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12-03-88/T\OSB17

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.

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Filename: 12-03-88/T\OSB17
Read OK: ✔
File size: 1CF7 bytes
Load address: 0000
Exec address: 0000
File contents
OSBITS - An Exploration of the BBC Micro at Machine Level

By Programmer

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


Part 17: Events


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 serviced that
interrupt.  When you had finished you returned to your
foreground task.

One of the strengths of the 6502 microprocessor is its
speedy 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.  When you press a key you generate an interrupt,
when the TV scan finishes a field and flies back to start
another picture an interrupt is generated.  The OS
interrupts every 10 milliseconds to update system clocks.

Although they are essentially the same, the BBC Micro OS
distinguishes between Events and Interrupts.  Events are
interrupts that have been 'packaged' to make them a little
easier to use, so we'll start there and go on interrupts
next time.

The vector intercept program in the last module would run
alongside any program.  Whenever a character was sent to the
VDU drivers the intercept program would spring into action. 
Similarly when an event occurs the 6502 jumps down the event
vector to find whatever code you might have put there.  The
'event' that the intercept program responded to was caused
by the software but the principle is the same.  An event has
to be enabled by using OSBYTE (*FX) 14 and can be disabled
by using OSBYTE 13.

These are the events and their numbers.

    0   Output buffer becomes empty
    1   Input buffer becomes full

The buffer number (which is as selected by OSBYTE 21) will
be in the X register.  Event 1 is generated when the OS
fails to put a character in a buffer and the Y register will
hold that character.

    2   A character enters the input buffer

This is effectively caused by a key being pressed on the
keyboard.

    3   A to D conversion finished

This is caused by a A to D conversion being finished on any
channel that is enabled and the channel just converted will
be represented by the number in the Y register.

    4   Start of TV field blanking interval

The beam of the TV picture generated by the micro scans the
whole screen twice for each frame.  Each frame is made up of
two fields, each of 1/50th second duration.  This event is
generated whenever the beam leaves the bottom of the screen
and flies back to the top, i.e. once per field, 50 times a
second.  OSBYTE 19 forces the micro to wait for the flyback
to start.

    5   Interval timer crosses zero

This is a timer, not the TIME timer, read or set by OSWORD
3 and 4.  As the timer increases by one 100 times a second
you could use this event to print a background clock on the
screen.

    6   ESCAPE detected

When this event is enabled, the ESCAPE key has no effect
from BASIC or whatever language you are using.  The ESCAPE
could have come down the RS423 if that was enabled.

    7   RS423 error detected

The serial system has generated an error.

    8   Network event

Can only happen when Econet is attached to your machine.

    9   User event

Set aside for the user, who could also use Event 8 if no
Econet was present.

If one of these events is enabled with OSBYTE 14 (A=14
X=event number) then when the event itself occurs the micro
will JMP (evntv), i.e. it will jump to the address held in
the event vector &220 (and &221).  Unless you wish to chain
on down the event vector after your interception (and this
should not be neccessary) you must exit from your code with
an RTS, since before execution jumped down the vector the OS
set up the stack and memory in a way that expects this. In
effect this means that the OS expects a subroutine at the
address in the event vector.

There is a 'ReTurn from Interrupt' mnemonic that might at
first sight seem appropriate, since events are actually a
form of interrupt.  However the OS has set up the event in a
special way and you must RTS from an event, not RTI.

The address in the vector should be pointing to your event
routine.  As it enters your routine the accumulator is
holding the event number.  Your routine must save all the
registers (P A X  and Y) to the stack, and when you have
finished you should restore the registers before you exit.

The interrupt disable flag will be set by the OS
automatically before the micro reaches your code and you
should not enable interrupts.  By the same token you should
not use any OS routine that enables interrupts.  This means
you can use OSBYTE and some OSWORD calls during an event
routine but not others like OSCLI, OSRDCH and OSWRCH.

If you are absolutely certain that yours is the only event
routine then you should RTS rather than going down the
original vector.

