CMP

Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	C9	2/3 bytes	2 cycles*
Absolute	CD	3 bytes	4 cycles*
Absolute Long	CF	4 bytes	5 cycles*
Direct Page	C5	2 bytes	3 cycles*
Direct Page Indirect	D2	2 bytes	5 cycles*
Direct Page Indirect Long	C7	2 bytes	6 cycles*
Absolute Indexed by X	DD	3 bytes	4 cycles*
Absolute Long Indexed by X	DF	4 bytes	5 cycles*
Absolute Indexed by Y	D9	3 bytes	4 cycles*
Direct Page Indexed by X	D5	2 bytes	4 cycles*
Direct Page Indexed Indirect by X	C1	2 bytes	6 cycles*
Direct Page Indirect Indexed by Y	D1	2 bytes	5 cycles*
Direct Page Indirect Long Indexed by Y	D7	2 bytes	6 cycles*
Stack Relative	C3	2 bytes	4 cycles*
Stack Relative Indirect Indexed by Y	D3	2 bytes	7 cycles*

Flags Affected
N	V	M	X	D	I	Z	C
N	.	.	.	.	.	Z	C

CMP (Compare) is a 65x instruction that compares the value of the accumulator to something. Internally, this is done via a subtraction, but the difference is not stored anywhere. Thus, the purpose of CMP is to setup the flags. The accumulator serves as the minuend. CMP only does unsigned comparisons. In immediate addressing only, CMP is a total of 3 bytes long when the accumulator is 16 bits wide. Unlike SBC, the carry flag does not affect CMP.

If the subtraction required a borrow, the carry flag is cleared (otherwise it is set).

The decimal mode flag has no effect on the behavior of CMP.

Syntax

CMP #const
CMP addr
CMP long
CMP dp
CMP (dp)
CMP [dp]
CMP addr, X
CMP long, X
CMP addr, Y
CMP dp, X
CMP (dp, X)
CMP (dp), Y
CMP [dp], Y
CMP sr, S
CMP (sr, S), Y

CPA is an alternative mnemonic. The 65c816 datasheet mentions an alternative mnemonic "CMA" which might be an alias for "CMP #0".

Cycle Penalties

• In all addressing modes, CMP takes an extra cycle when the accumulator is 16 bits wide
• In direct page addressing modes, CMP takes an extra cycle when the low byte of the direct page register is nonzero
• In both Absolute Indexed and DP Indirect Indexed by Y, CMP takes an extra cycle if adding the index crosses a page boundary

See Also

• CPX
• CPY
• CMP (Super FX)

External Links

• Eyes & Lichty, page 445 on CMP
• Labiak, page 134 on CMP
• MCS6500 Manual page 45 on CMP: https://archive.org/details/mos_microcomputers_programming_manual/page/n60
• Carr, page 255 on CMP
• Leventhal page 3-56 on CMP: https://archive.org/details/6502-assembly-language-programming/page/n105
• snes9x implementation of CMP: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L304
• undisbeliever on CMP: https://undisbeliever.net/snesdev/65816-opcodes.html#cmp-compare-accumulator-with-memory
• Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#CMP
• Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.1.2