e 67 20 74 68 72 6f 75 67 | by going throug| 00000fb0 68 20 74 68 65 20 62 72 65 61 6b 0d 76 65 63 74 |h the break.vect| 00000fc0 6f 72 2e 0d 0d 49 66 20 74 68 65 20 62 72 65 61 |or...If the brea| 00000fd0 6b 20 66 6c 61 67 20 77 61 73 20 6e 6f 74 20 73 |k flag was not s| 00000fe0 65 74 20 74 68 65 6e 20 61 20 68 61 72 64 77 61 |et then a hardwa| 00000ff0 72 65 20 69 6e 74 65 72 72 75 70 74 20 77 61 73 |re interrupt was| 00001000 0d 74 68 65 20 63 61 75 73 65 20 6f 66 20 74 68 |.the cause of th| 00001010 65 20 69 6e 74 65 72 72 75 70 74 2e 20 20 54 68 |e interrupt. Th| 00001020 69 73 20 69 73 20 74 68 65 20 63 6f 6e 64 69 74 |is is the condit| 00001030 69 6f 6e 20 77 65 20 61 72 65 0d 73 74 75 64 79 |ion we are.study| 00001040 69 6e 67 20 69 6e 20 74 68 69 73 20 6d 6f 64 75 |ing in this modu| 00001050 6c 65 2e 0d 0d 48 61 76 69 6e 67 20 64 65 74 65 |le...Having dete| 00001060 72 6d 69 6e 65 64 20 74 68 61 74 20 61 6e 20 49 |rmined that an I| 00001070 52 51 20 68 61 73 20 69 6e 64 65 65 64 20 6f 63 |RQ has indeed oc| 00001080 63 75 72 72 65 64 20 74 68 65 20 36 35 30 32 0d |curred the 6502.| 00001090 6a 75 6d 70 73 20 74 6f 20 74 68 65 20 63 6f 6e |jumps to the con| 000010a0 74 65 6e 74 73 20 6f 66 20 74 68 65 20 49 52 51 |tents of the IRQ| 000010b0 31 56 20 76 65 63 74 6f 72 2e 20 20 55 6e 74 69 |1V vector. Unti| 000010c0 6c 20 6e 6f 77 20 74 68 69 73 0d 68 61 73 20 68 |l now this.has h| 000010d0 61 70 70 65 6e 65 64 20 61 75 74 6f 6d 61 74 69 |appened automati| 000010e0 63 61 6c 6c 79 20 61 6e 64 20 74 68 65 20 75 73 |cally and the us| 000010f0 65 72 20 64 6f 65 73 20 6e 6f 74 20 6e 65 65 64 |er does not need| 00001100 20 74 6f 0d 77 6f 72 72 79 20 61 62 6f 75 74 20 | to.worry about | 00001110 69 74 2e 20 20 4e 6f 77 20 74 68 6f 75 67 68 20 |it. Now though | 00001120 74 68 65 20 4f 53 20 69 73 20 70 72 65 70 61 72 |the OS is prepar| 00001130 69 6e 67 20 74 6f 20 68 61 6e 64 20 6f 76 65 72 |ing to hand over| 00001140 0d 74 6f 20 69 6e 74 65 72 72 75 70 74 20 68 61 |.to interrupt ha| 00001150 6e 64 6c 69 6e 67 20 72 6f 75 74 69 6e 65 73 20 |ndling routines | 00001160 61 6e 64 20 74 68 65 20 66 6f 6c 6c 6f 77 69 6e |and the followin| 00001170 67 20 69 73 20 6b 6e 6f 77 6e 0d 61 62 6f 75 74 |g is known.about| 00001180 20 74 68 65 20 73 74 61 74 65 20 6f 66 20 74 68 | the state of th| 00001190 65 20 6d 69 63 72 6f 20 62 65 63 61 75 73 65 20 |e micro because | 000011a0 74 68 65 20 4f 53 20 68 61 73 20 73 65 74 20 69 |the OS has set i| 000011b0 74 20 75 70 0d 74 68 69 73 20 77 61 79 3a 0d 0d |t up.this way:..| 000011c0 54 68 65 20 76 61 6c 75 65 20 6f 66 20 74 68 65 |The value of the| 000011d0 20 61 63 63 75 6d 75 6c 61 74 6f 72 20 61 74 20 | accumulator at | 000011e0 74 68 65 20 74 69 6d 65 20 6f 66 20 74 68 65 20 |the time of the | 000011f0 69 6e 74 65 72 72 75 70 74 20 69 73 0d 73 74 6f |interrupt is.sto| 00001200 72 65 64 20 69 6e 20 6d 65 6d 6f 72 79 20 6c 6f |red in memory lo| 00001210 63 61 74 69 6f 6e 20 26 46 43 20 69 6e 20 74 68 |cation &FC in th| 00001220 65 20 49 2f 4f 20 70 72 6f 63 65 73 73 6f 72 2e |e I/O processor.| 00001230 20 20 54 68 65 0d 73 74 61 74 75 73 20 72 65 67 | The.status reg| 00001240 69 73 74 65 72 20 61 6e 64 20 74 68 65 20 72 65 |ister and the re| 00001250 74 75 72 6e 20 61 64 64 72 65 73 73 20 28 74 68 |turn address (th| 00001260 65 20 61 64 64 72 65 73 73 20 6f 66 20 74 68 65 |e address of the| 00001270 0d 69 6e 73 74 72 75 63 74 69 6f 6e 20 61 62 6f |.instruction abo| 00001280 75 74 20 74 6f 20 62 65 20 61 63 74 69 6f 6e 65 |ut to be actione| 00001290 64 20 77 68 65 6e 20 74 68 65 20 69 6e 74 65 72 |d when the inter| 000012a0 72 75 70 74 0d 6f 63 63 75 72 72 65 64 29 20 61 |rupt.occurred) a| 000012b0 72 65 20 73 74 6f 72 65 64 20 6f 6e 20 74 68 65 |re stored on the| 000012c0 20 73 74 61 63 6b 20 72 65 61 64 79 20 66 6f 72 | stack ready for| 000012d0 20 61 6e 20 52 54 49 20 28 52 65 54 75 72 6e 0d | an RTI (ReTurn.| 000012e0 66 72 6f 6d 20 49 6e 74 65 72 72 75 70 74 29 20 |from Interrupt) | 000012f0 61 74 20 74 68 65 20 65 6e 64 20 6f 66 20 74 68 |at the end of th| 00001300 65 20 69 6e 74 65 72 72 75 70 74 20 63 6f 64 65 |e interrupt code| 00001310 2e 20 20 54 68 65 0d 6f 72 69 67 69 6e 61 6c 20 |. 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It| 00002130 20 63 61 6e 20 71 75 69 63 6b 6c 79 20 63 68 65 | can quickly che| 00002140 63 6b 20 69 66 20 61 6e 0d 69 6e 74 65 72 72 75 |ck if an.interru| 00002150 70 74 20 63 61 6d 65 20 66 72 6f 6d 20 61 20 70 |pt came from a p| 00002160 61 72 74 69 63 75 6c 61 72 20 64 65 76 69 63 65 |articular device| 00002170 20 73 69 6e 63 65 20 65 61 63 68 20 64 65 76 69 | since each devi| 00002180 63 65 0d 68 61 73 20 61 20 66 6c 61 67 20 74 68 |ce.has a flag th| 00002190 61 74 20 69 73 20 73 65 74 20 77 68 65 6e 20 61 |at is set when a| 000021a0 6e 79 20 69 6e 74 65 72 72 75 70 74 20 63 6f 6d |ny interrupt com| 000021b0 65 73 20 66 72 6f 6d 20 69 74 2e 0d 54 68 69 73 |es from it..This| 000021c0 20 69 73 20 62 65 73 69 64 65 73 20 61 20 66 6c | is besides a fl| 000021d0 61 67 20 74 6f 20 69 64 65 6e 74 69 66 79 20 74 |ag to identify t| 000021e0 68 65 20 61 63 74 75 61 6c 20 70 61 72 74 20 6f |he actual part o| 000021f0 66 20 74 68 65 0d 64 65 76 69 63 65 20 74 68 61 |f the.device tha| 00002200 74 20 63 61 75 73 65 64 20 74 68 65 20 69 6e 74 |t caused the int| 00002210 65 72 72 75 70 74 2e 0d 0d 53 6f 20 74 68 65 20 |errupt...So the | 00002220 4f 53 20 6f 6e 6c 79 20 68 61 73 20 74 6f 20 6c |OS only has to l| 00002230 6f 6f 6b 20 61 74 20 74 68 65 20 66 6c 61 67 20 |ook at the flag | 00002240 6f 6e 20 65 61 63 68 20 64 65 76 69 63 65 20 61 |on each device a| 00002250 6e 64 0d 74 68 65 6e 20 63 68 65 63 6b 20 74 68 |nd.then check th| 00002260 65 20 66 6c 61 67 20 72 65 67 69 73 74 65 72 20 |e flag register | 00002270 6f 6e 20 74 68 65 20 64 65 76 69 63 65 20 74 68 |on the device th| 00002280 61 74 20 73 61 79 73 20 69 74 0d 63 61 75 73 65 |at says it.cause| 00002290 64 20 74 68 65 20 69 6e 74 65 72 72 75 70 74 2e |d the interrupt.