28-03-88/T\OSB19

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28-03-88/T\OSB19

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 » telesoftware6.adl
Filename:	28-03-88/T\OSB19
Read OK:	✔
File size:	3F29 bytes
Load address:	0000
Exec address:	FFFFFFFF

File contents

OSBITS - An Exploration of the BBC Micro at Machine Level

By Programmer

..........................................................


Part 19: Interrupts II  -  Palette Switching using Timers


In the last module we looked at interrupts and I explained
how the OS sets up the registers and memory when it detects
an interrupt.

There are two things to bear in mind about writing interrupt
handling code.  Firstly it must be short, or else some of
the OS housekeeping stops, and it must not re-enable
interrupts.  In fact if you are able to write what is called
re-entrant code, which means your code can be called from
within itself without crashing, then you could enable
interrupts.  This would be achieved by using the stack for
all your storage instead of fixed memory locations. 
Personally I can't make that work during interrupts, so I
always work with interrupts disabled, which is how the OS
sets things up.

One side effect of this is that OS calls that enable
interrupts can not be used during interrupt routines.  This
pretty well means everything except maybe OSBYTE and OSWORD. 
You certainly couldn't use OSWRCH or OSRDCH.

OSWORD 12 (&C) writes to the palette.  This controls the
relationship between the logical colour (the colour number)
and the actual colour (the colour displayed).  You may
remember that from 'BASIC' you use VDU 19 to do this, but
VDU 19 can not be used during an interrupt because VDU
statements use OSWRCH.

What I intend to do in this module is to use OSWORD 12 to
switch the palette for colour 1 in mode 3 so fast that we
can have areas of the screen with different colours in an
otherwise monochromatic mode.

We can detect the field sync pulse using an event and
that will be used to load a timer that waits until the beam 
is at the top of frame before generating an interrupt.  That
interrupt sets the first colour change and loads the timer
for the width of that colour band (in microseconds) and so
the process repeats down the screen.  This whole process is
repeated 50 times every second as a background task.

Okay, so first an explanation of field sync.  Any television
signal has to contain synchronisation pulses along with the
picture information.  That allows the display in the
receiver to be in step with the scanning in the camera, or
in our case, the computer.  In 625 line 50 hertz TV systems
(like PAL, SECAM and their RGB versions) there is a pulse
every 64 microseconds which marks the space between lines,
and is called the line sync pulse.

Every 20 milliseconds there is a second pulse called the
field sync pulse (again these figures only apply to PAL and
SECAM TV signals by the way, not the American/Japanese
standard called NTSC)  This synchronises the fields of the
source and monitor together.  A field is the display of
312.5 lines (in a 625 line signal) and two fields, slightly
displaced vertically, make up a frame.  So there are 50
fields per second, 25 frames per second.  The reasons for
this field/frame business are related to how little flicker
you can see in a picture without refreshing (re-scanning)
the picture so often that your signal bandwidth (the amount
of information carried per unit of time) becomes too big.

All you need to know for computing is that a BBC Micro
refreshes the screen 25 times per second in two fields, each
of a 50th of a second duration.  The field sync pulse starts
when the scanning of the electron beam has reached the
bottom of one field and starts to 'fly back' to the top of
the screen ready for the next field to begin.  This flyback
is also known as the vertical blanking interval (VBI) or
just the vertical interval.  (The term 'blanking' is used
because the beam is turned off during this flyback to stop
you seeing slanted lines across the screen.  If you do see
them then your monitor is not set up correctly!)

You are probably familiar with *FX 19.  This call (OSBYTE
19) forces program execution to wait for the start of the
next vertical interval.  Event number 4 occurs at this point
and we can use event 4 to synchronise the palette switching
that we are going to do here.

Since this event occurs at the start of the VBI we have to
add a delay so that the beam can reach the top of the next
frame before the routine proper can start.

So what do we use to do all this timing?  The VIA includes a
couple of timers, known as T1 and T2.  Essentially the
difference between the two timers is that T1 can be used to
generate pulses or a square wave from a pin of the VIA (and
hence of the user port) while T2 can count down under the
influence of pulses being fed to a pin of the VIA (and user
port).  Both timers can be used to generate an interrupt
after a specified delay.  These timings are done as counts
at 1 MHz, i.e. in microseconds.  That makes these timers
much more accurate that the BASIC TIME function, which in
any case stops during interrupts.

I'll briefly describe the modes of T1 and T2.