Your event routine will probably be expecting only one
event, so you should check the value in A to see if the
event that has caused the jump to your routine is the one
you expect.  If it is not then you should jump to the
original contents of the vector.

The program in this module uses the keyboard event (number
2) to make a click when a key is pressed.  The code is
assembled in the I/O processor only at &A00 as before. 
Let's go through the program.

The setting up routine is from line 210 to line 420.  In it
the vectors are changed (if they haven't been changed
already) with the old vector contents being stored in
'old_evntv'.  OSWORD 8 is then used to set up a suitable
envelope from the parameter list in lines 760 to 890.  The
setting up procedure for OSWORD was given in module 5.

The actual event code is very simple.  It lies between lines
440 and 690.  First we check that this is event 2 and if not
we jump out straight away down the old vector.  (If you are
feeling devious you could remove this trap and see what
happens when you enable other events with *FX 14.)

All registers are saved onto the stack, including the status
register (very important).  The decimal flag is cleared just
in case the foreground routine was in decimal mode.  (It
would return to decimal mode since the status register will
be restored at the end of the routine.)

Then OSWORD 7 is called which makes the sound using the
envelope (number 1) set in the set-up code.  The registers
are restored and the code jumps down the old vector.  You
can not assume that no one else would use event 2 just
because you have.

If you want to save the routine to disk then copy the *SAVE
line across.  This has to run in the I/O processor so you
will not be allowed to run this assembly in a second
processor and the disc saving command puts the address as
&FFFF0A00.  No OS calls are allowed inside events in the
second processor but, as with the vector program, this will
run in the I/O processor no-matter where your foreground
program is.

The BASIC program shell enables the event so you should hear
the key clicks.  To disable them enter *FX13,2.  If you
later want to run in the routine from disc then typing
*kclick followed by *FX 14,2 will do it.

In the next module we progress onto interrupts proper and
I'll show you how to write a mouse driver.

