Saturn buses width ?

[Dr.Wily]

Hello,

I'm doing some research on Saturn hardware. I can't find any solid information on what is the width of the saturn buses ?

I think :

A bus : 32 bits --> SCU <-> cart connector/CD drive
B bus : 32 bit --> SCU <-> VDP1/2, SCSP/68k
C bus : 32 bit --> SCU <-> Ms SH-2 <-> Sl SH-2

I'm not sure about the A bus. Any clarification would be welcome 🙂

thanks

 

[antime]

There are block diagrams in the service manual and the "Introduction to Saturn Game Development" document. The A-bus is 16 bits wide, B-bus is 16 bits (though the SCSP has an only 8-bit wide B-bus interface), and C-bus is 32 bits.

 

[Dr.Wily]

OK thanks !

I think you're referring to this :

Sorry I'am dumb but there is no clearly writed the bus width. Do I need to refer to the data/address lines to determine the width of each bus?

Does this mean that the VDP1 and VDP2 only have a 16-bit bus ? SegaRetro talk about a 48 bit bus for VDP 1 and no info for VDP 2 (apart from a terse piece of information of 16-bit access per memory cycle).

 

[antime]

The bus width is determined by the number of data lines. E.g. for the A-bus, the diagram in your picture lists 16 data lines (AD0-AD15) and 26 address lines (AA0-AA25). So the bus has a 2^26 bytes, or 64MB address space, and can transfer 16 bits at a time. 32-bit transfers will be split up into two consecutive 16-bit transfers.

The B-bus is different in that it's some kind of multiplexed or packet-oriented bus, there's no separate address and data lines, just BD0-BD15 and some control lines. The bus also has devices with all 16 lines (VDP1, VDP2), and only 8 (SCSP). I have a vague memory it might be described in some patent, but I couldn't find the file now.

 

[Dr.Wily]

Very thank you for your input. I was reading Segaretro and I know they have a bias with numbers, the bigger the better.

I read this passage that talks about 3 32-bit buses.

I'm inclined to agree with you, but when in doubt, who's right?

 

[fafling]

That passage describes the SCU DSP and it is correct, aside from the "do the math" 128 bit fantasy bus.