T1 - One shot mode

A duration is loaded into the registers, low byte first then
high byte.  When the high order byte is loaded the counter
starts to count down.  On reaching zero an interrupt is
generated.  Loading the high byte also resets the interrupt
flag.

T1 - Free running

In this case whenever the timer hits zero and generates the
interrupt the timer reloads itself with the value held in
what are called its latches.  It was in fact one of these
latches that was written to when you thought you were
writing to the low byte counter.  (You think you're
confused?) You can generate a rectangular wave form from PB7
of the VIA (which appears as a pin on the user port) if you
want to.  If you write to the latches while the counter is
counting down your value will be transferred across at the
next interrupt.  This allows you to pre-set the duration of
the next pulse and it is possible to emulate infra-red
control signals in this way to control your video.

T2 - One shot

Pretty well the same as for T1

T2 -  Pulse counting

T2 can count a preset number of pulses and then generate an
interrupt.

A data sheet for the 6522 VIA will explain this all more
fully, or an Advanced User Guide.  I shall only explain in
detail the operation we will use in this module.

Using a VIA timer as an interval timer involves two
operations.  First we must set up the timer and load it with
the interval (in microseconds) that we wish to count off. 
Then we must monitor the interrupts until we detect one
generated by our timer.

Loading the timer is done by writing to addresses within the
area of memory known as Sheila.  Sheila is a page of memory
running from &FE00 to &FEFF and in it live such pieces of
hardware as the CRTC video controller chip, the Tube, serial
and video ULA chips, the AtoD converter and others.  There
are 11 different devices on Sheila.  We are here interested
in the User VIA which lives from &FE60 to &FE6F.  The base
address for the VIA registers is &FE60 and in the program in
this module, B/osb19, that address is given a variable name
'via'.  [In fact, 32 bytes are allocated for the VIA
although it only uses 16 of them so that register X of the
VIA can be found at &FE6X and &FE7X identically.  You can
prove this by changing the value of 'via' in the program to
&FE70 and you will find no difference in the result.]

As explained in the last module speed is important while
using interrupts and so we will read and write direct rather
than use the OSBYTE calls (numbers 150 and 151) that are
available.

The VIA registers we need for this operation are as follows:

Register 11 - The auxiliary control register

With this register we select the mode of operation of the
timers and shift register.  To put T1 into its 'one shot'
mode we have to make sure that the top two bits of the
register are either both clear or just bit 7 is set.  In the
latter case the voltage on pin PB7 of the VIA will go low
when the timer is started and go back high when it times
out.  This produces a pulse of programmable width.  It
doesn't actually matter whether we set off a pulse on PB7 or
not in this application, but it is probably tidier not to,
so we must clear bits 6 and 7. Lines 540-560 in B/osb19 are
where this is done.

Register 14 - The interrupt enable register

To enable an interrupt we must write to bit 7 of this
register and also to the bit controlling the interrupt we
want to enable.  In the case of T1 it is bit 6.  In lines
570 and 580 we write &C0 to this register and so set T1
interrupts.  (To disable an interrupt you leave bit 7 clear
and write a 1 to the relevant bit or bits.)

Register 4 - Timer 1 low order counter
Register 5 - Timer 1 high order counter

Register 4 is loaded with the lower byte of the two byte
timing value first.  At this time you are actually loading a
latch which holds the value.  When you load the higher byte
of the timer value into register 5 the value in the lower
latch is transferred into the lower counter and the timer
starts counting down.  This loading operation is carried out
in lines 830-860 and 1380-1410.

Register 13 - The interrupt flag register

On receiving an interrupt we can quickly check bit 6 of this
register (using BIT in line 1220) to see if a T1 interrupt
has occurred.  This flag is cleared either by writing a 1
into it, or by reloading the timer.  In this program the
latter method is used since we have to reload the timer
anyway.

Let's go through the program.  B/osb19 takes logical colour
1 and changes its actual colour depending on where,
vertically on the screen, you are.  The width of the
horizontal bands of colour and the colours used are held in
a block of memory and can be modified.  The effect will be
visible in any mode except Mode 7 as graphics and text are
drawn in colour 1.

Since we are making an interrupt routine the code has to
live in the I/O processor and the program checks to make
sure that a second processor is not in use.  I have used
OSBYTE 234 for that this time.  On exit X is 0 if no second
processor is being used and &FF if one is.  There should not
be another result.  This method should be more fool-proof
than simply checking the value of PAGE.

