My favorite part of the 6502 is that you can write the instruction set as 4 columns by 8 rows, and see the really simple pattern. https://pastebin.com/rCN932a0

My favorite part of the 6502 is that you can write the instruction set as 4 columns by 8 rows, and see the really simple pattern. https://pastebin.com/rCN932a0 https://pastebin.com/rCN932a0 When you see it in rows and columns, you can see how you have these addressing modes that correspond to the row number most of the time: 0: X,indirect 1: zeropage 2: immediate 3: absolute 4: indirect,Y 5: zeropage,X 6: absolute,Y 7: absolute,X Then you can see that Column 2 is the arithmetic instructions: ORA, AND, EOR, ADC, STA, LDA, CMP, SBC And column 3 is the Read-Modify-Write instructions: ASL, ROL, LSR, ROR, STX, LDX, DEC, INC. And that fourth column of invalid instructions turns out to be a combination of the second and third column, effectively doing an arithmetic operation and a RMW operation at the same time, even for addressing modes normally not supported.