Date   : Thu, 08 Feb 1996 01:22:54 +0000 (GMT)
From   : Tom Seddon <T.W.Seddon@...>
Subject: That System VIA

Perhaps unsurprisingly, I'm having a few problems with the implementation of 
the System VIA. It's all written and put in now: I've wrestled with 
DJGPP's interrupt functions, made a key translation table, sorted out 
some silly bugs in the 'write IER' routine, and it compiles fine. 
Unfortunately, IT DOES NOT WORK!!! Help!!

I handle IRQs by having a variable called 'IntRequests'. Every time a
device (ie the system VIA) wants an interrupt, it sets its registers
appropriately, and increments this variable. This is used by the 6502
emulator: every X cycles, or after every instruction, the processor checks
this variable, and if it's greater than 0 (and the I flag is clear) it
services one interrupt, decrements the variable, and continues. 

Is this correct?! It seems to be the way both XBeeb and BeebEm do it. 

Should I let the interrupts from the VIA pile up? I've added an 
'interrupts outstanding' indicator to the disassembly output, and when 
BASIC starts (after a long period of OS initialisation with I set) there 
are 20 interrupts outstanding. Is this normal? It implies that BASIC is 
taking 20 seconds to start up (20 t1 countdowns) which is odd... or am I 
missing something? (It wouldn't surprise me)

And once I get this sorted out--there's still the keyboard to go! (I've 
written the code, but it doesn't work--maybe it's related to the problems 
above).

I hate the VIA.

--Tom

     .                     ** Tom Seddon **                     .
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