SVP NOT SSP1601?

[fuzzbuzz]

So, recent claims have stated the SVP chip is a Samsung DSP Core model SSP1601. The basis for this information is "recently unearthed documents"

I make mistakes, but I'm pretty sure the whopping 3 seconds of research I've done proves this claim wrong.

Compare these two images. One is the SVP (in the middle of the board), and the other is the diagram for a SSP1601.

en.wikipedia.org/wiki/Image:Virtuaracing_cartridge.jpg
www.hacking-cult.org/images/svp2.png

I don't have a degree in computer science or engineering, but I'm pretty sure, that when I counted, the SSP1601 has 17 pins on the bottom side, and the SVP at least 38-40.

So, I'm pretty much convinced that the SVP isn't a SSP1601 at all. However, the machine code looks a lot like Samsung "SAM" assembly code, so I wouldn't rule out that it's not from samsung at all.

[Tom Maneiro]

It could be a SSP1601 highly customized for Sega. They may have licensed the core from Samsung, and then put it onto their own chip (just take a look on any Capcom arcade board: all chips say "CAPCOM", but none of them are exclusive designs, just relabeled parts OR customized parts).

[fuzzbuzz]

I suppose anything is possible at this point, but if based on the SSP's archetecture I would doubt it would havr over double the number of pins. It just seems like a waste, it only makes sense to me if it was a different chip. :/

[Tom Maneiro]

After all, the SVP IS a SSP1601, and it's now emulated

[GiGaBiTe]

Most support chips that sega used were in-house fabs and customized for their needs, so they could turn out quite different than a stock unmodified chip. Take a look at the genesis VDP, it eventually had the Z80, PSG and itself all in one chip.

I suspect the reason it had more pins is either some of them aren't used and are there to keep reverse-engineering hard. Or that there is actually some other chip integrated into it.

[jlf65]

gigabite said:

Most support chips that sega used were in-house fabs and customized for their needs, so they could turn out quite different than a stock unmodified chip. Take a look at the genesis VDP, it eventually had the Z80, PSG and itself all in one chip.

I suspect the reason it had more pins is either some of them aren't used and are there to keep reverse-engineering hard. Or that there is actually some other chip integrated into it.

Actually, the VDP has extra pins because they're used on the arcade boards to hook to an external RAMDAC. SEGA would use a couple VDPs in pairs to gain more colors on arcade machines that way.

[GiGaBiTe]

jlf65 said:

Actually, the VDP has extra pins because they're used on the arcade boards to hook to an external RAMDAC. SEGA would use a couple VDPs in pairs to gain more colors on arcade machines that way.

Adding more VDP's isn't going to give you more colors. If you want a higher color resolution, you have to add more VRAM. I don't think that the genesis has external CRAM or VRAM on the board, otherwise you could theoretically add more RAM and have more available colors if the VDP supported it (which it should).

[jlf65]

gigabite said:

jlf65 said:

Actually, the VDP has extra pins because they're used on the arcade boards to hook to an external RAMDAC. SEGA would use a couple VDPs in pairs to gain more colors on arcade machines that way.

Adding more VDP's isn't going to give you more colors. If you want a higher color resolution, you have to add more VRAM. I don't think that the genesis has external CRAM or VRAM on the board, otherwise you could theoretically add more RAM and have more available colors if the VDP supported it (which it should).

I've only been loosely following the thread on the arcade boards over at SpritesMind.net, but IIRC, adding another VDP allowed them to fetch the other "half" of the pixel data so that instead of internally looking up four bits in one of four internal palettes, you're now looking up eight bits in an external RAMDAC.

Something like that. The point is, the "extra" line are a digital output that bypasses the internal palette lookup, or something along those lines. They aren't used on the Genesis, but are on arcade boards made around the same time.

[GiGaBiTe]

Even if you were looking up larger chunks of data, if you don't have the VRAM to hold that data, it isn't going to work. You can only store so much color and sprite information as the VRAM can hold.

You can use larger color palettes IF you significantly decrease the screen resolution (ex: if screen was 320x224, make it 160x80 for double the colors) but you could hardly do anything with that.

[jlf65]

gigabite said:

Even if you were looking up larger chunks of data, if you don't have the VRAM to hold that data, it isn't going to work. You can only store so much color and sprite information as the VRAM can hold.

Which is why you use two VDPs - each one controls their own 64K of VRAM. One VDP would have half the data in its VRAM, and the other would have the other half in its VRAM. Remember, the 68000 doesn't access the VRAM directly, it goes through the VDP registers. You'd simply have a second VDP mapped in to a new memory location so you can access its memory the same way you do the normal VDP.