And here we see values of 0x66000 being written in 32-bit chunks which agrees with the DSP seeing 0x0006 occasionally since it reads in 16-bit chunks. I logged the program counter so now it was time to disassemble the problematic code:
A quick glance at the disassembly shows that the DSP RAM init value is in register D0 (0x6000 is the DSP's floating point representation of zero) and the DSP RAM address is in A2. The values are written 32-bits at a time in a somewhat unrolled loop with D3 as the loop counter. The problem is clear - the init value in D0 is set with a MOVE.W instruction which only writes the 16 lower order bits of the register. The upper 16 bits is uninitialized data that just happens to have the value 0x0006 at runtime.
Fixing the code isn't hard. We just need to replace the MOVE.L instructions in the init loop with MOVE.W's so the DSP RAM is initialized in 16-bit (where we have a known value) chunks instead. The loop counter will need to be increased by a factor of 2 so the same amount of data is written.
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