Author
| 16kB and 64kB VRAM on V9938
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manuel
msx guru
Posts: 3447 |  Posted: March 19 2005, 14:35 |
Hi again,
After Daniel Vik told me he tested the behaviour of 16kB VRAM on V9938 (like on the SVI-738), I tried to implement it in openMSX as well. Daniel found that non-existing VRAM reads return simply 0xFF and writes are ignored (not mirrorred...).
So, I tried to run some normal MSX2 software on a machine with 16k VRAM and a V9938.
Did anyone ever test this on a real system? Maybe halfway during a SVI-738 upgrade?
What I see on openMSX is some mirroring effect (despite not having that implemented): part of the bottom screen is mirroring the top part. E.g., when trying the MSX2 bootup logo with 16k VRAM, I see part of this logo at the bottom after it stopped scrolling.
Do you guys have some test programs for these cases with known output?
E.g.: NYYRIKKI used to have a screen 5 picture viewer for the SVI-738. Anything more? And where can I download this nowadays?
I'll keep you updated if I find out more.
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NYYRIKKI
msx master
Posts: 1509 | Posted: March 19 2005, 15:26 |
Try going to Downloads / Utilitys / Graphics...  |
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manuel
msx guru
Posts: 3447 | Posted: March 19 2005, 15:56 |
Some questions mth came up with:
- 16kB VRAM is that 2x8kB or 1x16kB?
- what about the VRAM selection bit in R#8?
Any idea?
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Algorythms
msx freak
Posts: 175 | Posted: March 19 2005, 16:18 |
On 738 it's 2 x 8
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manuel
msx guru
Posts: 3447 | Posted: March 19 2005, 17:53 |
2x8K VRAM, is that (16K x 4) x 2 or (8K x 4) x 4 or (8K x 1) x 16?
RAM ICs seem to be either 1 bit wide or 4 bits wide...
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manuel
msx guru
Posts: 3447 | Posted: March 19 2005, 18:06 |
<Derpaux> 16x4x2
<Quibus> thanks
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flyguille
msx master
Posts: 1202 | Posted: March 19 2005, 21:43 |
<Quibus> sounds like a final fantasy character.
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manuel
msx guru
Posts: 3447 | Posted: March 20 2005, 00:10 |
Can someone with a 16k VRAM machine (e.g. SVI-738) give me the output of:
?VDP(9)
when in basic?
I'm especially interested in bits 2 and 3 (VRAM addressing).
Thanks!
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manuel
msx guru
Posts: 3447 | Posted: March 20 2005, 00:12 |
oh, never mind. It should give 0, but it won't because the V9938 registers can't be programmed in basic on a SVI-738  |
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manuel
msx guru
Posts: 3447 | Posted: March 20 2005, 13:57 |
OK, about V9938/58 VDP register 8: this is my theory:
- bit 2 is always 0 on MSX
- bit 3 is 0 by default and set to 1 by all 128k VRAM machines and kept to 0 by 16k VRAM machines. Not sure about 64k VRAM machines though.
Can someone confirm or deny (parts of) this theory? What about 64k VRAM?
Can someone explain to me in detail what happens when I set bit 3 to 0 on a normal 128k VRAM MSX2? What address lines are discarded? In other words: how is the change in address lines?
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