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Krill
Registered: Apr 2002
Posts: 3098 |
$D016 bit 5
There is this mysterious bit early on in the KERNAL reset routine:
FCEF: 8E 16 D0 STX $D016 ; VIC: Control Register 2 with X being anything in [0..5].
Now, some people [who?] claim that without this store to $d016, some [which?] cartridges won't start [citation needed].
However, $D016 apparently used to have a mysterious "reset bit" in supposedly early VIC-II revisions (those with only 5 not 9 luma steps, and then the early ones of those).
| Bit 5 | Reset-Bit: 1 = Stop VIC (no Video Out, no RAM |
| | refresh, no bus access) | Could this be the reason why some things on some machines (?) won't start without that store to $D016?
Do any machines still exist where setting $D016 to, say, $f8 would crash them (when running code from RAM)?
Or did this "reset bit" never exist? =) |
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chatGPZ
Registered: Dec 2001
Posts: 11523 |
Quote:I think we could find out by disassembling MAXBasic
It writes 0 first (at the beginning of the reset routine), then 8 (in the VICII register init loop). So it doesn't toggle bit5. |
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Monte Carlos
Registered: Jun 2004
Posts: 370 |
According to C64 Wiki the 6566 is a revision used in NTSC machines. So not wonder if the PAL world wonders about the reset bit.
https://www.c64-wiki.de/wiki/VIC |
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tlr
Registered: Sep 2003
Posts: 1814 |
Quote: According to C64 Wiki the 6566 is a revision used in NTSC machines. So not wonder if the PAL world wonders about the reset bit.
https://www.c64-wiki.de/wiki/VIC
The 6566 is used solely in the MAX Machine which is only available in NTSC, yes. |
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Fungus
Registered: Sep 2002
Posts: 752 |
I have never seen one in the wild with a 6566. I have seen R4 6567 (less lumas) and R5 6A though. |
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chatGPZ
Registered: Dec 2001
Posts: 11523 |
As said, its only used in the MAX Machine, it wouldn't work in a C64 even. THAT part is clear :) |
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Fungus
Registered: Sep 2002
Posts: 752 |
Yeah, someone should correct that in that wiki. |
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