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 37 3a 20  |.......Part 17: |
00000090  45 76 65 6e 74 73 0d 0d  0d 48 65 72 65 20 79 6f  |Events...Here yo|
000000a0  75 2c 20 61 72 65 20 72  65 61 64 69 6e 67 20 61  |u, are reading a|
000000b0  20 6d 6f 64 75 6c 65 20  6f 66 20 4f 53 62 69 74  | module of OSbit|
000000c0  73 2c 20 77 68 65 6e 20  74 68 65 20 74 65 6c 65  |s, when the tele|
000000d0  70 68 6f 6e 65 0d 72 69  6e 67 73 2e 20 20 59 6f  |phone.rings.  Yo|
000000e0  75 20 73 74 6f 70 20 72  65 61 64 69 6e 67 2c 20  |u stop reading, |
000000f0  61 6e 73 77 65 72 20 74  68 65 20 27 70 68 6f 6e  |answer the 'phon|
00000100  65 20 61 6e 64 20 74 61  6c 6b 20 74 6f 0d 77 68  |e and talk to.wh|
00000110  6f 65 76 65 72 20 69 73  20 72 69 6e 67 69 6e 67  |oever is ringing|
00000120  2e 20 20 57 68 65 6e 20  79 6f 75 20 68 61 76 65  |.  When you have|
00000130  20 66 69 6e 69 73 68 65  64 20 79 6f 75 20 70 75  | finished you pu|
00000140  74 20 74 68 65 0d 27 70  68 6f 6e 65 20 64 6f 77  |t the.'phone dow|
00000150  6e 20 61 6e 64 20 73 74  61 72 74 20 72 65 61 64  |n and start read|
00000160  69 6e 67 20 61 67 61 69  6e 2e 0d 0d 52 65 61 64  |ing again...Read|
00000170  69 6e 67 20 77 61 73 20  79 6f 75 72 20 6d 61 69  |ing was your mai|
00000180  6e 2c 20 6f 72 20 66 6f  72 65 67 72 6f 75 6e 64  |n, or foreground|
00000190  20 74 61 73 6b 2c 20 62  75 74 20 69 74 20 77 61  | task, but it wa|
000001a0  73 0d 69 6e 74 65 72 72  75 70 74 65 64 20 62 79  |s.interrupted by|
000001b0  20 74 68 65 20 74 65 6c  65 70 68 6f 6e 65 2e 20  | the telephone. |
000001c0  20 54 68 65 20 72 69 6e  67 69 6e 67 20 6f 66 20  | The ringing of |
000001d0  74 68 65 20 27 70 68 6f  6e 65 20 77 61 73 0d 61  |the 'phone was.a|
000001e0  6e 20 69 6e 74 65 72 72  75 70 74 20 61 6e 64 20  |n interrupt and |
000001f0  62 79 20 61 6e 73 77 65  72 69 6e 67 20 74 68 65  |by answering the|
00000200  20 27 70 68 6f 6e 65 20  79 6f 75 20 73 65 72 76  | 'phone you serv|
00000210  69 63 65 64 20 74 68 61  74 0d 69 6e 74 65 72 72  |iced that.interr|
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00000230  61 64 20 66 69 6e 69 73  68 65 64 20 79 6f 75 20  |ad finished you |
00000240  72 65 74 75 72 6e 65 64  20 74 6f 20 79 6f 75 72  |returned to your|
00000250  0d 66 6f 72 65 67 72 6f  75 6e 64 20 74 61 73 6b  |.foreground task|
00000260  2e 0d 0d 4f 6e 65 20 6f  66 20 74 68 65 20 73 74  |...One of the st|
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00000280  35 30 32 20 6d 69 63 72  6f 70 72 6f 63 65 73 73  |502 microprocess|
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000002c0  69 61 6c 6c 79 20 70 69  65 63 65 73 20 6f 66 0d  |ially pieces of.|
000002d0  68 61 72 64 77 61 72 65  20 69 6e 74 65 72 72 75  |hardware interru|
000002e0  70 74 20 74 68 65 20 36  35 30 32 20 61 6e 64 20  |pt the 6502 and |
000002f0  74 68 65 20 4f 53 20 68  61 73 20 61 20 62 75 69  |the OS has a bui|
00000300  6c 74 20 69 6e 0d 70 65  63 6b 69 6e 67 20 6f 72  |lt in.pecking or|
