Re: Plus/4 CPU

From: Ruud Baltissen (
Date: 2001-02-08 12:19:10

Hallo Nicholas,

> PS: I have headaches when I think about the recently discovered behaviour
> writes/reads to unconnected bits of adresses 0/1. IIRC, the data direction
> bit is connected, but the data port bit itself is not. Hard to emulate
with a
> 6522 that has both registers in a single chip.

I didn't understand some things or am not sure I understood it right.

Writing to and reading from the direction register returns all 8 bits right,
Writing to and reading from the data register only returns the connected
bits right, yes?
If so what do the other bits, return a (H)?
I also assume that the above only concerns the 7501/8501 as AFAIK the 6510
and 8502 behave right, ie. returning all bits right for all registers.

If the above is right, is there any software using this feature? IMHO I
don't see any negative point in using a 6522 which does not behave like
that. FYI, in 1988 I constructed a board with a 6502, 6526, buffers and glue
logic for the C64. Worked fine. But lost the board during a move (??
changing house).

Groetjes, Ruud

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