CD side
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andi.b #0xE2,0x8003.w /* Priority Mode = off, 2M mode */
move.b #'I,0x800F.w /* send MD init handshake to MD */
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| wait for MD init handshake from CD
0:
cmpi.b #'I,0x200F(a5)
bne.b 0b
bset #1,0x2003(a5) /* give Sub-CPU Word RAM */
move.b #'B,0x200E(a5) /* main comm port - do boot */
If you're trying to use 1M mode (two blocks of 128KB of word ram), you may be using the wrong address.
Also, you need to set an extra register telling the BIOS how the data will be transferred. See page 26 of the CD hardware manual. But in general, if you are using the CPU for the CD data transfer (as opposed to DMA), you do this before reading
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/* set CDC Mode destination device to Sub-CPU */
andi.w #0xF8FF,0x8004.w
ori.w #0x0300,0x8004.w