Compare Accumulator

Compare Accumulator with Memory
Tags:

CMP subtracts the data at the address in the operand from the contents of the accumulator, setting the n, z & c flags based on the result. The Accumulator & Memory are unaffected by this operation.

Data size

On all processors, the data added from memory is 8-bit. However, for 16-bit processors with the m flag is clear then the data added is 16-bit with the low-order 8-bits at the effective address and the high-order 8-bits at the effective address plus one.

Flags Affected
Flags
n-----zc
nSet if most-significant bit of result is set
zSet if result is zero
cSet if register value greater than or equal or Cleared if less than memory value
Instructions
SyntaxOpcode Available on: # of # of Addressing Mode
(hex) 6502 65C02 65816 bytes cycles
CMP #constC9 x x x 21 22 Immediate
CMP addrCD x x x 3 42 Absolute
CMP longCF x 4 52 Absolute Long
CMP dpC5 x x x 2 32, 3 Direct Page
CMP (dp)D2 x x 2 52, 3 Direct Page Indirect
CMP [dp]C7 x 2 62, 3 Direct Page Indirect Long
CMP addr,XDD x x x 3 42, 4 Absolute Indexed X
CMP long,XDF x 4 52 Absolute Long Indexed X
CMP addr,YD9 x x x 3 42, 4 Absolute Indexed Y
CMP dp,XD5 x x x 2 42, 3 Direct Page Indexed X
CMP (dp,X)C1 x x x 2 62, 3 Direct Page Indexed Indirect X
CMP (dp),YD1 x x x 2 52, 3, 4 Direct Page Indirect Indexed Y
CMP [dp],YD7 x 2 62, 3 Direct Page Indirect Long Indexed Y
CMP sr,SC3 x 2 42 Stack Relative
CMP (sr,S),YD3 x 2 72 Stack Relative Indirect Indexed Y

Notes:

  1. 65816: Add 1 byte if m=0 (16-bit memory/accumulator)
  2. 65816: Add 1 cycle if m=0 (16-bit memory/accumulator)
  3. 65816: Add 1 cycle if low byte of Direct Page register is not 0
  4. Add 1 cycle if adding index crosses a page boundary
Last modified November 5, 2021: Add instruction categories (6b74ff9)