| 000022a0 20 20 49 74 20 64 6f 65 73 6e 27 74 20 68 61 76 | It doesn't hav| 000022b0 65 20 74 6f 20 63 68 65 63 6b 20 61 6c 6c 20 74 |e to check all t| 000022c0 68 65 0d 66 6c 61 67 73 20 6f 6e 20 61 6c 6c 20 |he.flags on all | 000022d0 74 68 65 20 64 65 76 69 63 65 73 2e 20 20 54 68 |the devices. 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It wil| 00002460 6c 20 74 68 65 6e 0d 63 68 65 63 6b 20 74 68 65 |l then.check the| 00002470 20 64 65 76 69 63 65 20 69 74 73 65 6c 66 20 74 | device itself t| 00002480 6f 20 69 64 65 6e 74 69 66 79 20 74 68 65 20 70 |o identify the p| 00002490 61 72 74 69 63 75 6c 61 72 20 63 61 75 73 65 2e |articular cause.| 000024a0 20 0d 49 66 20 74 68 69 73 20 69 73 20 6f 6e 65 | .If this is one| 000024b0 20 77 68 69 63 68 20 74 68 65 20 4f 53 20 65 78 | which the OS ex| 000024c0 70 65 63 74 73 20 74 6f 20 73 65 72 76 69 63 65 |pects to service| 000024d0 2c 20 69 74 20 77 69 6c 6c 20 64 6f 0d 73 6f 20 |, it will do.so | 000024e0 61 6e 64 20 74 68 65 6e 20 72 65 74 75 72 6e 2e |and then return.| 000024f0 20 20 49 66 20 69 74 20 69 73 20 6f 6e 65 20 77 | If it is one w| 00002500 68 69 63 68 20 74 68 65 20 4f 53 20 64 6f 65 73 |hich the OS does| 00002510 6e 27 74 0d 72 65 63 6f 67 6e 69 73 65 20 69 74 |n't.recognise it| 00002520 20 70 61 73 73 65 73 20 69 74 20 6f 6e 20 74 68 | passes it on th| 00002530 72 6f 75 67 68 20 49 52 51 32 2e 0d 0d 49 74 20 |rough IRQ2...It | 00002540 69 73 20 70 6f 73 73 69 62 6c 65 20 74 6f 20 73 |is possible to s| 00002550 74 6f 70 20 74 68 65 20 4f 53 20 73 65 72 76 69 |top the OS servi| 00002560 63 69 6e 67 20 61 6e 20 69 6e 74 65 72 72 75 70 |cing an interrup| 00002570 74 20 69 74 0d 77 6f 75 6c 64 20 6e 6f 72 6d 61 |t it.would norma| 00002580 6c 6c 79 20 64 6f 20 62 79 20 75 73 69 6e 67 20 |lly do by using | 00002590 4f 53 42 59 54 45 20 32 33 31 20 74 6f 20 32 33 |OSBYTE 231 to 23| 000025a0 33 20 77 68 69 63 68 20 63 61 6e 20 66 6f 72 63 |3 which can forc| 000025b0 65 0d 74 68 65 20 4f 53 20 74 6f 20 70 61 73 73 |e.the OS to pass| 000025c0 20 61 20 70 61 72 74 69 63 75 6c 61 72 20 69 6e | a particular in| 000025d0 74 65 72 72 75 70 74 20 74 68 72 6f 75 67 68 20 |terrupt through | 000025e0 49 52 51 32 20 66 6f 72 20 74 68 65 0d 75 73 65 |IRQ2 for the.use| 000025f0 72 20 74 6f 20 64 65 61 6c 20 77 69 74 68 2e 20 |r to deal with. | 00002600 20 54 68 69 73 20 69 73 20 6b 6e 6f 77 6e 20 61 | This is known a| 00002610 73 20 6d 61 73 6b 69 6e 67 20 74 68 65 20 49 52 |s masking the IR| 00002620 51 20 61 6e 64 20 79 6f 75 0d 73 68 6f 75 6c 64 |Q and you.should| 00002630 20 63 6f 6e 73 75 6c 74 20 79 6f 75 72 20 61 64 | consult your ad| 00002640 76 61 6e 63 65 64 20 67 75 69 64 65 20 66 6f 72 |vanced guide for| 00002650 20 64 65 74 61 69 6c 73 2e 20 20 5b 54 68 65 72 | details. [Ther| 00002660 65 20 61 72 65 0d 65 78 61 6d 70 6c 65 20 70 72 |e are.example pr| 00002670 6f 67 72 61 6d 73 20 77 68 69 63 68 20 75 73 65 |ograms which use| 00002680 20 49 52 51 20 6d 61 73 6b 69 6e 67 20 6f 6e 20 | IRQ masking on | 00002690 70 61 67 65 73 20 33 30 36 20 61 6e 64 20 33 30 |pages 306 and 30| 000026a0 38 0d 6f 66 20 42 72 61 79 2c 20 44 69 63 6b 65 |8.of Bray, Dicke| 000026b0 6e 73 20 61 6e 64 20 48 6f 6c 6d 65 73 2e 5d 0d |ns and Holmes.].| 000026c0 0d 49 20 68 6f 70 65 20 79 6f 75 20 77 69 6c 6c |.I hope you will| 000026d0 20 73 65 65 20 77 68 79 20 69 6e 74 65 72 72 75 | see why interru| 000026e0 70 74 73 20 61 72 65 20 61 20 27 72 69 73 6b 79 |pts are a 'risky| 000026f0 27 20 61 72 65 61 20 66 6f 72 0d 65 78 70 65 72 |' area for.exper| 00002700 69 6d 65 6e 74 61 74 69 6f 6e 2c 20 73 69 6e 63 |imentation, sinc| 00002710 65 20 69 74 20 69 73 20 61 6c 6c 20 74 6f 6f 20 |e it is all too | 00002720 65 61 73 79 20 74 6f 20 6d 61 6b 65 20 74 68 65 |easy to make the| 00002730 0d 63 6f 6d 70 75 74 65 72 20 68 61 6e 67 20 75 |.computer hang u| 00002740 70 2e 20 20 48 6f 77 65 76 65 72 2c 20 69 74 20 |p. However, it | 00002750 69 73 20 75 6e 6c 69 6b 65 6c 79 20 74 6f 20 65 |is unlikely to e| 00002760 78 70 6c 6f 64 65 20 73 6f 20 74 68 65 0d 77 6f |xplode so the.wo| 00002770 72 73 74 20 74 68 61 74 20 63 61 6e 20 68 61 70 |rst that can hap| 00002780 70 65 6e 20 69 73 20 74 68 61 74 20 79 6f 75 20 |pen is that you | 00002790 77 69 6c 6c 20 68 61 76 65 20 74 6f 20 70 72 65 |will have to pre| 000027a0 73 73 20 42 52 45 41 4b 0d 61 6e 64 20 73 74 61 |ss BREAK.and sta| 000027b0 72 74 20 61 67 61 69 6e 2e 0d 0d 49 20 74 68 69 |rt again...I thi| 000027c0 6e 6b 20 69 74 20 69 73 20 62 65 73 74 20 74 6f |nk it is best to| 000027d0 20 70 72 6f 63 65 65 64 20 66 72 6f 6d 20 68 65 | proceed from he| 000027e0 72 65 20 77 69 74 68 20 65 78 61 6d 70 6c 65 73 |re with examples| 000027f0 2e 20 20 49 0d 70 6c 61 6e 20 74 6f 20 73 68 6f |. I.plan to sho| 00002800 77 20 79 6f 75 20 6d 6f 72 65 20 6f 66 20 74 68 |w you more of th| 00002810 65 20 56 49 41 20 74 69 6d 65 72 73 20 69 6e 20 |e VIA timers in | 00002820 74 68 65 20 6e 65 78 74 20 6d 6f 64 75 6c 65 0d |the next module.