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Shadow
Account closed
Registered: Apr 2002
Posts: 355 |
How to find a C64 capable of HW-scroll?
I have had my C64-C for more than 20 years, and I love the thing.
However, it always, ALWAYS locks up on hardware scrolling effects (VSP/AGSP or whatever it is called).
That meant that I could never watch for exaple the Horizon demos with large x/y scrolling pictures back in the day.
Then there was the 'pc-style' period from 1994 and the rest of the nineties where demos wasn't using such tricks much more, and all was good.
Lately though I have seen more and more demos use the hardware scrolling tricks again, which means lockups for me and not being able to watch the demo.
The fact that I am pissed of that people use code that is not compatible with all C64s will probably not make much difference, so I guess I am forced to track down another C64, but how do I make sure that it can cope with hardware scrolling?
I think it is a question the average eBay seller is unable to answer, but maybe there is something I should look for? Are old breadbox ones generally better? |
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chatGPZ
Registered: Dec 2001
Posts: 11303 |
you can check for this bug much more easily... clear the whole memory (use some $a5 pattern or so, not just zeros) then execute inc $d011; jmp *-3 ... the c64 will crash very quickly, or atleast show random bytes in memory after a while. |
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WVL
Registered: Mar 2002
Posts: 889 |
Quote: I've spoken to SLC quite much about this and he spoke with the fpga64 guy. It's quite certain it's not a memory refresh cycle being missed. Fact 1) A single missed refresh cycle is not enough to distrupt the memory contents, it's just to quick. Fact 2) Doing a full page read directly after VSP will refresh the memory and should therefore prevent the bug, it doesn't.
The fpga64 guys however found some other very interesting issues when switching from chars to bitmap (commonly done when u VSP actually). Strange power spikes (very short) occurs on the memory line, and perhaps these will destroy the memory contents. What has to be done is to measure RAS and CAS during mode-switch and check if they're the reason for the bug. If this is the reason then a HW-fix must be applied (a simple HF-filter actually on RAS & CAS).
I'll keep u notified once I've persuaded my boss to measure this for me. ;)
I can confirm that refreshing the memory by executing a full page of code does not help, I tried it so it's bogus. I also dont think it's a memory-refresh thing.
Another reason why I think this : I swapped the memory for a crash-prone c64 with the memory of a non-crash prone c64. They still behaved the same. Then, I also swapped the VICS! They still behaved the same!!! :P |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Never seen the problem on any of my old breadboxes, so I would say go for one of those. That way you'll also get a proper SID.
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assiduous
Account closed
Registered: Jun 2007
Posts: 343 |
Quote:That way you'll also get a proper SID. what is a "proper" Sid ? if its the one which is closer to the original specs then it must be the 8580;) |
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Fungus
Registered: Sep 2002
Posts: 675 |
WVL: That is weird, I had a pal c64 (breadbox) that crashed constantly with anything using agsp/vsp and even fli. I had a few spare vics of different revisions, and I swapped in a r7 or r8 (don't remember now) and the problem completely disappeared.
I had various NTSC machines also crash with vsp, breadboxes, 64-c, 128 and 128DCR.
some worked, and some did not, of ALL of them.
So this being limited to later revision 64's is bs in my opinion cause I had early breadboxes that crashed like hell, and they seemed to be the most prone to it.
Generally I would swap to later vic revisions and these problems would go away.
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Devia
Registered: Oct 2004
Posts: 401 |
The old R3 VICs tend to run pretty hot, sometimes resulting in really weird bugs. Typically pixel noise to start with and then weird crashes. This is easily proven temperature related by watching the pixel noise disappear the moment you lay a heat sink or even a big fat greasy finger on the poor VIC.
Later revisions (still 6569) are a lot cooler.
This AGSP memory corruption thing has me confused... It doesn't appear to be single bit errors, but rather 4 bit single-bit rols, meaning I see MSB of corrupt nibbles being ROL'ed into LSB of same nibble without affecting the middle 2 bits.
Unfortunately I cant seem to find a c64 that's very suceptible to this bug, so it takes forever to hunt the corruptions down.
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SIDWAVE
Account closed
Registered: Apr 2002
Posts: 2238 |
I've never seen a demo crash on my first C64.
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Shadow
Account closed
Registered: Apr 2002
Posts: 355 |
OK, this is just plain weird. I have tested with "1 Year Totally Stoned", and the AGSP part runs without problem (tested three times, let it run for a minute or so)!
However the vector part that followed messed up two times out of three tests, almost as running the AGSP before it has somehow put the computer in an unstable state... However there is a loading break in between, so that seems unlikely that any memory corruption would carry over. Strangeness...
Oh and Jackasser - had I lived closer to Lund I would totally have brought my machine for some measuring. Perhaps if there is another BFP/LCP nearby sometime...
-edit- OK, now I was able to watch Royal Arte from start to finish without problems as well! I used to get lockups in the DYCP part. Maybe my C64 got the feeling that I was going to replace it and got its act together! :D |
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HCL
Registered: Feb 2003
Posts: 727 |
@Shadow: Yeah, i think you're right about the feelings of your c64 ;). But that test you made sort of sums up my view of this problem also. VSP on the top of the screen, like the bitmap mover (AGSP.. hate that abbreviation..) in "1 Year Totally Stoned" is less error prone than VSP somewhere in the middle of the screen, like the vector-part after. Strange but true. |
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Shadow
Account closed
Registered: Apr 2002
Posts: 355 |
HCL: Ahh.. The vector part uses VSP as well, didn't realize that. |
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