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Forums > C64 Coding > Writes to $00 / $01
2008-09-11 11:24
JackAsser

Registered: Jun 2002
Posts: 2014
Writes to $00 / $01

When you write to $00 and $01, are these writes also issued on the memory bus?

To test on the real thing, simply enable bitmap mode and point the VIC-bank to #$03 ($0000-$3fff). Then write to $00 and $01, if the memory writes are issued then the two first bytes on the bit map will change. I don't have my setup up and running, could someone please test it.

@Gideon: If this works then this will enable you to know indirectly the state of $00 and $01 and thus be able to switch I/O-Address decoding on/off for f.e. SID-emulation.
2008-09-11 12:31
chatGPZ

Registered: Dec 2001
Posts: 11386
Quote:
To test on the real thing, simply enable bitmap mode and point the VIC-bank to #$03 ($0000-$3fff). Then write to $00 and $01, if the memory writes are issued then the two first bytes on the bit map will change. I don't have my setup up and running, could someone please test it.


that was tested long ago... marko did it if i recall correctly. (yes it works)
2008-09-11 12:41
JackAsser

Registered: Jun 2002
Posts: 2014
@Groepaz: \o/ Hurray for SID-emulation core in 1541u then! (and other I/O stuff).
2008-09-11 13:29
Frantic

Registered: Mar 2003
Posts: 1648
http://codebase64.org/doku.php?id=magazines:chacking7#hiding_ki..
2008-09-11 14:43
Oswald

Registered: Apr 2002
Posts: 5094
mmm... this makes me remember an article which claimed that if you read 00/01 on the same cycle the VIC does you will read the ram (iirc)
2008-09-11 17:16
Krill

Registered: Apr 2002
Posts: 2980
Sounds like nonsense. :)

The 6510 just catches any read-accesses on $00 and $01 and gives you the contents of their respective registers in the CPU, no matter what the VIC is currently doing. Besides, the CPU and the VIC access the bus on different edges of the clock signal. (Except when the CPU is stalled and the VIC does 2 reads per clock cycle.)
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