Direct Video Memory Access (DVMA) for V9938

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By Eugeny_Brychkov

Paragon (1184)

Eugeny_Brychkov's picture

21-10-2010, 11:19

www.ccas.ru/brychkov/inventor-corner/tn-DVMA-MSX-view.jpg www.ccas.ru/brychkov/inventor-corner/tn-DVMA-MSX-installed.jpg www.ccas.ru/brychkov/inventor-corner/tn-DVMA-128K-versus-16K.jpg www.ccas.ru/brychkov/inventor-corner/tn-DVMA-close-up.jpg

Source is here

"Those, who are familiar with architecture and programming of MSX computers and other devices based on Texas Instrument's TMS and Yamaha's V99x8 family video processors, know that while these VDPs provide remarkable graphics flexibility and capabilities, they are not lack of serious drawbacks which are missing in the models like Commodore or Spectrum. Those issues include ported VDP access, no option for CPU to access VDP memory directly, and serious video problems when over-clocking the system (e.g. installing Z80B at 7MHz speed).

I've designed and built prototype of the Direct Video Memory Access (DVMA) board, which allows to map video memory to the CPU addressable space window, uses hardware synchronization between CPU and VDP (so no "NOP" is required between VDP access commands), and what's even more important - graphics performs excellently in the over-clocked system. It holds 128K of video memory based on fast SRAM chips.

I have taken MSX1 Yamaha YIS503IIR machine, upgraded it to MSX2: soldered some logic on board, upgraded memory board, modified and programmed MSX2 bios to work without RTC chip. This model is best suitable because it has CX7 version 1 board with VRAM memory board connector (native YIS503IIIR has main board of version 2 without such connector).

With this board, CPU command 'LD (HL),A' is enough to draw on the screen."

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By Huey

Prophet (2676)

Huey's picture

21-10-2010, 11:37

Nice. The one thing that comes to my mind is: What if this was implemented 20 years ago as a new standard for msx.

By marlon-B

Expert (88)

marlon-B's picture

21-10-2010, 11:37

Excellent achievement !!Smile

I believe this modification in combination with my hardware blitter would produce astonishing results...

nice work !

By wolf_

Ambassador_ (9903)

wolf_'s picture

21-10-2010, 12:06

The real challenge is how to get this board into the hands of every active MSX user.

By MicroTech

Champion (385)

MicroTech's picture

21-10-2010, 14:33

... I'm astonished... really cool.
Maybe I lost something but I can't find how is VRAM mapped in z80 addressing space?

Question (maybe not strictly related to DVMA): is it possible to use real DMA chip in MSX architecture?

By caro

Champion (511)

caro's picture

21-10-2010, 15:06

ADVRAM - the variant of the circuit of direct access to memory VDP was already made:
www.msx.org/forumtopic5524.html

By RetroTechie

Paragon (1563)

RetroTechie's picture

21-10-2010, 16:35

Excellent achievement !!Smile
Second that! Eugeny, can you make a schematic & put online, and perhaps a description of how to use from a programmer's point of view?

Was also thinking about ADVRAM - do you know this, and if so: how does it compare to your solution? Also: I spotted "-15" markings on the SRAMs in the pix... I assume these are fast cache SRAMs where -15 actually means ~15 ns (and not 150) access time. Is this speed needed, or would this perhaps also work with more common speeds like 55 or 70 ns (low power!) SRAMs? Question Let's say for standard 3.58 MHz, or 7 MHz machine.

By Eugeny_Brychkov

Paragon (1184)

Eugeny_Brychkov's picture

21-10-2010, 20:34

A this time I prefer to keep distribution of the schematic under control as it is still under further development.

I can not find any useful hardware info about ADVRAM, and most important - about how it works, so can't compare...

I did not try with 55/70ns SRAMs, but it may not work well simply because V9938 full memory cycle length is 279 ns and it may be impossible to insert host read/write cycle between screen update read cycles.

By Manuel

Ascended (18238)

Manuel's picture

21-10-2010, 20:36

ADVRAM is implemented in openMSX, so you can see how it works there Tongue

By Eugeny_Brychkov

Paragon (1184)

Eugeny_Brychkov's picture

21-10-2010, 20:38

I need information about how its hardware works, not about how to program it.

By dhau

Paragon (1570)

dhau's picture

21-10-2010, 20:45

Eugeny: schematics with explanations, or it doesn't exist! Smile Smile Smile

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