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Krill
Registered: Apr 2002
Posts: 2940 |
$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: 248 |
I personally am sure that i don't mix it up with $d030 because i actually wrote one (lost) intro that uses the "switch mhz mode in the border" trick. |
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Jetboy
Registered: Jul 2006
Posts: 265 |
> And you don't mix it up with $d030?
There is such possibility.
Fiddling with bit 5 in vice 128 does not matter (both c128 and c64 modes). I do not have real HW to check. |
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GeirS
Registered: Jan 2014
Posts: 4 |
Quoting chatGPZTo me that hints at it perhaps being removed from later editions of the PRG too. Maybe :)
I can confirm that the textual description of the reset bit has been removed from Appendix N in my printed PRG (first edition, fourth printing - 1983.) There's just some extra empty space between the "Reset" heading and the next "Theory of operation" heading. |
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chatGPZ
Registered: Dec 2001
Posts: 11290 |
Ha! |
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ws
Registered: Apr 2012
Posts: 248 |
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: 1762 |
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: 11290 |
Someone on Lemon confirmed that setting bit5 indeed blanks the screen on a MAX Machine |
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tlr
Registered: Sep 2003
Posts: 1762 |
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: 11290 |
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: 1762 |
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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