Alpha-II - Programming Information: SNES to APU data transfer

Transferring data from ROM to the SNES APU

The method of transferring data to the sound module of the SNES is a bit screwy, but it works. I've read many docs, looked at a lot of code, and nothing I've seen seems to explain the method to this madness in a straightforward way. I'm hoping I can shed some light on the matter.

During the transfer process, the following ports apply:

$2140 = Status

$2141 = Command / Data

$2142-3 = Address

Initializing the transfer

1. Wait for port $2140 to be $AA and port $2141 to be $BB. (This means the ROM program is through initializing, and is ready to begin a transfer.)

2. Write any value other than 0 to $2141. (A value of 0 has a different meaning which will be explained later.)

3. Write the destination address of the transfer to ports $2142 and $2143, with the low byte written to $2142.

4. Write $CC to port $2140.

5. Wait for $2140 to be $CC.

Transferring data

1. Write the first byte to be transferred to port $2141.

2. Write $00 to port $2140.

3. Wait for $2140 to be $00.

4. Write the next byte to be transferred to port $2141.

5. Increase the value in $2140 by 1, and write it back.

6. Wait for $2140 to be the same as the value written.

7. Goto step 4 until all bytes are transferred

Beginning another transfer

1. Write a non-zero value to port $2141.

2. Write the destination address of the transfer to ports $2142 and $2143, with the low byte written to $2142.

3. Increase the value in $2140 by 2. If it's 0, increase it again. Write it back.

4. Wait for $2140 to be the same as the value written, then go to the section "Transferring data".

Ending transfers

1. Write 0 to port $2141.

2. Write the address to begin execution to ports $2142 and $2143, with the low byte written to $2142.

3. Increase the value in $2140 by 2, and write it back.

Summary

Between transfers, a non-zero value sent to $2141 means to begin a transfer, and a zero value means to end a transfer and begin execution.

16-bit addresses are written to $2142-3.

During transfers, in $2140 a value less than means the APU is busy, a value equal means it's ready for the next byte. Writing a value one greater means the next byte ($2141) is ready to be stored. Writing a value two or more greater means to end the transfer and apply the command in $2141. (Because of the internal workings of the ROM program, the value in $2140 must be non-zero if the command is to start another transfer.)

Example

Init: Start: Next: Wait: ;Wait for SPC700 to initialize -- ReP #$30 LdA #$BBAA Cmp $2140 BNE Init ;Initialize transfer ------------ LdA #SPCExec StA $2142 LdA #$1CC StA $2140 SeP #$20 Cmp $2140 BNE Start ;Send data ---------------------- LdX #0 LdY #SPCSize LdA SPCData,X StA $2141 TXA StA $2140 Cmp $2140 BNE Wait InX DeY BNE Next ;Begin execution ---------------- ReP #$20 LdA #SPCExec StA $2142 SeP #$20 LdA #0 StA $2141 LdA $2140 AdC #2 StA $2140 ;(16-bit A and X Y) ;Send starting address ;Write $CC to 2140 and !0 to 2141 ;(8-bit A) ;Wait for SPC700 to give the go ;X indexes the current byte being sent ;Y = Number of bytes that need to be sent ;Send byte ;Wait for transfer ;(16-bit A) ;Starting address of execution ;(8-bit A) ;Jmp to execution address ;Break transfer cycle

1.	Wait for port $2140 to be $AA and port $2141 to be $BB. (This means the ROM program is through initializing, and is ready to begin a transfer.)
2.	Write any value other than 0 to $2141. (A value of 0 has a different meaning which will be explained later.)
3.	Write the destination address of the transfer to ports $2142 and $2143, with the low byte written to $2142.
4.	Write $CC to port $2140.
5.	Wait for $2140 to be $CC.

1.	Write the first byte to be transferred to port $2141.
2.	Write $00 to port $2140.
3.	Wait for $2140 to be $00.
4.	Write the next byte to be transferred to port $2141.
5.	Increase the value in $2140 by 1, and write it back.
6.	Wait for $2140 to be the same as the value written.
7.	Goto step 4 until all bytes are transferred

1.	Write a non-zero value to port $2141.
2.	Write the destination address of the transfer to ports $2142 and $2143, with the low byte written to $2142.
3.	Increase the value in $2140 by 2. If it's 0, increase it again. Write it back.
4.	Wait for $2140 to be the same as the value written, then go to the section "Transferring data".

1.	Write 0 to port $2141.
2.	Write the address to begin execution to ports $2142 and $2143, with the low byte written to $2142.
3.	Increase the value in $2140 by 2, and write it back.