RE: C64 Kernal disassembly question

From: Baltissen, GJPAA (Ruud) (
Date: 2004-05-13 07:47:56

Hallo Jim,

> (just check ATN when you start clocking the byte in),

The ATN signal triggers the 15x1 to start clocking in the data. Without ATN
the 15x1 doesn't do anything at all. The rest of the transfer is based on
the protocol.

> If not, it looks like I could just use 3 IRQ lines...

I don't know the AVR at all but looking at the interrupthandling of 6502,
Z80 and 80x86, I would not use IRQ for the DATA and CLK line: handling the
IRQ could eat up more time then you can aford. So: IRQ for ATN, bitbanging
for DATA and CLK

> But, since I may have RS232 ints coming into the controller,

Here you can have a problem: the moment you are receiving/sending a byte
from/to the IEC-bus, an IRQ is out of the question otherwise you mess up the
whole timing. But IMHO there is nothing against pausing the transfer between
two bytes to poll the RS232 register to see if something has happened and
maybe even serve a request.

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