Re: PCcard (2)

From: Marko Mäkelä (
Date: 2001-04-10 11:56:33

Hello Ruud,

> IMHO bad programming and that does not count.
> On second thought: how can a instruction like STA $D880,X store something in
> $D7XX even when X is, let's say, $CC?

It does not.  Hmm, let's think.  When someone wants to initialize the
chars to one color, and the top line to some other color, he could use a
single "loop: LDX#0:...:DEX:BNE loop" to initialize everything.  Then the
offset for the top-line STA would be $d700+40=$d728, I think.  That'd
normally work, since writes to $d7xx are sort of NOPs.

> So the moment you willingly generate an IRQ with this 6821, you are unable
> to address it anymore. Then how do you disable this IRQ? Otherwise you won't
> be able anymore to use the 6821 at all.

That's the idea: the circuit will be disabled until the next system RESET.
It's the same thing on the C128.  Once you enable the C64 mode, there is
no return (without using RESET).

On the other hand, the C65 boots up in C64 mode by default.  They use a
magic sequence of writes (and maybe reads) to some 4567 VIC-III registers
to enable C65 mode.  That's why the RESET routine of the C64 mode ROM in
the C65 is modified from the original.

> (I understand that the 6821's IRQ is NOT connected to the system IRQ)

You understood correctly.  I think I also used a pull-up resistor on that


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