| 00002830 62 75 74 20 66 6f 72 20 6e 6f 77 20 49 20 77 69 |but for now I wi| 00002840 6c 6c 20 69 6e 69 74 69 61 74 65 20 79 6f 75 20 |ll initiate you | 00002850 69 6e 74 6f 20 74 68 65 20 6c 69 74 74 6c 65 20 |into the little | 00002860 6b 6e 6f 77 6e 20 73 70 6f 72 74 0d 6f 66 20 6d |known sport.of m| 00002870 6f 75 73 65 20 64 72 69 76 69 6e 67 2e 20 20 41 |ouse driving. A| 00002880 20 6d 6f 75 73 65 20 64 72 69 76 65 72 20 69 73 | mouse driver is| 00002890 20 61 20 70 69 65 63 65 20 6f 66 20 63 6f 64 65 | a piece of code| 000028a0 20 74 68 61 74 0d 6b 65 65 70 73 20 74 72 61 63 | that.keeps trac| 000028b0 6b 20 6f 66 20 74 68 65 20 6d 6f 76 65 6d 65 6e |k of the movemen| 000028c0 74 20 6f 66 20 61 20 6d 6f 75 73 65 20 28 69 2e |t of a mouse (i.| 000028d0 65 2e 20 61 20 70 6f 69 6e 74 69 6e 67 0d 64 65 |e. a pointing.de| 000028e0 76 69 63 65 29 20 61 74 74 61 63 68 65 64 20 74 |vice) attached t| 000028f0 6f 20 74 68 65 20 6d 69 63 72 6f 20 61 6e 64 20 |o the micro and | 00002900 65 6e 61 62 6c 65 73 20 79 6f 75 20 74 6f 20 69 |enables you to i| 00002910 6d 70 6c 65 6d 65 6e 74 20 61 0d 70 6f 69 6e 74 |mplement a.point| 00002920 65 72 20 73 79 73 74 65 6d 20 6f 6e 20 74 68 65 |er system on the| 00002930 20 73 63 72 65 65 6e 2e 20 20 49 20 73 68 61 6c | screen. I shal| 00002940 6c 20 75 73 65 20 74 68 65 20 41 4d 58 20 6d 6f |l use the AMX mo| 00002950 75 73 65 20 61 73 0d 6d 79 20 65 78 61 6d 70 6c |use as.my exampl| 00002960 65 2c 20 61 6c 74 68 6f 75 67 68 20 74 68 65 20 |e, although the | 00002970 4d 65 67 61 6d 6f 75 73 65 20 75 73 65 73 20 74 |Megamouse uses t| 00002980 68 65 20 73 61 6d 65 20 63 6f 64 65 2e 20 20 45 |he same code. E| 00002990 76 65 6e 0d 69 66 20 79 6f 75 20 64 6f 20 6e 6f |ven.if you do no| 000029a0 74 20 70 6f 73 73 65 73 20 61 20 6d 6f 75 73 65 |t posses a mouse| 000029b0 20 49 20 68 6f 70 65 20 79 6f 75 20 77 69 6c 6c | I hope you will| 000029c0 20 66 69 6e 64 20 74 68 65 0d 65 78 61 6d 70 6c | find the.exampl| 000029d0 65 20 75 73 65 66 75 6c 2e 0d 0d 54 68 69 73 20 |e useful...This | 000029e0 6d 6f 75 73 65 20 69 73 20 63 6f 6e 6e 65 63 74 |mouse is connect| 000029f0 65 64 20 74 6f 20 74 68 65 20 6d 69 63 72 6f 27 |ed to the micro'| 00002a00 73 20 75 73 65 72 20 70 6f 72 74 20 61 6e 64 20 |s user port and | 00002a10 73 6f 20 74 6f 0d 74 68 65 20 75 73 65 72 20 56 |so to.the user V| 00002a20 49 41 20 62 65 68 69 6e 64 20 69 74 2e 20 20 41 |IA behind it. A| 00002a30 73 20 74 68 65 20 6d 6f 75 73 65 20 69 73 20 6d |s the mouse is m| 00002a40 6f 76 65 64 20 61 20 73 6d 61 6c 6c 0d 76 6f 6c |oved a small.vol| 00002a50 74 61 67 65 20 28 61 6c 73 6f 20 74 61 6b 65 6e |tage (also taken| 00002a60 20 66 72 6f 6d 20 74 68 65 20 75 73 65 72 20 70 | from the user p| 00002a70 6f 72 74 29 20 69 73 20 61 70 70 6c 69 65 64 20 |ort) is applied | 00002a80 74 6f 20 66 6f 75 72 0d 77 69 72 65 73 20 69 6e |to four.wires in| 00002a90 20 74 68 65 20 63 6f 6e 6e 65 63 74 69 6e 67 20 | the connecting | 00002aa0 63 61 62 6c 65 20 69 6e 20 73 75 63 68 20 61 20 |cable in such a | 00002ab0 77 61 79 20 74 68 61 74 20 79 6f 75 20 63 61 6e |way that you can| 00002ac0 0d 64 65 74 65 63 74 20 74 68 65 20 64 69 72 65 |.detect the dire| 00002ad0 63 74 69 6f 6e 20 6f 66 20 6d 6f 74 69 6f 6e 20 |ction of motion | 00002ae0 61 74 20 61 6e 79 20 69 6e 73 74 61 6e 74 2e 20 |at any instant. | 00002af0 20 54 68 65 20 6d 6f 75 73 65 0d 64 6f 65 73 6e | The mouse.doesn| 00002b00 27 74 20 74 65 6c 6c 20 79 6f 75 20 77 68 65 72 |'t tell you wher| 00002b10 65 20 69 74 20 69 73 2c 20 69 74 20 74 65 6c 6c |e it is, it tell| 00002b20 73 20 79 6f 75 20 69 6e 20 77 68 69 63 68 0d 64 |s you in which.d| 00002b30 69 72 65 63 74 69 6f 6e 20 69 74 20 69 73 20 6d |irection it is m| 00002b40 6f 76 69 6e 67 2e 0d 0d 54 68 65 20 77 61 76 65 |oving...The wave| 00002b50 66 6f 72 6d 20 6f 66 20 74 68 65 20 76 6f 6c 74 |form of the volt| 00002b60 61 67 65 20 69 73 20 72 65 63 74 61 6e 67 75 6c |age is rectangul| 00002b70 61 72 2c 20 6c 69 6b 65 20 61 20 73 71 75 61 72 |ar, like a squar| 00002b80 65 0d 77 61 76 65 2c 20 61 6e 64 20 69 66 20 77 |e.wave, and if w| 00002b90 65 20 63 61 6c 6c 20 74 68 65 20 63 6f 6e 6e 65 |e call the conne| 00002ba0 63 74 69 6f 6e 73 20 58 31 20 59 31 20 58 32 20 |ctions X1 Y1 X2 | 00002bb0 61 6e 64 20 59 32 20 74 68 65 0d 66 6f 6c 6c 6f |and Y2 the.follo| 00002bc0 77 69 6e 67 20 72 75 6c 65 73 20 61 70 70 6c 79 |wing rules apply| 00002bd0 2e 0d 0d 49 66 20 74 68 65 20 6d 6f 75 73 65 20 |...If the mouse | 00002be0 6d 6f 76 65 73 20 6c 65 66 74 20 74 6f 20 72 69 |moves left to ri| 00002bf0 67 68 74 20 28 6f 72 20 76 69 63 65 20 76 65 72 |ght (or vice ver| 00002c00 73 61 29 20 61 20 77 61 76 65 66 6f 72 6d 0d 69 |sa) a waveform.i| 00002c10 66 20 61 70 70 6c 69 65 64 20 74 6f 20 74 68 65 |f applied to the| 00002c20 20 58 31 20 61 6e 64 20 58 32 20 77 69 72 65 73 | X1 and X2 wires| 00002c30 20 73 75 63 68 20 74 68 61 74 20 69 66 20 69 74 | such that if it| 00002c40 20 6d 6f 76 65 73 20 6f 6e 65 0d 77 61 79 20 74 | moves one.way t| 00002c50 68 65 20 73 69 67 6e 61 6c 73 20 61 72 65 20 68 |he signals are h| 00002c60 69 67 68 20 61 74 20 74 68 65 20 73 61 6d 65 20 |igh at the same | 00002c70 74 69 6d 65 20 61 6e 64 20 6c 6f 77 20 61 74 20 |time and low at | 00002c80 74 68 65 0d 73 61 6d 65 20 74 69 6d 65 20 28 69 |the.same time (i| 00002c90 2e 65 2e 20 69 6e 20 70 68 61 73 65 29 20 61 6e |.e. in phase) an| 00002ca0 64 20 69 66 20 69 74 20 6d 6f 76 65 73 20 74 