00000310  64 65 72 20 66 6f 72 20  74 68 65 73 65 20 64 65  |der for these de|
00000320  76 69 63 65 73 20 73 6f  20 69 74 20 6b 6e 6f 77  |vices so it know|
00000330  73 20 69 6e 20 77 68 69  63 68 20 6f 72 64 65 72  |s in which order|
00000340  0d 74 6f 20 72 65 73 70  6f 6e 64 2e 20 20 57 68  |.to respond.  Wh|
00000350  65 6e 20 79 6f 75 20 70  72 65 73 73 20 61 20 6b  |en you press a k|
00000360  65 79 20 79 6f 75 20 67  65 6e 65 72 61 74 65 20  |ey you generate |
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00000380  65 6e 20 74 68 65 20 54  56 20 73 63 61 6e 20 66  |en the TV scan f|
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00000730  65 72 73 2e 0d 0d 20 20  20 20 30 20 20 20 4f 75  |ers...    0   Ou|
00000740  74 70 75 74 20 62 75 66  66 65 72 20 62 65 63 6f  |tput buffer beco|
00000750  6d 65 73 20 65 6d 70 74  79 0d 20 20 20 20 31 20  |mes empty.    1 |
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00000780  20 62 75 66 66 65 72 20  6e 75 6d 62 65 72 20 28  | buffer number (|
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000007a0  63 74 65 64 20 62 79 20  4f 53 42 59 54 45 20 32  |cted by OSBYTE 2|
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000007e0  74 65 64 20 77 68 65 6e  20 74 68 65 20 4f 53 0d  |ted when the OS.|
000007f0  66 61 69 6c 73 20 74 6f  20 70 75 74 20 61 20 63  |fails to put a c|
00000800  68 61 72 61 63 74 65 72  20 69 6e 20 61 20 62 75  |haracter in a bu|
00000810  66 66 65 72 20 61 6e 64  20 74 68 65 20 59 20 72  |ffer and the Y r|
00000820  65 67 69 73 74 65 72 20  77 69 6c 6c 0d 68 6f 6c  |egister will.hol|
00000830  64 20 74 68 61 74 20 63  68 61 72 61 63 74 65 72  |d that character|
00000840  2e 0d 0d 20 20 20 20 32  20 20 20 41 20 63 68 61  |...    2   A cha|
00000850  72 61 63 74 65 72 20 65  6e 74 65 72 73 20 74 68  |racter enters th|
00000860  65 20 69 6e 70 75 74 20  62 75 66 66 65 72 0d 0d  |e input buffer..|
00000870  54 68 69 73 20 69 73 20  65 66 66 65 63 74 69 76  |This is effectiv|
00000880  65 6c 79 20 63 61 75 73  65 64 20 62 79 20 61 20  |ely caused by a |
00000890  6b 65 79 20 62 65 69 6e  67 20 70 72 65 73 73 65  |key being presse|
000008a0  64 20 6f 6e 20 74 68 65  0d 6b 65 79 62 6f 61 72  |d on the.keyboar|
000008b0  64 2e 0d 0d 20 20 20 20  33 20 20 20 41 20 74 6f  |d...    3   A to|
000008c0  20 44 20 63 6f 6e 76 65  72 73 69 6f 6e 20 66 69  | D conversion fi|
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000008e0  63 61 75 73 65 64 20 62  79 20 61 20 41 20 74 6f  |caused by a A to|
000008f0  20 44 20 63 6f 6e 76 65  72 73 69 6f 6e 20 62 65  | D conversion be|
00000900  69 6e 67 20 66 69 6e 69  73 68 65 64 20 6f 6e 20  |ing finished on |
00000910  61 6e 79 0d 63 68 61 6e  6e 65 6c 20 74 68 61 74  |any.channel that|
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00000930  74 68 65 20 63 68 61 6e  6e 65 6c 20 6a 75 73 74  |the channel just|
00000940  20 63 6f 6e 76 65 72 74  65 64 20 77 69 6c 6c 0d  | converted will.|
00000950  62 65 20 72 65 70 72 65  73 65 6e 74 65 64 20 62  |be represented b|