The IRQ and Event vectors are saved and the new intercept
addresses loaded.  You will notice that interrupts are
disabled during the latter operation.  I am using IRQ1
because intercepts of IRQ2 are a little too variable for the
kind of operation here.  You should change the address in
line 110 from &204 to &206 to see the effect I mean.

The bands will flicker rather violently, or at least they do
on my machine.  I suspect that this is due to the time taken
for the OS to finish servicing whatever interrupt occurred
just prior to our timer interrupt.  Our interrupt is
'stored' until interrupts are enabled again, because the
interrupt flags on the VIA remain set, and then it is
serviced by our routine.  This delay, which would be very
variable, leads to delays in implimenting our palette
switching routine and hence causes the flickering.  [The
bands will also 'bounce' when a disc is accessed, this is
due to the NMI routines servicing the disc drive.]

Line 650 to line 970 is the code that lies in the event
vector.  We are waiting for event 4, which will be activated
later.  When we have confirmed that it is indeed event 4,
the vertical sync pulse event, we save registers, force
binary mode and re-set a colour counter flag to zero.  This
flag, called 'colours' is counting the number of the band we
are going to draw on the screen so that the correct colour
can be allocated.

I could have coloured the first band from here but I have
decided to switch the top band in just above the top of
screen.  The time delay between the sync and the top of
screen is found by trial and error and I found it to be &560
microseconds on my machine.  The counter is loaded with this
value and started (lines 830 to 860) and the event intercept
is now over and we exit.

Next I have put the block of memory in which parameters and
flags are stored.  You can actually put this kind of block
anywhere you like within your code, as long as it lies out
of the way of program execution.

A block of 5 bytes at 'para_block' is the parameter block
for the OSWORD call.  I have pre-loaded the first byte with
a '1' which is the logical colour we will be switching. 
Should you want to switch another colour then change this
byte.

You will see that at 'colours' I have put the counter flag
and that after it comes a list of 6 bytes, each containing
the number of a colour.  You can modify this list if you
want to change the colours of the bands.  This code draws up
to six bands but could be easily modified to draw more just
by adding more lines of 'colours' in this section of memory
and adding equivalent 'coltimes' below.  The 'coltimes'
bytes are the length of the bands in units of 256
microseconds.  However, since the interrupt code takes a
finite time to execute you can not shrink a band to nothing
by entering zero.  Try that and see the effect.  Note also
that the final time in the list is &FF or 255.  This is to
make sure that the bottom of the final band is not reached
before the vertical sync.  The last number in the 'coltimes'
block should always be &FF.

Then we have the interrupt code, from line 1170 to line
1610.  First it quickly checks to see if this is a timer 1
interrupt.  Since the accumulator value is safely held in
&FC we can corrupt the accumulator.  This means that this
fast check can be made without saving and restoring
registers.  As this code is sitting as top priority in the
interrupt vector we must very quickly pass on any interrupts
we do not want.  By using BIT we put the value of bit six
straight into the overflow flag and can do a quick branch on
overflow set (BVS) if T1 has timed out and this is indeed
our interrupt.

At line 1280 registers are saved and binary mode is forced. 
Then we take the current value of the colours counter and
transfer it into the X register.  We can now use this to
count along the block of colours and of 'coltimes' to load
the timer and change the colour.  The timer is loaded first,
and restarted (lines 1380 to 1410).  Then OSWORD 12 (&C) is
used to reset the palette for colour 1.

Finally the colour counter is incremented, registers are
restored and we exit with an RTI.

There is no need to check that we have reached the final
band of colour.  This is set to be so wide (255*256
microseconds, which is about 65 milliseconds) that it could
never be finished before the sync event resets everything
ready for the next field.  These sync pulses are only 20
milliseconds apart.

As a useful routine this is obviously incomplete.  You would
have to have some control over the colours and width of the
bands.  Obviously you can write to the values of 'colours'
and 'coltimes' with appropriate offsets.  After running the
assembly just enter 'PRINT colours' and 'PRINT coltimes' to
find those values.  To write to those locations across the
tube, from a second processor, use OSWORD 6.  Here, at the
block of memory pointed to by X and Y (XY) you should put
the 32 bit address of the memory to be written.  As it is in
the I/O processor the top 16 bits should be set.  In XY+4
you will put the byte to be written.

The bit of BASIC at the end of the program in this module
enables the event (*FX 14 4) and prints up some coloured
text in Mode 3 to show you what it looks like.  It's
switching of this kind that enables you to mix several modes
on the screen at once, as in 'Elite'.

