65816 Disassembly - 8 and 16 bit modes

Today I made JMON a little smarter about 65816 disassembly. It now handles 8-bit and 16-bit instructions, looking at SEP and REP instructions to determine what mode the CPU is currently in. Of course, it cannot always get this correct as it doesn't know if code could be called when the CPU is in a specified mode. It starts in all 8-bit mode. This is similar to how the CC65 assembler works in "auto" mode unless you use assembler directives to explicitly set the mode.

While I was testing this I noticed the SEP instruction was disassembled as CPX. There is actually an error in the Western Design Center manual in Chapter 19, Instruction Lists, which I had copied when I entered the op codes. The byte $E2 is SEP but is listed as CPX.

Here is sample disassembly output from JMON showing all 4 combinations of 8 and 16-bit accumulator and index register modes (I've manually added spaces to highlight where the mode changes).

JMON MONITOR 0.97 BY JEFF TRANTER
? U 60B9
60B9   E2 30       SEP   #$30
60BB   09 12       ORA   #$12
60BD   29 12       AND   #$12
60BF   49 12       EOR   #$12
60C1   69 12       ADC   #$12
60C3   89 12       BIT   #$12
60C5   A9 12       LDA   #$12
60C7   C9 12       CMP   #$12
60C9   E9 12       SBC   #$12
60CB   A0 12       LDY   #$12
60CD   A2 12       LDX   #$12
60CF   C0 12       CPY   #$12
60D1   E0 12       CPX   #$12


60D3   C2 30       REP   #$30
60D5   09 34 12    ORA   #$1234
60D8   29 34 12    AND   #$1234
60DB   49 34 12    EOR   #$1234
60DE   69 34 12    ADC   #$1234
60E1   89 34 12    BIT   #$1234
60E4   A9 34 12    LDA   #$1234
60E7   C9 34 12    CMP   #$1234
60EA   E9 34 12    SBC   #$1234
60ED   A0 34 12    LDY   #$1234
60F0   A2 34 12    LDX   #$1234
60F3   C0 34 12    CPY   #$1234
60F6   E0 34 12    CPX   #$1234


60F9   C2 20       REP   #$20
60FB   E2 10       SEP   #$10
60FD   09 34 12    ORA   #$1234
6100   29 34 12    AND   #$1234
6103   49 34 12    EOR   #$1234
6106   69 34 12    ADC   #$1234
6109   89 34 12    BIT   #$1234
610C   A9 34 12    LDA   #$1234
610F   C9 34 12    CMP   #$1234
6112   E9 34 12    SBC   #$1234
6115   A0 12       LDY   #$12
6117   A2 12       LDX   #$12
6119   C0 12       CPY   #$12
611B   E0 12       CPX   #$12


611D   E2 20       SEP   #$20
611F   C2 10       REP   #$10
6121   09 12       ORA   #$12
6123   29 12       AND   #$12
6125   49 12       EOR   #$12
6127   69 12       ADC   #$12
6129   89 12       BIT   #$12
612B   A9 12       LDA   #$12
612D   C9 12       CMP   #$12
612F   E9 12       SBC   #$12
6131   A0 34 12    LDY   #$1234
6134   A2 34 12    LDX   #$1234
6137   C0 34 12    CPY   #$1234
613A   E0 34 12    CPX   #$1234