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Krill
Registered: Apr 2002
Posts: 2980 |
$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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ws
Registered: Apr 2012
Posts: 251 |
i was trying to remember more and then i came across the ECM mode + Bitmap-mode-enabled-at-the-same-time-"bug" again which actually does the black screen thing - but it does not crash. Perhaps the screen blackening happened and i was unable to do anything onscreen and identified that as a halt (i had no monitoring back in the day, let alone real knowledge :shrug:)
so i am not sure if it is actually true that the C128 halted, i just read this thread and remembered "wait i experienced exactly this!" and hastily commented - while i actually should only have asked if this was tried an a C128.
so my apologies, maybe that was a mixed up memory. but it surely was not d030. anyways. maybe i'll dig out the c128 i got in 2015 and try there again later - ofcourse it will most likely not be the same HW revision. Nevermind. |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: i was trying to remember more and then i came across the ECM mode + Bitmap-mode-enabled-at-the-same-time-"bug" again which actually does the black screen thing - but it does not crash. Perhaps the screen blackening happened and i was unable to do anything onscreen and identified that as a halt (i had no monitoring back in the day, let alone real knowledge :shrug:)
so i am not sure if it is actually true that the C128 halted, i just read this thread and remembered "wait i experienced exactly this!" and hastily commented - while i actually should only have asked if this was tried an a C128.
so my apologies, maybe that was a mixed up memory. but it surely was not d030. anyways. maybe i'll dig out the c128 i got in 2015 and try there again later - ofcourse it will most likely not be the same HW revision. Nevermind.
Thanks for clarifying.
Illegal vic modes only blacks the screen area. The RES bit should kill the video signal completely, no sync. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
Someone on Lemon confirmed that setting bit5 indeed blanks the screen on a MAX Machine |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: Someone on Lemon confirmed that setting bit5 indeed blanks the screen on a MAX Machine
Cool! Would be interesting to know the exact chip version/revision used.
Also, does it require a reset using that but or can it be kept 0 forever? |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
Quote:Also, does it require a reset using that but or can it be kept 0 forever?
You mean, will a program keep running, or machine crash (require reset to come back)? |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: Quote:Also, does it require a reset using that but or can it be kept 0 forever?
You mean, will a program keep running, or machine crash (require reset to come back)?
I mean: does it require a reset using that bit to start operating correctly after power up? |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
I still don't understand :) You mean the reset routine would set and then unset the bit? (I think we could find out by disassembling MAXBasic or MultiMAX) |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: I still don't understand :) You mean the reset routine would set and then unset the bit? (I think we could find out by disassembling MAXBasic or MultiMAX)
Yes.
Or perhaps starting the thing without a cart or with a dummy cart and see what happens? |
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oziphantom
Registered: Oct 2014
Posts: 490 |
Quote: i was trying to remember more and then i came across the ECM mode + Bitmap-mode-enabled-at-the-same-time-"bug" again which actually does the black screen thing - but it does not crash. Perhaps the screen blackening happened and i was unable to do anything onscreen and identified that as a halt (i had no monitoring back in the day, let alone real knowledge :shrug:)
so i am not sure if it is actually true that the C128 halted, i just read this thread and remembered "wait i experienced exactly this!" and hastily commented - while i actually should only have asked if this was tried an a C128.
so my apologies, maybe that was a mixed up memory. but it surely was not d030. anyways. maybe i'll dig out the c128 i got in 2015 and try there again later - ofcourse it will most likely not be the same HW revision. Nevermind.
The C128 does have a test bit that stops output, but its on D030 and doesn't use the D016 one as far as I know. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
Quote:Or perhaps starting the thing without a cart or with a dummy cart and see what happens?
Then it simply does nothing :) But yes, it'd be interesting if the monitor syncs or not in that case
Quote:The C128 does have a test bit that stops output, but its on D030
Technically the test bit in $d030 doesn't "stop output" - the line counter increases by 1 every cycle when it is set, it still produces a video signal though. |
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