I should also mention that there is another way of dealing
with external hardware besides letting the hardware
interrupt the processor.  The OS can poll the hardware at
intervals, probably using an interrupt routine.  By polling
we mean the OS effectively asks for information, rather than
being told it.  You might use this method to get replies
from a device connected via the RS423 for example.

So there you have a couple of forays into interrupts. 
They're not particularly fearsome.  The only thing to
remember is that it is easy to crash the machine and so have
your BREAK key handy.  Next time we head off into more
mathematics as numbers with a decimal point appear.

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 39 3a 20  |.......Part 19: |
00000090  49 6e 74 65 72 72 75 70  74 73 20 49 49 20 20 2d  |Interrupts II  -|
000000a0  20 20 50 61 6c 65 74 74  65 20 53 77 69 74 63 68  |  Palette Switch|
000000b0  69 6e 67 20 75 73 69 6e  67 20 54 69 6d 65 72 73  |ing using Timers|
000000c0  0d 0d 0d 49 6e 20 74 68  65 20 6c 61 73 74 20 6d  |...In the last m|
000000d0  6f 64 75 6c 65 20 77 65  20 6c 6f 6f 6b 65 64 20  |odule we looked |
000000e0  61 74 20 69 6e 74 65 72  72 75 70 74 73 20 61 6e  |at interrupts an|
000000f0  64 20 49 20 65 78 70 6c  61 69 6e 65 64 0d 68 6f  |d I explained.ho|
00000100  77 20 74 68 65 20 4f 53  20 73 65 74 73 20 75 70  |w the OS sets up|
00000110  20 74 68 65 20 72 65 67  69 73 74 65 72 73 20 61  | the registers a|
00000120  6e 64 20 6d 65 6d 6f 72  79 20 77 68 65 6e 20 69  |nd memory when i|
00000130  74 20 64 65 74 65 63 74  73 0d 61 6e 20 69 6e 74  |t detects.an int|
00000140  65 72 72 75 70 74 2e 0d  0d 54 68 65 72 65 20 61  |errupt...There a|
00000150  72 65 20 74 77 6f 20 74  68 69 6e 67 73 20 74 6f  |re two things to|
00000160  20 62 65 61 72 20 69 6e  20 6d 69 6e 64 20 61 62  | bear in mind ab|
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00000190  6f 64 65 2e 20 20 46 69  72 73 74 6c 79 20 69 74  |ode.  Firstly it|
000001a0  20 6d 75 73 74 20 62 65  20 73 68 6f 72 74 2c 20  | must be short, |
000001b0  6f 72 20 65 6c 73 65 20  73 6f 6d 65 20 6f 66 0d  |or else some of.|
000001c0  74 68 65 20 4f 53 20 68  6f 75 73 65 6b 65 65 70  |the OS housekeep|
000001d0  69 6e 67 20 73 74 6f 70  73 2c 20 61 6e 64 20 69  |ing stops, and i|
000001e0  74 20 6d 75 73 74 20 6e  6f 74 20 72 65 2d 65 6e  |t must not re-en|
000001f0  61 62 6c 65 0d 69 6e 74  65 72 72 75 70 74 73 2e  |able.interrupts.|
00000200  20 20 49 6e 20 66 61 63  74 20 69 66 20 79 6f 75  |  In fact if you|
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00000240  65 2c 20 77 68 69 63 68  20 6d 65 61 6e 73 20 79  |e, which means y|
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00000260  63 61 6c 6c 65 64 20 66  72 6f 6d 0d 77 69 74 68  |called from.with|
00000270  69 6e 20 69 74 73 65 6c  66 20 77 69 74 68 6f 75  |in itself withou|
00000280  74 20 63 72 61 73 68 69  6e 67 2c 20 74 68 65 6e  |t crashing, then|
00000290  20 79 6f 75 20 63 6f 75  6c 64 20 65 6e 61 62 6c  | you could enabl|
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000002b0  68 69 73 20 77 6f 75 6c  64 20 62 65 20 61 63 68  |his would be ach|
