🚀 go-pugleaf

>>                                                                 Also, if
>>    I hook 08H instead of 1CH, I could theoretically call the original 08H
>>    at 18.2 hz, but the system locks all the same.  So it appears that 08H
>>    MUST be called every timer pulse.  
>
>INT 08H does the following things:
>	1) Increment a running count of clock ticks at 0040:006C
>	2) Decrements the byte at 0040:0040, which controls the timing of
>	   the diskette motor
>	3) Calls interrupt 1CH

There's one more, very important thing that Int 08H does: it clears the
interrupt!  This is done by twiddling the 8259A interrupt controllers.
Sorry, but I don't know exactly how.  The bottom line is, if you are
going to set up a timer interrupt routine that doesn't always call the
standard BIOS routine, then you must clear the interrupt yourself.
Otherwise, your system will definitely hang.

I discovered this by using Turbo Debugger to disassemble the BIOS' Int 8
routine.  You can use DOS' DEBUG to do this too.  Find the Int 8 routine
via its interrupt vector and disassemble.  The code for resetting the
8259A should be in there (look for accesses to IO ports 20H and/or 21H.)
Alternatively, perhaps someone else in this group knows how to do it.

  Bob Alexander      | The more the government does,
  boba@hpwala.hp.com | the better libertarianism looks.
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