68 |d if it moves th| 00002cb0 65 20 6f 74 68 65 72 20 74 68 65 0d 73 69 67 6e |e other the.sign| 00002cc0 61 6c 73 20 61 72 65 20 64 69 66 66 65 72 65 6e |als are differen| 00002cd0 74 20 61 74 20 61 6e 79 20 69 6e 73 74 61 6e 74 |t at any instant| 00002ce0 20 28 69 2e 65 2e 20 6f 75 74 20 6f 66 20 70 68 | (i.e. out of ph| 00002cf0 61 73 65 29 2e 20 0d 53 69 6d 69 6c 61 72 6c 79 |ase). .Similarly| 00002d00 20 66 6f 72 20 74 68 65 20 59 20 77 69 72 65 73 | for the Y wires| 00002d10 20 77 68 65 6e 20 74 68 65 20 6d 6f 75 73 65 20 | when the mouse | 00002d20 6d 6f 76 65 73 20 62 61 63 6b 77 61 72 64 73 20 |moves backwards | 00002d30 61 6e 64 0d 66 6f 72 77 61 72 64 73 2e 0d 0d 58 |and.forwards...X| 00002d40 31 20 61 6e 64 20 59 31 20 61 72 65 20 63 6f 6e |1 and Y1 are con| 00002d50 6e 65 63 74 65 64 20 74 6f 20 74 68 65 20 74 77 |nected to the tw| 00002d60 6f 20 69 6e 74 65 72 72 75 70 74 20 6c 69 6e 65 |o interrupt line| 00002d70 73 20 6f 6e 20 74 68 65 0d 75 73 65 72 20 70 6f |s on the.user po| 00002d80 72 74 20 73 6f 20 74 68 65 79 20 63 61 6e 20 62 |rt so they can b| 00002d90 65 20 75 73 65 64 20 74 6f 20 67 65 6e 65 72 61 |e used to genera| 00002da0 74 65 20 61 6e 20 69 6e 74 65 72 72 75 70 74 20 |te an interrupt | 00002db0 66 72 6f 6d 0d 74 68 65 20 75 73 65 72 20 56 49 |from.the user VI| 00002dc0 41 2e 20 20 57 65 20 77 69 6c 6c 20 73 65 74 20 |A. We will set | 00002dd0 74 68 65 20 56 49 41 20 74 6f 20 67 65 6e 65 72 |the VIA to gener| 00002de0 61 74 65 20 74 68 69 73 0d 69 6e 74 65 72 72 75 |ate this.interru| 00002df0 70 74 20 77 68 65 6e 65 76 65 72 20 74 68 65 20 |pt whenever the | 00002e00 56 49 41 20 6c 69 6e 65 20 64 65 74 65 63 74 73 |VIA line detects| 00002e10 20 61 20 70 6f 73 69 74 69 76 65 20 67 6f 69 6e | a positive goin| 00002e20 67 0d 76 6f 6c 74 61 67 65 20 28 69 2e 65 2e 20 |g.voltage (i.e. | 00002e30 74 68 65 20 66 72 6f 6e 74 20 70 6f 72 63 68 20 |the front porch | 00002e40 6f 66 20 74 68 65 20 72 65 63 74 61 6e 67 75 6c |of the rectangul| 00002e50 61 72 20 77 61 76 65 66 6f 72 6d 29 0d 66 72 6f |ar waveform).fro| 00002e60 6d 20 65 69 74 68 65 72 20 6c 69 6e 65 2e 0d 0d |m either line...| 00002e70 57 68 65 6e 20 74 68 65 20 69 6e 74 65 72 72 75 |When the interru| 00002e80 70 74 20 6f 63 63 75 72 73 20 77 65 20 6c 6f 6f |pt occurs we loo| 00002e90 6b 20 61 74 20 74 68 65 20 63 6f 72 72 65 73 70 |k at the corresp| 00002ea0 6f 6e 64 69 6e 67 0d 73 65 63 6f 6e 64 20 6c 69 |onding.second li| 00002eb0 6e 65 20 28 58 32 20 6f 72 20 59 32 29 20 74 6f |ne (X2 or Y2) to| 00002ec0 20 73 65 65 20 69 66 20 69 74 20 69 73 20 68 69 | see if it is hi| 00002ed0 67 68 20 6f 72 20 6c 6f 77 20 61 74 20 74 68 61 |gh or low at tha| 00002ee0 74 0d 6d 6f 6d 65 6e 74 20 61 6e 64 20 68 65 6e |t.moment and hen| 00002ef0 63 65 20 64 65 64 75 63 65 20 77 68 69 63 68 20 |ce deduce which | 00002f00 77 61 79 20 74 68 65 20 6d 6f 75 73 65 20 69 73 |way the mouse is| 00002f10 20 6d 6f 76 69 6e 67 2e 0d 0d 41 73 20 49 20 68 | moving...As I h| 00002f20 61 76 65 20 73 61 69 64 2c 20 74 68 65 20 6c 69 |ave said, the li| 00002f30 6e 65 73 20 6f 6e 20 74 68 65 20 75 73 65 72 20 |nes on the user | 00002f40 70 6f 72 74 20 61 72 65 20 63 6f 6e 6e 65 63 74 |port are connect| 00002f50 65 64 20 74 6f 0d 74 68 65 20 75 73 65 72 20 56 |ed to.the user V| 00002f60 49 41 2e 20 20 49 6e 20 66 61 63 74 20 74 68 65 |IA. In fact the| 00002f70 20 75 73 65 72 20 70 6f 72 74 20 63 6f 6e 6e 65 | user port conne| 00002f80 63 74 73 20 74 6f 20 74 68 65 20 27 42 27 0d 73 |cts to the 'B'.s| 00002f90 69 64 65 20 6f 66 20 74 68 69 73 20 64 65 76 69 |ide of this devi| 00002fa0 63 65 20 61 6e 64 20 74 68 65 20 70 61 72 61 6c |ce and the paral| 00002fb0 6c 65 6c 20 70 72 69 6e 74 65 72 20 63 6f 6e 6e |lel printer conn| 00002fc0 65 63 74 73 20 74 6f 20 74 68 65 0d 6f 74 68 65 |ects to the.othe| 00002fd0 72 2e 20 20 54 68 65 20 56 49 41 20 69 73 20 61 |r. The VIA is a| 00002fe0 20 64 65 76 69 63 65 20 63 61 6c 6c 65 64 20 61 | device called a| 00002ff0 20 36 35 32 32 20 61 6e 64 20 69 6e 66 6f 72 6d | 6522 and inform| 00003000 61 74 69 6f 6e 20 6f 6e 0d 68 6f 77 20 74 6f 20 |ation on.how to | 00003010 75 73 65 20 69 74 20 63 61 6e 20 62 65 20 67 6c |use it can be gl| 00003020 65 61 6e 65 64 20 28 77 69 74 68 20 61 20 6c 69 |eaned (with a li| 00003030 74 74 6c 65 20 64 69 66 66 69 63 75 6c 74 79 29 |ttle difficulty)| 00003040 20 66 72 6f 6d 0d 74 68 65 20 73 70 65 63 69 66 | from.the specif| 00003050 69 63 61 74 69 6f 6e 20 73 68 65 65 74 73 20 66 |ication sheets f| 00003060 6f 72 20 74 68 65 20 64 65 76 69 63 65 2e 20 20 |or the device. | 00003070 42 72 61 79 2c 20 44 69 63 6b 65 6e 73 20 61 6e |Bray, Dickens an| 00003080 64 0d 48 6f 6c 6d 65 73 20 28 41 64 76 61 6e 63 |d.Holmes (Advanc| 00003090 65 64 20 55 73 65 72 20 47 75 69 64 65 29 20 61 |ed User Guide) a| 000030a0 6c 73 6f 20 72 65 70 72 69 6e 74 73 20 74 68 65 |lso reprints the| 000030b0 20 69 6e 66 6f 72 6d 61 74 69 6f 6e 0d 66 72 6f | information.fro| 000030c0 6d 20 74 68 65 20 73 70 65 63 20 73 68 65 65 74 |m the spec sheet| 000030d0 2e 0d 0d 49 6e 20 62 72 69 65 66 20 74 68 65 20 |...In brief the | 000030e0 64 65 76 69 63 65 20 69 73 20 73 65 74 20 75 70 |device is set up| 000030f0 20 62 79 20 70 6f 6b 69 6e 67 20 62 69 74 73 20 | by poking bits | 00003100 69 6e 20 69 74 73 0d 72 65 67 69 73 74 65 72 73 |in its.registers| 00003110 2e 20 20 4f 6e 20 74 68 65 20 42 42 43 20 4d 69 |. On the BBC Mi| 00003120 63 72 6f 20 74 68 6f 73 65 20 72 65 67 69 73 74 |cro those regist| 00003130 65 72 73 20 6c 69 76 65 20 69 6e 20 74 68 65 0d |ers live in the.