CMP (CoMPare)¶

A - [operand]

CMP compares the accumulator register (A) to an operand. It subtracts the operand from A and sets the flags accordingly, but discards the result. The flags are set as in the SBC instruction.

The CMP instruction is often used before a branch instruction.

The flags that are set by CMP can be interpreted differently based on whether the operands are signed or unsigned.

Addressing Modes¶

Mode	Syntax	Bytes	Cycles
Immediate	CMP #const	`C9 nn`	2
Zero Page	CMP zp	`C5 nn`	3
Zero Page,X	CMP zp,X	`D5 nn`	4
Absolute	CMP addr	`CD ll hh`	4
Absolute,X	CMP addr,X	`DD ll hh`	4-5
Absolute,Y	CMP addr,Y	`D9 ll hh`	4-5
(Indirect,X)	CMP (zp,X)	`C1 nn`	6
(Indirect),Y	CMP (zp),Y	`D1 nn`	5-6

Flags Affected¶

N (Negative) – Set if the result of the comparison has the most significant bit set (i.e., is negative); otherwise, it is cleared.
Z (Zero) – Set if the result of the comparison is zero; otherwise, it is cleared.
C (Carry) – Set if A is greater than or equal to the operand; otherwise, it is cleared.

Examples¶

Compare to Immediate Value¶

    LDA #10    ; load A with constant 10
    CMP #5     ; compare A with constant 5

Compare to Value in Zero Page¶

    LDA #10    ; load A with constant 10
    CMP $00    ; compare A with the value at address $00

Compare to Value in Absolute Addressing¶

    LDA #10    ; load A with constant 10
    CMP $2000  ; compare A with the value at address $2000