Re: Looking for YMF262

From: Nicolas Welte (
Date: 1999-07-02 10:57:13

Richard Atkinson wrote:
> Hmm. now that I have worked out the bizarre implementation of the FM sound
> chip on the 65xx bus, I doubt very much that a Soundblaster card could be
> grafted onto the Commodore bus without at least some modifications on the
> Soundblaster pcb.

Why? The ISA bus should also have the separated READ and WRITE lines,
and all the address lines are there. The interface to the SB card could
be similar to the C64-OPL2 interface, with an added address encoder for
the SB card. Instead of just decoding the C64 addresses to the OPL2 chip
select you would have to create an ISA bus I/O address for it. Somewhat
dependent of the R/W signals of course, because of that strange mapping
on the Sound Expander. Doing it with discrete TTL components might
actually be hard, but it should be possible to use some programmable
logic like GALs. Not that I had any experience with them :-(

> Here's the proposed software interface ('supported' register addresses):
> $DF40 Primary index register    write only (220 on PC)
> $DF48 Secondary index register  write only (222 on PC)
> $DF50 Primary data register     write only (221 on PC)
> $DF58 Secondary data register   write only (223 on PC)
> $DF60 Status register           read only  (220 on PC)

Looks good. I don't know about the compatibility with the original
software, of course. There are 2 significant address bits on the SFX
side, plus the status register bit and the state of the R/W line. The PC
side also only has 2 significant register bits. The extra case of
reading the status register can be handled. As it seems, the address
lines can be directly connected: C64 A4 with SB A0 and C64 A3 with SB
A1. C64 A5 and R/W is checked for read accesses to the SB status
register. The address lines A4 and A3 already match.

BTW, where are the SB DAC and ADC registers on the PC? These could also
be mapped into the C64 I/O space to give possibility of 8bit sampling.
Only in PIO mode, I guess, because a DMA interface would be to

> The two registers labelled 'Secondary' are new. Of course, in actual fact
> there are mirrors all over the DFxx address range, just as there are in
> the original Sound Expander.

Where are the mirrors? Which lines are decoded at all?

> Expect scanned-in copies of hand-drawn schematics tomorrow at an ftp site
> near you ;)

I looking forward to it :-) This will also answer the question of the
mirror registers.

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