| 00003140 61 72 65 61 20 6f 66 20 6d 65 6d 6f 72 79 20 6b |area of memory k| 00003150 6e 6f 77 6e 20 61 73 20 53 68 65 69 6c 61 20 61 |nown as Sheila a| 00003160 6e 64 20 74 68 65 20 75 73 65 72 20 56 49 41 20 |nd the user VIA | 00003170 6c 69 76 65 73 20 66 72 6f 6d 0d 26 46 45 36 30 |lives from.&FE60| 00003180 20 74 6f 20 26 46 45 36 46 2e 20 20 54 68 65 72 | to &FE6F. Ther| 00003190 65 20 61 72 65 20 31 36 20 72 65 67 69 73 74 65 |e are 16 registe| 000031a0 72 73 20 61 73 20 66 6f 6c 6c 6f 77 73 3a 0d 0d |rs as follows:..| 000031b0 20 26 46 45 36 30 20 20 49 2f 4f 20 66 6f 72 20 | &FE60 I/O for | 000031c0 74 68 65 20 27 42 27 20 73 69 64 65 20 6f 66 20 |the 'B' side of | 000031d0 74 68 65 20 56 49 41 0d 20 20 20 20 20 31 20 20 |the VIA. 1 | 000031e0 49 2f 4f 20 66 6f 72 20 74 68 65 20 27 41 27 20 |I/O for the 'A' | 000031f0 73 69 64 65 20 6f 66 20 74 68 65 20 56 49 41 0d |side of the VIA.| 00003200 20 20 20 20 20 32 20 20 43 6f 6e 74 72 6f 6c 20 | 2 Control | 00003210 6f 66 20 74 68 65 20 64 69 72 65 63 74 69 6f 6e |of the direction| 00003220 20 6f 66 20 64 61 74 61 20 6f 6e 20 74 68 65 20 | of data on the | 00003230 27 42 27 20 73 69 64 65 0d 20 20 20 20 20 33 20 |'B' side. 3 | 00003240 20 43 6f 6e 74 72 6f 6c 20 6f 66 20 74 68 65 20 | Control of the | 00003250 64 69 72 65 63 74 69 6f 6e 20 6f 66 20 64 61 74 |direction of dat| 00003260 61 20 6f 6e 20 74 68 65 20 27 41 27 20 73 69 64 |a on the 'A' sid| 00003270 65 0d 20 20 20 20 20 34 20 20 54 69 6d 65 72 20 |e. 4 Timer | 00003280 31 20 63 6f 6e 74 72 6f 6c 0d 20 20 20 20 20 35 |1 control. 5| 00003290 20 20 54 69 6d 65 72 20 31 20 63 6f 6e 74 72 6f | Timer 1 contro| 000032a0 6c 0d 20 20 20 20 20 36 20 20 54 69 6d 65 72 20 |l. 6 Timer | 000032b0 31 20 63 6f 6e 74 72 6f 6c 0d 20 20 20 20 20 37 |1 control. 7| 000032c0 20 20 54 69 6d 65 72 20 31 20 63 6f 6e 74 72 6f | Timer 1 contro| 000032d0 6c 0d 20 20 20 20 20 38 20 20 54 69 6d 65 72 20 |l. 8 Timer | 000032e0 32 20 63 6f 6e 74 72 6f 6c 0d 20 20 20 20 20 39 |2 control. 9| 000032f0 20 20 54 69 6d 65 72 20 32 20 63 6f 6e 74 72 6f | Timer 2 contro| 00003300 6c 0d 20 20 20 20 20 41 20 20 53 68 69 66 74 20 |l. A Shift | 00003310 72 65 67 69 73 74 65 72 0d 20 20 20 20 20 42 20 |register. B | 00003320 20 41 75 78 69 6c 69 61 72 79 20 63 6f 6e 74 72 | Auxiliary contr| 00003330 6f 6c 0d 20 20 20 20 20 43 20 20 50 65 72 69 70 |ol. C Perip| 00003340 68 65 72 61 6c 20 63 6f 6e 74 72 6f 6c 0d 20 20 |heral control. | 00003350 20 20 20 44 20 20 49 6e 74 65 72 72 75 70 74 20 | D Interrupt | 00003360 66 6c 61 67 73 0d 20 20 20 20 20 45 20 20 49 6e |flags. E In| 00003370 74 65 72 72 75 70 74 20 65 6e 61 62 6c 69 6e 67 |terrupt enabling| 00003380 0d 20 20 20 20 20 46 20 20 53 61 6d 65 20 61 73 |. F Same as| 00003390 20 72 65 67 69 73 74 65 72 20 31 20 62 75 74 20 | register 1 but | 000033a0 77 69 74 68 6f 75 74 20 68 61 6e 64 73 68 61 6b |without handshak| 000033b0 69 6e 67 0d 0d 54 68 65 72 65 27 73 20 61 20 6c |ing..There's a l| 000033c0 6f 74 20 69 6e 20 74 68 65 20 56 49 41 73 2e 20 |ot in the VIAs. | 000033d0 20 49 27 6c 6c 20 73 68 6f 77 20 79 6f 75 20 61 | I'll show you a| 000033e0 6e 20 61 70 70 6c 69 63 61 74 69 6f 6e 0d 75 73 |n application.us| 000033f0 69 6e 67 20 61 20 74 69 6d 65 72 20 69 6e 20 74 |ing a timer in t| 00003400 68 65 20 6e 65 78 74 20 6d 6f 64 75 6c 65 2e 20 |he next module. | 00003410 20 54 68 65 20 73 68 69 66 74 20 72 65 67 69 73 | The shift regis| 00003420 74 65 72 20 69 73 0d 75 73 65 64 20 66 6f 72 20 |ter is.used for | 00003430 63 6f 75 6e 74 69 6e 67 20 70 75 6c 73 65 73 2e |counting pulses.| 00003440 20 20 54 68 65 20 61 75 78 69 6c 69 61 72 79 20 | The auxiliary | 00003450 63 6f 6e 74 72 6f 6c 20 73 65 74 73 0d 70 61 72 |control sets.par| 00003460 61 6d 65 74 65 72 73 20 66 6f 72 20 74 68 65 20 |ameters for the | 00003470 74 69 6d 65 72 73 20 61 6e 64 20 73 68 69 66 74 |timers and shift| 00003480 20 72 65 67 69 73 74 65 72 20 61 6e 64 20 74 68 | register and th| 00003490 65 0d 70 65 72 69 70 68 65 72 61 6c 20 63 6f 6e |e.peripheral con| 000034a0 74 72 6f 6c 20 73 65 74 73 20 75 70 20 74 68 65 |trol sets up the| 000034b0 20 77 61 79 20 74 68 65 20 69 6e 74 65 72 72 75 | way the interru| 000034c0 70 74 20 6c 69 6e 65 73 0d 72 65 73 70 6f 6e 64 |pt lines.respond| 000034d0 20 74 6f 20 74 68 65 20 76 6f 6c 74 61 67 65 73 | to the voltages| 000034e0 20 61 70 70 6c 69 65 64 20 74 6f 20 74 68 65 6d | applied to them| 000034f0 2e 0d 0d 54 6f 20 61 63 63 65 73 73 20 74 68 65 |...To access the| 00003500 73 65 20 73 65 63 74 69 6f 6e 73 20 6f 66 20 6d |se sections of m| 00003510 65 6d 6f 72 79 20 79 6f 75 20 77 6f 75 6c 64 20 |emory you would | 00003520 6e 6f 72 6d 61 6c 6c 79 20 75 73 65 0d 4f 53 42 |normally use.OSB| 00003530 59 54 45 20 77 69 74 68 20 41 20 62 65 74 77 65 |YTE with A betwe| 00003540 65 6e 20 31 34 36 20 61 6e 64 20 31 35 31 2e 20 |en 146 and 151. | 00003550 20 54 68 65 73 65 20 63 61 6c 6c 73 20 72 65 61 | These calls rea| 00003560 64 20 66 72 6f 6d 0d 61 6e 64 20 77 72 69 74 65 |d from.and write| 00003570 20 74 6f 20 74 68 65 20 6d 65 6d 6f 72 79 2d 6d | to the memory-m| 00003580 61 70 70 65 64 20 49 2f 4f 20 28 46 72 65 64 2c |apped I/O (Fred,| 00003590 20 4a 69 6d 20 61 6e 64 20 53 68 65 69 6c 61 29 | Jim and Sheila)| 000035a0 20 61 73 0d 66 6f 6c 6c 6f 77 73 2e 0d 0d 20 20 | as.follows... | 000035b0 20 20 4f 53 42 59 54 45 20 31 34 36 20 2d 20 52 | OSBYTE 146 - R| 000035c0 