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Krill
Registered: Apr 2002
Posts: 2854 |
$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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Martin Piper
Registered: Nov 2007
Posts: 645 |
Is it refresh related? Yes it is, as per the datasheets:
Datasheet 1: http://datasheet.elcodis.com/pdf2/73/0/730070/hm4864-2.pdf
"*Any cycle* in which *_RAS signal* occurs *refreshes* the entire row."
It doesn't matter if it is _RAS only or not, it is still a refresh.
Datasheet 1: https://www.datasheetarchive.com/pdf/download.php?id=8e4d8a58a2..
"Strobing each of the 256 row addresses with *_RAS* causes all bits in each row to be *refreshed*"
Post #33 in https://csdb.dk/forums/?roomid=7&topicid=89189&showallposts=1
At -59.81us to -59.8us where RAS goes from 1 to 0, per the datasheets, is enough to start the process of *refresh* for that row address.
What this means is that any _RAS (low RAS) is by definition refresh related. |
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Martin Piper
Registered: Nov 2007
Posts: 645 |
The NEC PD4164-2 datasheet also confirms it is refresh related... https://datasheetspdf.com/pdf-file/1098100/NEC/UPD4164-2/1
Mentioned by the C64 service manual page 22: https://www.retro-kit.co.uk/user/custom/Commodore/C64/manuals/C..
Refreshing is accomplished by:
Performing *_RAS only* refresh cycles
hidden refresh cycles
or *normal read* or write cycles
So any VIC reads that use _RAS (obviously) are refreshing, hence the problem is refresh related. As I said, refreshing doesn't just happen during the 5 VIC refresh slots per raster line. |
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Jetboy
Registered: Jul 2006
Posts: 227 |
OMG! OMG! THERE IS PERSON WRONG ON THE INTERNET! |
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Monte Carlos
Registered: Jun 2004
Posts: 351 |
So, is this thread actually becoming out of topic? The actual question was about a d016 bit and not HSP. |
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Monte Carlos
Registered: Jun 2004
Posts: 351 |
It's good to have different sights on the same thing, although if it gets a one man show, the constructive discussion suffers a lot. |
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Krill
Registered: Apr 2002
Posts: 2854 |
What iAN CooG and Monte Carlos said.
Quoting GroepazAFAIK this bit is only needed/used in the VICII of the MAX Machine. If at all :) Quoting tlrThe reset bit does not do anything on the 6569R1. I'm guessing early 6567's, maybe only prototypes even?
How likely is it that some (NTSC) C-64 have one of those early VIC-II revisions?
And if unlikely, why could a cartridge fail to start without that store to $d016? |
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chatGPZ
Registered: Dec 2001
Posts: 11145 |
Quote:How likely is it that some (NTSC) C-64 have one of those early VIC-II revisions?
"uncommon but not rare" :) The first batch(es) had the early VICIIs (obviously) - whats probably unlikely is that someone is still using these (64 cycles and everything)
Quote:And if unlikely, why could a cartridge fail to start without that store to $d016?
Could be as simple as "some broken code relies on the value in the register", who knows - i'd like to know who made those claims about which cartridges though (that would give us a chance to answer that question, perhaps). |
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Krill
Registered: Apr 2002
Posts: 2854 |
Quoting Groepaz i'd like to know who made those claims about which cartridges though (that would give us a chance to answer that question, perhaps). Will ask them to chime in. One of the best current coders on this machine, though, with many games under the belt. =)
Was stumped as well by a bug report of one of the testers and the fix.
Some sort of (yet unemulated) VIC-II startup state (unrelated to the reset bit) that only gets unlocked by a store to $D016 is ruled out? |
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chatGPZ
Registered: Dec 2001
Posts: 11145 |
I'd never rule out weird things like that.... That strange behaviour with one of the sprites that some VICII show was also quite unexpected :)
But as said, without knowing more details its all (a bit pointless) speculation. Once we know these, we can knock up tests and ask people to run them - and we can find out what exactly is happening. |
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Martin Piper
Registered: Nov 2007
Posts: 645 |
http://janklingel.de/wordpress/wp-content/uploads/2020/08/MOS-6..
"The RES bit (bit 5) of register $d016 has no function on the VIC 6567/6569 examined as yet. On the 6566, this bit is used to stop the VIC."
6566...
Wikipedia: "MOS Technology 6566 – designed for SRAM/non-muxed address lines (used in the MAX Machine)"
So... SRAM, no need for refresh, by normal reading or by scheduling forced row address _RAS strobes. |
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