SPC700/Driver Upload

In order to use main audio (Blargg's trick notwithstanding), the 5A22 must first upload a sound driver to the SPC700 over the following four ports:

Official Port Name	Port Name (SPC side)	Port Address (SPC side)	Port Name (5A22 side)	Port Address (5A22 side)	Data
PORT0	CPUIO0	00F4h	APUIO0	2140h	The SPC IPL writes AAh here when ready, kick and kickback (as described by fullsnes) also go through here
PORT1	CPUIO1	00F5h	APUIO1	2141h	The SPC IPL writes BBh here when ready, the 5A22 writes which command ("transfer" or "entry" as described by fullsnes) it wants the IPL to run here, and the driver data is uploaded through here too
PORT2	CPUIO2	00F6h	APUIO2	2142h	The 5A22 writes the low byte of the upload destination ARAM address here, and the low byte of the entrypoint into the SPC program once all blocks have been uploaded
PORT3	CPUIO3	00F7h	APUIO3	2143h	The 5A22 writes the high byte of the upload destination ARAM address here, and the high byte of the entrypoint into the SPC program once all blocks have been uploaded.

The official Super Nintendo development manual describes the driver upload process in Appendix D of Book I. [1] Fullsnes has pseudocode for the upload procedure in https://problemkaputt.de/fullsnes.htm#snesapumaincpucommunicationport, and anomie's SPC700 Doc also lists a very similar 11 step procedure.

For the purposes of uploading to the SPC, the sound driver SPC700 machine code is divided into some number of contiguous data blocks. Each block has a four byte header which consists of:

• the block size (2 bytes)
• the target address where the block will end up in ARAM (another 2 bytes) [2]

References

• [1] https://archive.org/details/SNESDevManual/book1/page/n236
• [2] page D-3 "Data Block Organization" of the official dev manual