00000960  79 20 74 68 65 20 6e 75  6d 62 65 72 20 69 6e 20  |y the number in |
00000970  74 68 65 20 59 20 72 65  67 69 73 74 65 72 2e 0d  |the Y register..|
00000980  0d 20 20 20 20 34 20 20  20 53 74 61 72 74 20 6f  |.    4   Start o|
00000990  66 20 54 56 20 66 69 65  6c 64 20 62 6c 61 6e 6b  |f TV field blank|
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000009b0  65 20 62 65 61 6d 20 6f  66 20 74 68 65 20 54 56  |e beam of the TV|
000009c0  20 70 69 63 74 75 72 65  20 67 65 6e 65 72 61 74  | picture generat|
000009d0  65 64 20 62 79 20 74 68  65 20 6d 69 63 72 6f 20  |ed by the micro |
000009e0  73 63 61 6e 73 20 74 68  65 0d 77 68 6f 6c 65 20  |scans the.whole |
000009f0  73 63 72 65 65 6e 20 74  77 69 63 65 20 66 6f 72  |screen twice for|
00000a00  20 65 61 63 68 20 66 72  61 6d 65 2e 20 20 45 61  | each frame.  Ea|
00000a10  63 68 20 66 72 61 6d 65  20 69 73 20 6d 61 64 65  |ch frame is made|
00000a20  20 75 70 20 6f 66 0d 74  77 6f 20 66 69 65 6c 64  | up of.two field|
00000a30  73 2c 20 65 61 63 68 20  6f 66 20 31 2f 35 30 74  |s, each of 1/50t|
00000a40  68 20 73 65 63 6f 6e 64  20 64 75 72 61 74 69 6f  |h second duratio|
00000a50  6e 2e 20 20 54 68 69 73  20 65 76 65 6e 74 20 69  |n.  This event i|
00000a60  73 0d 67 65 6e 65 72 61  74 65 64 20 77 68 65 6e  |s.generated when|
00000a70  65 76 65 72 20 74 68 65  20 62 65 61 6d 20 6c 65  |ever the beam le|
00000a80  61 76 65 73 20 74 68 65  20 62 6f 74 74 6f 6d 20  |aves the bottom |
00000a90  6f 66 20 74 68 65 20 73  63 72 65 65 6e 0d 61 6e  |of the screen.an|
00000aa0  64 20 66 6c 69 65 73 20  62 61 63 6b 20 74 6f 20  |d flies back to |
00000ab0  74 68 65 20 74 6f 70 2c  20 69 2e 65 2e 20 6f 6e  |the top, i.e. on|
00000ac0  63 65 20 70 65 72 20 66  69 65 6c 64 2c 20 35 30  |ce per field, 50|
00000ad0  20 74 69 6d 65 73 20 61  0d 73 65 63 6f 6e 64 2e  | times a.second.|
00000ae0  20 20 4f 53 42 59 54 45  20 31 39 20 66 6f 72 63  |  OSBYTE 19 forc|
00000af0  65 73 20 74 68 65 20 6d  69 63 72 6f 20 74 6f 20  |es the micro to |
00000b00  77 61 69 74 20 66 6f 72  20 74 68 65 20 66 6c 79  |wait for the fly|
00000b10  62 61 63 6b 0d 74 6f 20  73 74 61 72 74 2e 0d 0d  |back.to start...|
00000b20  20 20 20 20 35 20 20 20  49 6e 74 65 72 76 61 6c  |    5   Interval|
00000b30  20 74 69 6d 65 72 20 63  72 6f 73 73 65 73 20 7a  | timer crosses z|
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00001a40  69 73 6b 20 74 68 65 6e  20 63 6f 70 79 20 74 68  |isk then copy th|
00001a50  65 20 2a 53 41 56 45 0d  6c 69 6e 65 20 61 63 72  |e *SAVE.line acr|
00001a60  6f 73 73 2e 20 20 54 68  69 73 20 68 61 73 20 74  |oss.  This has t|
00001a70  6f 20 72 75 6e 20 69 6e  20 74 68 65 20 49 2f 4f  |o run in the I/O|
00001a80  20 70 72 6f 63 65 73 73  6f 72 20 73 6f 20 79 6f  | processor so yo|
00001a90  75 0d 77 69 6c 6c 20 6e  6f 74 20 62 65 20 61 6c  |u.will not be al|
00001aa0  6c 6f 77 65 64 20 74 6f  20 72 75 6e 20 74 68 69  |lowed to run thi|
00001ab0  73 20 61 73 73 65 6d 62  6c 79 20 69 6e 20 61 20  |s assembly in a |