65 61 64 20 46 72 65 64 20 28 26 46 43 30 30 20 |ead Fred (&FC00 | 000035d0 74 6f 20 26 46 43 46 46 29 0d 20 20 20 20 4f 53 |to &FCFF). OS| 000035e0 42 59 54 45 20 31 34 37 20 2d 20 57 72 69 74 65 |BYTE 147 - Write| 000035f0 20 46 72 65 64 0d 20 20 20 20 4f 53 42 59 54 45 | Fred. OSBYTE| 00003600 20 31 34 38 20 2d 20 52 65 61 64 20 4a 69 6d 20 | 148 - Read Jim | 00003610 28 26 46 44 30 30 20 74 6f 20 26 46 44 46 46 29 |(&FD00 to &FDFF)| 00003620 0d 20 20 20 20 4f 53 42 59 54 45 20 31 34 39 20 |. OSBYTE 149 | 00003630 2d 20 57 72 69 74 65 20 4a 69 6d 0d 20 20 20 20 |- Write Jim. | 00003640 4f 53 42 59 54 45 20 31 35 30 20 2d 20 52 65 61 |OSBYTE 150 - Rea| 00003650 64 20 53 68 65 69 6c 61 20 28 26 46 45 30 30 20 |d Sheila (&FE00 | 00003660 74 6f 20 26 46 45 46 46 29 0d 20 20 20 20 4f 53 |to &FEFF). OS| 00003670 42 59 54 45 20 31 35 31 20 2d 20 57 72 69 74 65 |BYTE 151 - Write| 00003680 20 53 68 65 69 6c 61 0d 0d 49 6e 20 65 61 63 68 | Sheila..In each| 00003690 20 63 61 73 65 2c 20 6f 6e 20 65 6e 74 72 79 20 | case, on entry | 000036a0 58 20 63 6f 6e 74 61 69 6e 73 20 74 68 65 20 6f |X contains the o| 000036b0 66 66 73 65 74 20 77 69 74 68 69 6e 20 74 68 65 |ffset within the| 000036c0 20 70 61 67 65 0d 61 6e 64 2c 20 69 66 20 61 20 | page.and, if a | 000036d0 77 72 69 74 65 2c 20 59 20 63 6f 6e 74 61 69 6e |write, Y contain| 000036e0 73 20 74 68 65 20 62 79 74 65 2e 20 20 4f 6e 20 |s the byte. On | 000036f0 65 78 69 74 2c 20 69 66 20 61 20 72 65 61 64 2c |exit, if a read,| 00003700 20 59 0d 63 6f 6e 74 61 69 6e 73 20 74 68 65 20 | Y.contains the | 00003710 62 79 74 65 20 72 65 61 64 2e 20 20 54 68 69 73 |byte read. 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The| 000038b0 20 4f 53 42 59 54 45 20 63 61 6c 6c 73 20 61 6e | OSBYTE calls an| 000038c0 64 20 74 68 65 20 4f 53 57 4f 52 44 20 63 61 6c |d the OSWORD cal| 000038d0 6c 73 20 77 6f 72 6b 20 61 63 72 6f 73 73 0d 74 |ls work across.t| 000038e0 68 65 20 74 75 62 65 2c 20 69 2e 65 2e 20 66 72 |he tube, i.e. fr| 000038f0 6f 6d 20 61 20 73 65 63 6f 6e 64 20 70 72 6f 63 |om a second proc| 00003900 65 73 73 6f 72 2e 0d 0d 49 6e 20 74 68 65 20 63 |essor...In the c| 00003910 61 73 65 20 6f 66 20 69 6e 74 65 72 72 75 70 74 |ase of interrupt| 00003920 73 20 79 6f 75 72 20 69 6e 74 65 72 63 65 70 74 |s your intercept| 00003930 69 6e 67 20 63 6f 64 65 20 68 61 73 20 74 6f 20 |ing code has to | 00003940 62 65 0d 69 6e 20 74 68 65 20 49 2f 4f 20 70 72 |be.in the I/O pr| 00003950 6f 63 65 73 73 6f 72 2c 20 73 6f 20 63 72 6f 73 |ocessor, so cros| 00003960 73 2d 74 75 62 65 20 72 65 71 75 69 72 65 6d 65 |s-tube requireme| 00003970 6e 74 73 20 64 6f 20 6e 6f 74 20 63 6f 6d 65 0d |nts do not come.| 00003980 69 6e 74 6f 20 69 74 2e 20 20 49 66 20 77 65 20 |into it. If we | 00003990 74 61 6b 65 20 74 68 65 20 28 73 6d 61 6c 6c 29 |take the (small)| 000039a0 20 72 69 73 6b 20 74 68 61 74 20 53 68 65 69 6c | risk that Sheil| 000039b0 61 20 77 69 6c 6c 20 6d 6f 76 65 0d 69 6e 20 66 |a will move.in f| 000039c0 75 74 75 72 65 20 74 68 65 20 62 65 73 74 20 74 |uture the best t| 000039d0 68 69 6e 67 20 74 6f 20 64 6f 20 69 73 20 77 72 |hing to do is wr| 000039e0 69 74 65 20 61 6e 64 20 72 65 61 64 20 74 68 65 |ite and read the| 000039f0 20 6d 65 6d 6f 72 79 0d 6c 6f 63 61 74 69 6f 6e | memory.location| 00003a00 73 20 64 69 72 65 63 74 6c 79 2e 20 20 41 6e 79 |s directly. Any| 00003a10 20 69 6e 74 65 72 72 75 70 74 20 63 6f 64 65 20 | interrupt code | 00003a20 6d 75 73 74 20 62 65 20 73 68 6f 72 74 20 61 6e |must be short an| 00003a30 64 20 74 6f 0d 74 68 65 20 70 6f 69 6e 74 2c 20 |d to.the point, | 00003a40 73 6f 20 77 65 20 73 68 6f 75 6c 64 20 61 76 6f |so we should avo| 00003a50 69 64 20 75 6e 6e 65 63 65 73 73 61 72 79 20 4f |id unnecessary O| 00003a60 53 20 63 61 6c 6c 73 2e 20 20 49 6e 20 61 6e 79 |S calls. In any| 00003a70 0d 63 61 73 65 20 74 68 65 72 65 20 69 73 20 61 |.case there is a| 00003a80 20 72 65 73 74 72 69 63 74 69 6f 6e 20 6f 6e 20 | restriction on | 00003a90 77 68 69 63 68 20 4f 53 20 63 61 6c 6c 73 20 63 |which OS calls c| 00003aa0 61 6e 20 62 65 20 75 73 65 64 0d 64 75 72 69 6e |an be used.durin| 00003ab0 67 20 69 6e 74 65 72 72 75 70 74 73 2e 20 20 49 |g interrupts. I| 00003ac0 20 77 69 6c 6c 20 67 69 76 65 20 79 6f 75 20 6d | will give you m| 00003ad0 6f 72 65 20 64 65 74 61 69 6c 20 69 6e 20 74 68 |ore detail in th| 00003ae0 65 20 6e 65 78 74 0d 6d 6f 64 75 6c 65 2e 0d 0d |e next.module...| 00003af0 49 6e 20 74 68 69 73 20 6d 6f 64 75 6c 65 20 77 |In this module w| 00003b00 65 20 73 68 61 6c 6c 20 64 65 61 6c 20 64 69 72 |e shall deal dir| 00003b10 65 63 74 6c 79 20 77 69 74 68 20 74 68 65 20 72 |ectly with the r| 00003b20 65 67 69 73 74 65 72 73 20 6f 66 0d 74 68 65 20 |egisters of.the | 00003b30 56 49 41 2e 20 20 54 6f 20 73 65 74 20 75 70 20 |VIA. To set up | 00003b40 74 68 65 20 56 49 41 20 74 6f 20 64 65 74 65 63 |the VIA to detec| 00003b50 74 20 74 68 65 20 6d 6f 75 73 65 20 77 65 20 6d |t the mouse we m| 00003b60 75 73 74 20 64 6f 0d 74 68 72 65 65 20 74 68 69 |ust do.three thi| 00003b70 6e 67 73 2e 20 20 57 65 20 6d 75 73 74 20 65 6e |ngs. We must en| 00003b80 61 62 6c 65 20 74 68 65 20 69 6e 74 65 72 72 75 |able the interru| 00003b90 70 74 73 20 77 65 20 61 72 65 20 67 6f 69 6e 67 |pts we are going| 00003ba0 20 74 6f 0d 75 73 65 2c 20 73 65 74 20 75 70 20 | to.use, set up | 00003bb0 74 68 65 20 62 65 68 61 76 69 6f 75 72 20 6f 66 |the behaviour of| 00003bc0 20 74 68 65 20 69 6e 74 65 72 72 75 70 74 20 6c | the interrupt l| 00003bd0 69 6e 65 73 20 61 6e 64 20 73 65 74 20 75 70 0d |ines and set up.