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Saturn buses width ?
Dr.Wily - Apr 2, 2025
  Dr.Wily Apr 2, 2025
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 Apr 2, 2025
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 Apr 3, 2025
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 Apr 3, 2025
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 Apr 4, 2025
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 Apr 4, 2025
That passage describes the SCU DSP and it is correct, aside from the "do the math" 128 bit fantasy bus.