https://forums.atariage.com/topic/385516-fingerprinting-6502... https://forums.atariage.com/topic/385521-fingerprinting-6502...

https://csdb.dk/release/?id=248511

To understand, it helps if you write out the instruction table in columns, so here's the CMP and DEC instructions:

Byte C1: (add 4 to get to the next instruction in this table)

CMP X,ind [x indirect, read instruction's immediate value, add X, then read that pointer from zeropage, written like CMP ($nn,x)]

CMP zpg [zeropage, written like CMP $nn]

CMP # [immediate value, written like CMP #$nn]

CMP abs [absolute address, written like CMP $nnnn]

CMP ind,Y [indirect Y, read pointer from zeropage then add Y, written like CMP ($nnnn,Y)]

CMP zpg,X [zeropage plus X, add X to the zeropage address, written like CMP $nn,X]

CMP abs,Y [absolute address plus Y, add Y to the address, written like CMP $nnnn,Y]

CMP abs,X [absolute address plus X, add X to the address, written like CMP $nnnn,X]

So that's 8 possible addressing modes for this instruction.

Immediately afterwards:

Byte C2: (add 4 to get to the next instruction in this table)

???

DEC zpg

DEX

DEC abs

???

DEC zpg,X

???

DEC abs,X

That's 5 possible addressing modes. So where's "DEC X,ind", "DEC ind,Y", and "DEC abs,Y"? They don't exist.

Table for Byte C3 is 8 undocumented instructions that aren't supposed to be used. So people determined what the instruction did. Turns out, it's a combination of CMP and DEC, so people named the instruction "DCP".

Byte C3:

DCP X,ind

DCP zpg

???

DCP abs

DCP ind,Y

DCP zpg,X

DCP abs,Y

DCP abs,X

Unlike the "DEC" instruction, you have the "X,ind", "ind,Y", and "abs,Y" addressing modes available. So if you want to decrement memory, and don't care about your flags being correct (because it's also doing a CMP operation), you can use this DCP instruction.

Same idea with INC and SBC, you get the ISC instruction. For when you want to increment, and don't care about register A and flags afterwards.