| 00003be0 74 68 65 20 72 65 6c 65 76 61 6e 74 20 62 69 74 |the relevant bit| 00003bf0 73 20 6f 66 20 74 68 65 20 75 73 65 72 20 70 6f |s of the user po| 00003c00 72 74 20 74 6f 20 61 63 74 20 61 73 20 69 6e 70 |rt to act as inp| 00003c10 75 74 73 2e 0d 0d 54 68 65 20 66 69 72 73 74 20 |uts...The first | 00003c20 73 65 63 74 69 6f 6e 20 6f 66 20 74 68 65 20 6d |section of the m| 00003c30 61 63 68 69 6e 65 20 63 6f 64 65 20 69 6e 20 42 |achine code in B| 00003c40 2f 6f 73 62 31 38 20 72 65 73 65 74 73 20 74 68 |/osb18 resets th| 00003c50 65 0d 49 52 51 20 76 65 63 74 6f 72 2e 20 20 49 |e.IRQ vector. I| 00003c60 20 68 61 76 65 20 75 73 65 64 20 49 52 51 31 20 | have used IRQ1 | 00003c70 62 65 63 61 75 73 65 20 77 65 20 6e 65 65 64 20 |because we need | 00003c80 61 20 66 61 73 74 0d 72 65 73 70 6f 6e 73 65 20 |a fast.response | 00003c90 74 6f 20 74 68 65 20 6d 6f 75 73 65 20 69 6e 74 |to the mouse int| 00003ca0 65 72 72 75 70 74 2e 20 20 57 68 65 6e 20 79 6f |errupt. When yo| 00003cb0 75 20 68 61 76 65 20 73 65 65 6e 20 74 68 69 73 |u have seen this| 00003cc0 0d 63 6f 64 65 20 69 6e 20 61 63 74 69 6f 6e 20 |.code in action | 00003cd0 79 6f 75 20 73 68 6f 75 6c 64 20 63 68 61 6e 67 |you should chang| 00003ce0 65 20 74 68 65 20 76 61 6c 75 65 20 6f 66 20 27 |e the value of '| 00003cf0 69 72 71 31 27 20 69 6e 20 6c 69 6e 65 0d 31 33 |irq1' in line.13| 00003d00 30 20 74 6f 20 26 32 30 36 20 61 6e 64 20 63 6f |0 to &206 and co| 00003d10 6d 70 61 72 65 20 74 68 65 20 72 65 73 75 6c 74 |mpare the result| 00003d20 73 2e 20 20 5b 4e 6f 74 65 20 74 68 61 74 20 69 |s. [Note that i| 00003d30 66 20 79 6f 75 20 68 61 76 65 0d 61 6e 20 41 4d |f you have.an AM| 00003d40 58 20 6d 6f 75 73 65 20 63 68 69 70 20 69 6e 20 |X mouse chip in | 00003d50 79 6f 75 72 20 6d 61 63 68 69 6e 65 20 79 6f 75 |your machine you| 00003d60 20 6d 75 73 74 20 72 65 6d 6f 76 65 20 6f 72 20 | must remove or | 00003d70 64 69 73 61 62 6c 65 0d 69 74 20 62 65 66 6f 72 |disable.it befor| 00003d80 65 20 79 6f 75 20 63 61 6e 20 67 65 74 20 41 4e |e you can get AN| 00003d90 59 20 72 65 73 75 6c 74 20 66 72 6f 6d 20 75 73 |Y result from us| 00003da0 69 6e 67 20 49 52 51 32 20 61 74 20 26 32 30 36 |ing IRQ2 at &206| 00003db0 2e 5d 20 0d 0d 57 65 20 6d 75 73 74 20 65 6e 61 |.] ..We must ena| 00003dc0 62 6c 65 20 69 6e 74 65 72 72 75 70 74 73 20 66 |ble interrupts f| 00003dd0 72 6f 6d 20 74 68 65 20 74 77 6f 20 27 42 27 20 |rom the two 'B' | 00003de0 73 69 64 65 20 69 6e 74 65 72 72 75 70 74 0d 6c |side interrupt.l| 00003df0 69 6e 65 73 2c 20 6b 6e 6f 77 6e 20 61 73 20 43 |ines, known as C| 00003e00 42 31 20 61 6e 64 20 43 42 32 2e 20 20 54 6f 20 |B1 and CB2. 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Th| 00004520 65 20 66 69 6e 61 6c 0d 76 61 72 69 61 62 6c 65 |e final.variable| 00004530 20 73 70 61 63 65 20 68 6f 6c 64 73 20 74 68 65 | space holds the| 00004540 20 6f 6c 64 20 76 61 6c 75 65 20 6f 66 20 49 52 | old value of IR| 00004550 51 31 20 74 6f 20 61 6c 6c 6f 77 0d 63 68 61 69 |Q1 to allow.chai| 00004560 6e 69 6e 67 2e 0d 0d 57 68 65 6e 20 61 6e 79 20 |ning...When any | 00004570 69 6e 74 65 72 72 75 70 74 20 28 6f 74 68 65 72 |interrupt (other| 00004580 20 74 68 61 6e 20 61 6e 20 4e 4d 49 29 20 69 73 | than an NMI) is| 00004590 20 64 65 74 65 63 74 65 64 20 65 78 65 63 75 74 | detected execut| 000045a0 69 6f 6e 0d 6a 75 6d 70 73 20 64 6f 77 6e 20 74 |ion.jumps down t| 000045b0 68 65 20 49 52 51 31 20 76 65 63 74 6f 72 2e 20 |he IRQ1 vector. | 000045c0 20 54 68 65 20 66 69 72 73 74 20 74 68 69 6e 67 | The first thing| 000045d0 20 69 6e 20 74 68 61 74 20 76 65 63 74 6f 72 0d | in that vector.| 000045e0 69 73 20 6f 75 72 20 63 6f 64 65 20 68 65 72 65 |is our code here| 000045f0 2e 20 20 54 68 65 20 73 74 61 6e 64 61 72 64 20 |. 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If yo| 00004f80 75 20 77 65 72 65 20 74 6f 20 72 65 61 64 20 74 |u were to read t| 00004f90 68 65 0d 74 77 6f 20 62 79 74 65 73 20 6f 66 20 |he.two bytes of | 00004fa0 74 68 65 20 63 6f 6f 72 64 69 6e 61 74 65 73 20 |the coordinates | 00004fb0 73 65 70 61 72 61 74 65 6c 79 20 75 73 69 6e 67 |separately using| 00004fc0 20 3f 20 6f 70 65 72 61 74 6f 72 73 0d 79 6f 75 | ? operators.you| 00004fd0 20 77 6f 75 6c 64 20 67 65 74 20 73 6f 6d 65 20 | would get some | 00004fe0 69 6e 63 6f 72 72 65 63 74 20 72 65 73 75 6c 74 |incorrect result| 00004ff0 73 2c 20 74 72 79 20 69 74 20 61 6e 64 20 73 65 |s, try it and se| 00005000 65 2e 0d 0d 54 68 69 73 20 63 6f 64 65 20 77 69 |e...This code wi| 00005010 6c 6c 20 77 6f 72 6b 20 77 69 74 68 20 61 20 4d |ll work with a M| 00005020 61 72 63 6f 6e 69 20 54 72 61 63 6b 65 72 62 61 |arconi Trackerba| 00005030 6c 6c 2c 20 61 6c 74 68 6f 75 67 68 20 74 68 65 |ll, although the| 00005040 0d 70 69 6e 20 63 6f 6e 6e 65 63 74 69 6f 6e 73 |.pin connections| 00005050 20 61 72 65 20 64 69 66 66 65 72 65 6e 74 20 74 | are different t| 00005060 6f 20 74 68 65 20 41 4d 58 20 6d 6f 75 73 65 2e |o the AMX mouse.