000002c0  69 65 76 65 64 20 62 79  20 75 73 69 6e 67 20 74  |ieved by using t|
000002d0  68 65 20 73 74 61 63 6b  20 66 6f 72 0d 61 6c 6c  |he stack for.all|
000002e0  20 79 6f 75 72 20 73 74  6f 72 61 67 65 20 69 6e  | your storage in|
000002f0  73 74 65 61 64 20 6f 66  20 66 69 78 65 64 20 6d  |stead of fixed m|
00000300  65 6d 6f 72 79 20 6c 6f  63 61 74 69 6f 6e 73 2e  |emory locations.|
00000310  20 0d 50 65 72 73 6f 6e  61 6c 6c 79 20 49 20 63  | .Personally I c|
00000320  61 6e 27 74 20 6d 61 6b  65 20 74 68 61 74 20 77  |an't make that w|
00000330  6f 72 6b 20 64 75 72 69  6e 67 20 69 6e 74 65 72  |ork during inter|
00000340  72 75 70 74 73 2c 20 73  6f 20 49 0d 61 6c 77 61  |rupts, so I.alwa|
00000350  79 73 20 77 6f 72 6b 20  77 69 74 68 20 69 6e 74  |ys work with int|
00000360  65 72 72 75 70 74 73 20  64 69 73 61 62 6c 65 64  |errupts disabled|
00000370  2c 20 77 68 69 63 68 20  69 73 20 68 6f 77 20 74  |, which is how t|
00000380  68 65 20 4f 53 0d 73 65  74 73 20 74 68 69 6e 67  |he OS.sets thing|
00000390  73 20 75 70 2e 0d 0d 4f  6e 65 20 73 69 64 65 20  |s up...One side |
000003a0  65 66 66 65 63 74 20 6f  66 20 74 68 69 73 20 69  |effect of this i|
000003b0  73 20 74 68 61 74 20 4f  53 20 63 61 6c 6c 73 20  |s that OS calls |
000003c0  74 68 61 74 20 65 6e 61  62 6c 65 0d 69 6e 74 65  |that enable.inte|
000003d0  72 72 75 70 74 73 20 63  61 6e 20 6e 6f 74 20 62  |rrupts can not b|
000003e0  65 20 75 73 65 64 20 64  75 72 69 6e 67 20 69 6e  |e used during in|
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00000400  2e 20 20 54 68 69 73 0d  70 72 65 74 74 79 20 77  |.  This.pretty w|
00000410  65 6c 6c 20 6d 65 61 6e  73 20 65 76 65 72 79 74  |ell means everyt|
00000420  68 69 6e 67 20 65 78 63  65 70 74 20 6d 61 79 62  |hing except mayb|
00000430  65 20 4f 53 42 59 54 45  20 61 6e 64 20 4f 53 57  |e OSBYTE and OSW|
00000440  4f 52 44 2e 20 0d 59 6f  75 20 63 65 72 74 61 69  |ORD. .You certai|
00000450  6e 6c 79 20 63 6f 75 6c  64 6e 27 74 20 75 73 65  |nly couldn't use|
00000460  20 4f 53 57 52 43 48 20  6f 72 20 4f 53 52 44 43  | OSWRCH or OSRDC|
00000470  48 2e 0d 0d 4f 53 57 4f  52 44 20 31 32 20 28 26  |H...OSWORD 12 (&|
00000480  43 29 20 77 72 69 74 65  73 20 74 6f 20 74 68 65  |C) writes to the|
00000490  20 70 61 6c 65 74 74 65  2e 20 20 54 68 69 73 20  | palette.  This |
000004a0  63 6f 6e 74 72 6f 6c 73  20 74 68 65 0d 72 65 6c  |controls the.rel|
000004b0  61 74 69 6f 6e 73 68 69  70 20 62 65 74 77 65 65  |ationship betwee|
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000004d0  6c 6f 75 72 20 28 74 68  65 20 63 6f 6c 6f 75 72  |lour (the colour|
000004e0  20 6e 75 6d 62 65 72 29  0d 61 6e 64 20 74 68 65  | number).and the|
000004f0  20 61 63 74 75 61 6c 20  63 6f 6c 6f 75 72 20 28  | actual colour (|
00000500  74 68 65 20 63 6f 6c 6f  75 72 20 64 69 73 70 6c  |the colour displ|
00000510  61 79 65 64 29 2e 20 20  59 6f 75 20 6d 61 79 0d  |ayed).  You may.|
00000520  72 65 6d 65 6d 62 65 72  20 74 68 61 74 20 66 72  |remember that fr|
00000530  6f 6d 20 27 42 41 53 49  43 27 20 79 6f 75 20 75  |om 'BASIC' you u|
00000540  73 65 20 56 44 55 20 31  39 20 74 6f 20 64 6f 20  |se VDU 19 to do |
00000550  74 68 69 73 2c 20 62 75  74 0d 56 44 55 20 31 39  |this, but.VDU 19|
