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GI-Joe
Registered: Sep 2015
Posts: 15 |
unknown Register $D7FF
Hi,
i have found this code in a source:
ldy #$00
sty $d7ff
The D7FF bytes are not changed by the code.
Can someone explain this to me?
Thanxx
GI-Joe |
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GI-Joe
Registered: Sep 2015
Posts: 15 |
normal #$37 |
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chatGPZ
Registered: Dec 2001
Posts: 11116 |
We use $d7ff for the debug register in VICE :) |
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GI-Joe
Registered: Sep 2015
Posts: 15 |
Quote: We use $d7ff for the debug register in VICE :)
ok. Does this address give the value I wrote in it earlier ?
If so, I could use this for detection (vice)emulation ? |
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iAN CooG
Registered: May 2002
Posts: 3132 |
x64sc doesn't keep the value, but you get 0 and not 255 ($ff) as expected from an unconnected address and as written in Mapping the C64. |
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chatGPZ
Registered: Dec 2001
Posts: 11116 |
Quote:ok. Does this address give the value I wrote in it earlier ?
If so, I could use this for detection (vice)emulation ?
no and no. it's also disabled by default. if enabled, when something is written to $d7ff, the emulator quits with the value written as exitcode |
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TheRyk
Registered: Mar 2009
Posts: 2071 |
wasn't there something with SID registers/Mirror RAM (IO banked) being "WOM" (write only memory) by the book but in fact you _can_ read those at least for a frame or am I confusing stuff now? |
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iAN CooG
Registered: May 2002
Posts: 3132 |
Sid registers, $d400-1c, maybe true, but those 3 leftovers are unconnected, so whatever you write doesn't get stored anywhere even for a fraction of a second. |
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tlr
Registered: Sep 2003
Posts: 1714 |
Quote: Sid registers, $d400-1c, maybe true, but those 3 leftovers are unconnected, so whatever you write doesn't get stored anywhere even for a fraction of a second.
I believe it’s the last write you may read back for a while after it has been written. The adress, at least within the write only regs, should not matter.
I presume this behavior is due to the value being retained for a while on the SID’s floating internal data bus.
It might be that these retained values may also read back on the “unconnected” registers. |
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CyberBrain
Administrator
Posts: 392 |
Quote: wasn't there something with SID registers/Mirror RAM (IO banked) being "WOM" (write only memory) by the book but in fact you _can_ read those at least for a frame or am I confusing stuff now?
You might be thinking about the SID-posts in thread: https://csdb.dk/forums/?roomid=11&topicid=141335&showallposts=1
So yeah, a SID-write can be read back, but with lots of caveats :) (aka. not working like normal mem - see the thread for info)
(Of course, it was only speculated in that thread what would happen when reading/writing $D41D-$D41F (=$D7FD-$D7FF). Should be tested...) |
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ws
Registered: Apr 2012
Posts: 228 |
OMG. I just read that thread... i think my head exploded a little bit. |
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