CMP

Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	C9	2 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
	.	.	.	.	.

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. 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).

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

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: https://archive.org/details/0893037893ProgrammingThe65816/page/n471
• Labiak page 134 on CMP: https://archive.org/details/Programming_the_65816/page/n144
• MCS6500 Manual page 45 on CMP: https://archive.org/details/mos_microcomputers_programming_manual/page/n60
• Carr page 255 on CMP: https://archive.org/details/6502UsersManual/page/n268
• 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