Code

UploadAndExecuteSoundEngine:
    sei         ;Disable interrupts

@WaitReady1:
    lda $2140
    cmp #$AA
    bne @WaitReady1
@WaitReady2:
    lda $2141
    cmp #$BB
    bne @WaitReady2 ;"Is the SPC700 ready?"

    lda #$01
    sta $2141
    lda #$00
    sta $2142
    lda #$05
    sta $2143
    lda #$CC
    sta $2140       ;Send the transfer begin bytes

@WaitEcho1:
    lda $2140
    cmp #$CC
    bne @WaitEcho1  ;Wait for PORT0 to echo

    ldx SoundEngineEnd-SoundEngine
    ldy #$0000
@TransferFirst:
    lda SoundEngine, y
    sta $2141       ;Write the first byte to PORT1

    lda #$00
    sta $2140       ;Write 0 to PORT0

@WaitEcho2:
    lda $2140
    cmp #$00
    bne @WaitEcho2  ;Wait for PORT0 to echo

    dex
    iny

@TransferRest:
    lda SoundEngine, y
    sta $2141       ;Write the next byte to PORT1

    lda $2140
    inc a
    sta $2140       ;Increase the byte in PORT0 and send it back
    sta $0000

@WaitEcho3:
    lda $2140
    cmp $0000
    bne @WaitEcho3

    dex
    iny

    txa
    cmp #$00
    bne @TransferRest

    lda #$00
    sta $2141   ;0 in PORT1
    lda #$00
    sta $2142
    lda #$05
    sta $2143   ;Execute address in PORT2 and PORT3
    lda $2140
    clc
    adc #$02
    sta $2140   ;Increase PORT0 by 2 and store it in PORT0

    stz $0000
    lda #$00
    ldx #$0000
    ldy #$0000

    cli         ;Enable interrupts
    rtl