| 00005070 20 20 54 68 65 20 64 61 74 61 0d 73 68 65 65 74 | The data.sheet| 00005080 20 77 69 74 68 20 74 68 65 20 74 72 61 63 6b 65 | with the tracke| 00005090 72 62 61 6c 6c 20 77 69 6c 6c 20 61 64 76 69 73 |rball will advis| 000050a0 65 20 79 6f 75 20 6f 6e 20 77 68 69 63 68 20 70 |e you on which p| 000050b0 69 6e 73 20 61 72 65 0d 77 68 69 63 68 2e 20 20 |ins are.which. | 000050c0 43 42 31 20 61 6e 64 20 43 42 32 20 61 72 65 20 |CB1 and CB2 are | 000050d0 75 73 65 64 20 62 75 74 20 74 68 65 20 73 65 63 |used but the sec| 000050e0 6f 6e 64 20 6c 69 6e 65 73 20 61 72 65 0d 64 69 |ond lines are.di| 000050f0 66 66 65 72 65 6e 74 20 74 6f 20 74 68 6f 73 65 |fferent to those| 00005100 20 75 73 65 64 20 69 6e 20 74 68 69 73 20 70 72 | used in this pr| 00005110 6f 67 72 61 6d 2e 20 20 4c 69 6e 65 73 20 39 36 |ogram. Lines 96| 00005120 30 20 61 6e 64 20 31 32 34 30 0d 77 6f 75 6c 64 |0 and 1240.would| 00005130 20 6e 65 65 64 20 74 6f 20 62 65 20 63 68 61 6e | need to be chan| 00005140 67 65 64 20 74 6f 20 6c 6f 61 64 20 31 36 20 61 |ged to load 16 a| 00005150 6e 64 20 38 20 72 65 73 70 65 63 74 69 76 65 6c |nd 8 respectivel| 00005160 79 0d 62 65 63 61 75 73 65 20 62 69 74 73 20 34 |y.because bits 4| 00005170 20 61 6e 64 20 33 20 63 61 72 72 79 20 59 32 20 | and 3 carry Y2 | 00005180 61 6e 64 20 58 32 2e 0d 0d 4f 66 20 63 6f 75 72 |and X2...Of cour| 00005190 73 65 20 74 68 69 73 20 63 6f 64 65 20 64 6f 65 |se this code doe| 000051a0 73 20 6e 6f 74 20 69 6d 70 6c 65 6d 65 6e 74 20 |s not implement | 000051b0 61 20 70 72 6f 70 65 72 20 6d 6f 75 73 65 0d 70 |a proper mouse.p| 000051c0 6f 69 6e 74 65 72 20 69 6e 20 74 68 65 20 77 61 |ointer in the wa| 000051d0 79 20 41 63 6f 72 6e 27 73 20 56 46 53 20 6f 72 |y Acorn's VFS or| 000051e0 20 41 4d 58 27 73 20 53 75 70 65 72 2d 41 72 74 | AMX's Super-Art| 000051f0 20 52 4f 4d 20 64 6f 65 73 2c 0d 62 75 74 20 69 | ROM does,.but i| 00005200 74 20 73 68 6f 75 6c 64 20 67 69 76 65 20 79 6f |t should give yo| 00005210 75 20 61 20 62 61 73 69 63 20 69 64 65 61 20 6f |u a basic idea o| 00005220 66 20 68 6f 77 20 61 6e 20 69 6e 74 65 72 72 75 |f how an interru| 00005230 70 74 20 69 73 0d 68 61 6e 64 6c 65 64 2e 0d 0d |pt is.handled...| 00005240 59 6f 75 20 73 68 6f 75 6c 64 20 61 6c 73 6f 20 |You should also | 00005250 6e 6f 74 69 63 65 20 74 68 61 74 20 49 20 68 61 |notice that I ha| 00005260 76 65 20 70 75 74 20 74 68 65 20 63 6f 64 65 20 |ve put the code | 00005270 69 6e 20 74 68 65 0d 63 61 73 73 65 74 74 65 2f |in the.cassette/| 00005280 52 53 34 32 33 20 69 6e 70 75 74 20 62 75 66 66 |RS423 input buff| 00005290 65 72 2e 20 20 49 66 20 79 6f 75 20 75 73 65 20 |er. If you use | 000052a0 61 20 63 61 73 73 65 74 74 65 20 28 69 66 20 79 |a cassette (if y| 000052b0 6f 75 0d 64 6f 20 79 6f 75 20 73 68 6f 75 6c 64 |ou.do you should| 000052c0 20 62 65 20 73 61 76 69 6e 67 20 66 6f 72 20 61 | be saving for a| 000052d0 20 64 69 73 63 20 64 72 69 76 65 29 20 6f 72 20 | disc drive) or | 000052e0 68 61 76 65 20 61 20 6d 6f 64 65 6d 20 6f 72 0d |have a modem or.| 000052f0 73 6f 6d 65 74 68 69 6e 67 20 74 68 61 74 20 75 |something that u| 00005300 73 65 73 20 52 53 34 32 33 20 69 6e 70 75 74 73 |ses RS423 inputs| 00005310 20 74 68 65 6e 20 72 75 6e 6e 69 6e 67 20 74 68 | then running th| 00005320 69 73 20 63 6f 64 65 20 77 69 6c 6c 0d 63 72 61 |is code will.cra| 00005330 73 68 20 79 6f 75 72 20 63 6f 6d 70 75 74 65 72 |sh your computer| 00005340 2e 20 20 50 65 72 68 61 70 73 20 74 68 65 20 73 |. Perhaps the s| 00005350 61 66 65 73 74 20 74 68 69 6e 67 20 74 6f 20 64 |afest thing to d| 00005360 6f 20 77 6f 75 6c 64 0d 62 65 20 74 6f 20 72 61 |o would.be to ra| 00005370 69 73 65 20 50 41 47 45 20 62 79 20 26 31 30 30 |ise PAGE by &100| 00005380 20 28 32 35 36 29 20 62 79 74 65 73 20 61 6e 64 | (256) bytes and| 00005390 20 70 75 74 20 74 68 65 20 63 6f 64 65 20 69 6e | put the code in| 000053a0 0d 74 68 65 72 65 3a 20 62 75 74 20 6f 6e 20 6d |.there: but on m| 000053b0 6f 73 74 20 42 73 20 61 6e 64 20 42 2b 73 20 50 |ost Bs and B+s P| 000053c0 41 47 45 20 69 73 20 74 6f 6f 20 68 69 67 68 20 |AGE is too high | 000053d0 61 6e 79 77 61 79 2e 20 20 49 0d 6c 65 61 76 65 |anyway. I.leave| 000053e0 20 69 74 20 75 70 20 74 6f 20 79 6f 75 2e 0d 0d | it up to you...| 000053f0 52 65 6d 65 6d 62 65 72 20 74 68 61 74 20 74 68 |Remember that th| 00005400 65 20 76 65 63 74 6f 72 73 20 61 72 65 20 72 65 |e vectors are re| 00005410 2d 73 65 74 20 6f 6e 20 70 72 65 73 73 69 6e 67 |-set on pressing| 00005420 20 42 52 45 41 4b 2e 20 0d 41 6c 73 6f 20 79 6f | BREAK. .Also yo| 00005430 75 20 77 69 6c 6c 20 6e 6f 74 69 63 65 20 74 68 |u will notice th| 00005440 61 74 20 74 68 69 73 20 74 69 6d 65 20 49 20 68 |at this time I h| 00005450 61 76 65 6e 27 74 20 61 6c 6c 6f 77 65 64 20 66 |aven't allowed f| 00005460 6f 72 0d 74 68 65 20 63 6f 64 65 20 74 6f 20 62 |or.the code to b| 00005470 65 20 63 61 6c 6c 65 64 20 74 77 69 63 65 20 77 |e called twice w| 00005480 69 74 68 6f 75 74 20 61 20 42 52 45 41 4b 2e 20 |ithout a BREAK. | 00005490 20 44 6f 69 6e 67 20 74 68 69 73 0d 77 69 6c 6c | Doing this.will| 000054a0 20 6c 6f 63 6b 20 75 70 20 74 68 65 20 63 6f 6d | lock up the com| 000054b0 70 75 74 65 72 2e 0d 0d 4e 65 78 74 20 74 69 6d |puter...Next tim| 000054c0 65 20 77 65 20 77 69 6c 6c 20 73 65 74 20 75 70 |e we will set up| 000054d0 20 61 20 56 49 41 20 74 69 6d 65 72 20 74 6f 20 | a VIA timer to | 000054e0 70 61 6c 65 74 74 65 20 73 77 69 74 63 68 20 6f |palette switch o| 000054f0 6e 0d 74 68 65 20 73 63 72 65 65 6e 20 74 6f 20 |n.the screen to | 00005500 67 65 74 20 73 65 76 65 72 61 6c 20 63 6f 6c 6f |get several colo| 00005510 75 72 73 20 69 6e 20 6d 6f 64 65 20 33 2c 20 69 |urs in mode 3, i| 00005520 6e 20 61 20 6c 69 6d 69 74 65 64 0d 77 61 79 2e |n a limited.way.| 00005530 0d |.| 00005531
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20-03-88/T\OSB18.m4
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