rrr selects a register. iii selects either an index register for addressing or an second register operand, depending on addressing mode, selected by mm: mm=0: register (eg txa ) mm=1: immediate (eg lda #$nnnn ) mm=2: indexed (eg lda $nnnn,x ) mm=3: indirect (eg lda ($nnnn),z ) To get absolute addressing, link cleverly and use PC as the index register, or use a zero index. Perhaps address indexing should be disabled if PC is selected as the index??? Instruction set: 0000 rrr 0000 mm iii + adc mem,reg 0001 rrr 0000 mm iii + sbc mem,reg 0010 rrr 0000 mm iii + and mem,reg 0011 rrr 0000 mm iii + eor mem,reg 0100 rrr 0000 mm iii + ora mem,reg 0101 rrr 0000 mm iii + cmp mem,reg 0110 rrr 0000 mm iii + load mem,reg 0111 rrr 0000 mm iii store mem,reg 1000 000 nnnn mm iii * bitset #n,mem 1001 000 nnnn mm iii * bitclr #n,mem 1010 000 nnnn mm iii * rol #n,mem ; roll left; n= 0..15 1011 000 nnnn mm iii * rlc #n,mem ; roll left thru carry; n= 1..16 1100 000 nnnn mm iii addq #n,mem 1101 rrr nnnn mm iii bbs #n,reg,(mem) 1110 rrr nnnn mm iii bbc #n,reg,(mem) 1111 000 0000 mm iii jsr (mem) 1111 001 0000 00 000 rts 1111 010 0000 00 000 rti 1111 011 0000 mm iii push mem 1111 100 0000 mm iii pop mem *) if rrr is non zero, then substitute the low bits of register rrr for #n +) if nnnn is non zero, then roll left contents of mem by n before using Flags: carry,zero,negative,interupt-disable,oVerflow. (decimal???)