00000560  20 63 61 6e 20 6e 6f 74  20 62 65 20 75 73 65 64  | can not be used|
00000570  20 64 75 72 69 6e 67 20  61 6e 20 69 6e 74 65 72  | during an inter|
00000580  72 75 70 74 20 62 65 63  61 75 73 65 20 56 44 55  |rupt because VDU|
00000590  0d 73 74 61 74 65 6d 65  6e 74 73 20 75 73 65 20  |.statements use |
000005a0  4f 53 57 52 43 48 2e 0d  0d 57 68 61 74 20 49 20  |OSWRCH...What I |
000005b0  69 6e 74 65 6e 64 20 74  6f 20 64 6f 20 69 6e 20  |intend to do in |
000005c0  74 68 69 73 20 6d 6f 64  75 6c 65 20 69 73 20 74  |this module is t|
000005d0  6f 20 75 73 65 20 4f 53  57 4f 52 44 20 31 32 20  |o use OSWORD 12 |
000005e0  74 6f 0d 73 77 69 74 63  68 20 74 68 65 20 70 61  |to.switch the pa|
000005f0  6c 65 74 74 65 20 66 6f  72 20 63 6f 6c 6f 75 72  |lette for colour|
00000600  20 31 20 69 6e 20 6d 6f  64 65 20 33 20 73 6f 20  | 1 in mode 3 so |
00000610  66 61 73 74 20 74 68 61  74 20 77 65 0d 63 61 6e  |fast that we.can|
00000620  20 68 61 76 65 20 61 72  65 61 73 20 6f 66 20 74  | have areas of t|
00000630  68 65 20 73 63 72 65 65  6e 20 77 69 74 68 20 64  |he screen with d|
00000640  69 66 66 65 72 65 6e 74  20 63 6f 6c 6f 75 72 73  |ifferent colours|
00000650  20 69 6e 20 61 6e 0d 6f  74 68 65 72 77 69 73 65  | in an.otherwise|
00000660  20 6d 6f 6e 6f 63 68 72  6f 6d 61 74 69 63 20 6d  | monochromatic m|
00000670  6f 64 65 2e 0d 0d 57 65  20 63 61 6e 20 64 65 74  |ode...We can det|
00000680  65 63 74 20 74 68 65 20  66 69 65 6c 64 20 73 79  |ect the field sy|
00000690  6e 63 20 70 75 6c 73 65  20 75 73 69 6e 67 20 61  |nc pulse using a|
000006a0  6e 20 65 76 65 6e 74 20  61 6e 64 0d 74 68 61 74  |n event and.that|
000006b0  20 77 69 6c 6c 20 62 65  20 75 73 65 64 20 74 6f  | will be used to|
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00003370  6d 65 73 27 0d 62 6c 6f  63 6b 20 73 68 6f 75 6c  |mes'.block shoul|
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000033c0  30 20 74 6f 20 6c 69 6e  65 0d 31 36 31 30 2e 20  |0 to line.1610. |
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00003b70  72 69 74 74 65 6e 2e 0d  0d 54 68 65 20 62 69 74  |ritten...The bit|
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00003dd0  20 74 6f 20 67 65 74 20  72 65 70 6c 69 65 73 0d  | to get replies.|
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00003e60  72 6c 79 20 66 65 61 72  73 6f 6d 65 2e 20 20 54  |rly fearsome.  T|
00003e70  68 65 20 6f 6e 6c 79 20  74 68 69 6e 67 20 74 6f  |he only thing to|
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00003eb0  65 20 61 6e 64 20 73 6f  20 68 61 76 65 0d 79 6f  |e and so have.yo|
00003ec0  75 72 20 42 52 45 41 4b  20 6b 65 79 20 68 61 6e  |ur BREAK key han|
00003ed0  64 79 2e 20 20 4e 65 78  74 20 74 69 6d 65 20 77  |dy.  Next time w|
00003ee0  65 20 68 65 61 64 20 6f  66 66 20 69 6e 74 6f 20  |e head off into |
00003ef0  6d 6f 72 65 0d 6d 61 74  68 65 6d 61 74 69 63 73  |more.mathematics|
00003f00  20 61 73 20 6e 75 6d 62  65 72 73 20 77 69 74 68  | as numbers with|
00003f10  20 61 20 64 65 63 69 6d  61 6c 20 70 6f 69 6e 74  | a decimal point|
00003f20  20 61 70 70 65 61 72 2e  0d                       | appear..|
00003f29

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