00001ac0  73 65 63 6f 6e 64 0d 70  72 6f 63 65 73 73 6f 72  |second.processor|
00001ad0  20 61 6e 64 20 74 68 65  20 64 69 73 63 20 73 61  | and the disc sa|
00001ae0  76 69 6e 67 20 63 6f 6d  6d 61 6e 64 20 70 75 74  |ving command put|
00001af0  73 20 74 68 65 20 61 64  64 72 65 73 73 20 61 73  |s the address as|
00001b00  0d 26 46 46 46 46 30 41  30 30 2e 20 20 4e 6f 20  |.&FFFF0A00.  No |
00001b10  4f 53 20 63 61 6c 6c 73  20 61 72 65 20 61 6c 6c  |OS calls are all|
00001b20  6f 77 65 64 20 69 6e 73  69 64 65 20 65 76 65 6e  |owed inside even|
00001b30  74 73 20 69 6e 20 74 68  65 0d 73 65 63 6f 6e 64  |ts in the.second|
00001b40  20 70 72 6f 63 65 73 73  6f 72 20 62 75 74 2c 20  | processor but, |
00001b50  61 73 20 77 69 74 68 20  74 68 65 20 76 65 63 74  |as with the vect|
00001b60  6f 72 20 70 72 6f 67 72  61 6d 2c 20 74 68 69 73  |or program, this|
00001b70  20 77 69 6c 6c 0d 72 75  6e 20 69 6e 20 74 68 65  | will.run in the|
00001b80  20 49 2f 4f 20 70 72 6f  63 65 73 73 6f 72 20 6e  | I/O processor n|
00001b90  6f 2d 6d 61 74 74 65 72  20 77 68 65 72 65 20 79  |o-matter where y|
00001ba0  6f 75 72 20 66 6f 72 65  67 72 6f 75 6e 64 0d 70  |our foreground.p|
00001bb0  72 6f 67 72 61 6d 20 69  73 2e 0d 0d 54 68 65 20  |rogram is...The |
00001bc0  42 41 53 49 43 20 70 72  6f 67 72 61 6d 20 73 68  |BASIC program sh|
00001bd0  65 6c 6c 20 65 6e 61 62  6c 65 73 20 74 68 65 20  |ell enables the |
00001be0  65 76 65 6e 74 20 73 6f  20 79 6f 75 20 73 68 6f  |event so you sho|
00001bf0  75 6c 64 20 68 65 61 72  0d 74 68 65 20 6b 65 79  |uld hear.the key|
00001c00  20 63 6c 69 63 6b 73 2e  20 20 54 6f 20 64 69 73  | clicks.  To dis|
00001c10  61 62 6c 65 20 74 68 65  6d 20 65 6e 74 65 72 20  |able them enter |
00001c20  2a 46 58 31 33 2c 32 2e  20 20 49 66 20 79 6f 75  |*FX13,2.  If you|
00001c30  0d 6c 61 74 65 72 20 77  61 6e 74 20 74 6f 20 72  |.later want to r|
00001c40  75 6e 20 69 6e 20 74 68  65 20 72 6f 75 74 69 6e  |un in the routin|
00001c50  65 20 66 72 6f 6d 20 64  69 73 63 20 74 68 65 6e  |e from disc then|
00001c60  20 74 79 70 69 6e 67 0d  2a 6b 63 6c 69 63 6b 20  | typing.*kclick |
00001c70  66 6f 6c 6c 6f 77 65 64  20 62 79 20 2a 46 58 20  |followed by *FX |
00001c80  31 34 2c 32 20 77 69 6c  6c 20 64 6f 20 69 74 2e  |14,2 will do it.|
00001c90  0d 0d 49 6e 20 74 68 65  20 6e 65 78 74 20 6d 6f  |..In the next mo|
00001ca0  64 75 6c 65 20 77 65 20  70 72 6f 67 72 65 73 73  |dule we progress|
00001cb0  20 6f 6e 74 6f 20 69 6e  74 65 72 72 75 70 74 73  | onto interrupts|
00001cc0  20 70 72 6f 70 65 72 20  61 6e 64 0d 49 27 6c 6c  | proper and.I'll|
00001cd0  20 73 68 6f 77 20 79 6f  75 20 68 6f 77 20 74 6f  | show you how to|
00001ce0  20 77 72 69 74 65 20 61  20 6d 6f 75 73 65 20 64  | write a mouse d|
00001cf0  72 69 76 65 72 2e 0d                              |river..|
00001cf7
12-03-88/T\OSB17.m0
12-03-88/T\OSB17.m1
12-03-88/T\OSB17.m2
12-03-88/T\OSB17.m4
12-03-88/T\OSB17.m5

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