ijor
commented
3 years ago Jose,
You are performing a byte sized instruction. It is not $8401 - 1, it is $01 - 1. So the result is $00, and not $8400.
Also the FX68K behavior is correct. When performing a byte sized write, the byte data is replicated in the other half of the 16 bits data bus. That's what a real 68000 does and it is documented in Motorola hardware manual.
jotego
The subq instruction in the code:
is not executed correctly by fx68k. Look at the following signal dump:
The CPU has to read the value in memory address $FFF310, decreased it by one and write it back. The value read is $8401, but the value written back is $0000 instead of $8400.
This is how the same instructions look like through the J68_CPU module, to serve as a reference:
You can see how the ram_data bus is decreased by one in this case.
Could you please look into this bug?