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'''CMP''' (Compare) is a 65x instruction that compares the value of the [[accumulator]].
{| class="wikitable" style="float:right;clear:right;width:50%"
 
{| class="wikitable" style="float:right;clear:right;width:30%"
!colspan="8"|Basic Info
!colspan="8"|Basic Info
|+
|+
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|'''Speed'''
|'''Speed'''
|+
|+
|immediate
|[[Immediate]]
|C9
|C9
|2 bytes
|2 bytes*
|2 cycles
|2 cycles*
|+
|+
|absolute
|[[Absolute]]
|CD
|CD
|3 bytes
|3 bytes
|4 cycles
|4 cycles*
|+
|+
|absolute long
|[[Absolute Long]]
|CF
|CF
|4 bytes
|4 bytes
|5 cycles
|5 cycles*
|+
|+
|direct page
|[[Direct Page Addressing|Direct Page]]
|C5
|C5
|2 bytes
|2 bytes
|3 cycles
|3 cycles*
|+
|+
|direct page indirect
|[[Direct Page Indirect]]
|D2
|D2
|2 bytes
|2 bytes
|5 cycles
|5 cycles*
|+
|+
|direct page indirect long
|[[Direct Page Indirect Long]]
|C7
|C7
|2 bytes
|2 bytes
|6 cycles
|6 cycles*
|+
|+
|absolute indexed by X
|absolute indexed by X
|DD
|DD
|3 bytes
|3 bytes
|4 cycles
|4 cycles*
|+
|+
|absolute long indexed by X
|absolute long indexed by X
|DF
|DF
|4 bytes
|4 bytes
|5 cycles
|5 cycles*
|+
|+
|absolute indexed by Y
|absolute indexed by Y
|D9
|D9
|3 bytes
|3 bytes
|4 cycles
|4 cycles*
|+
|+
|direct page indexed by X
|direct page indexed by X
|D5
|D5
|2 bytes
|2 bytes
|4 cycles
|4 cycles*
|+
|+
|direct page indexed indirect by X
|direct page indexed indirect by X
|C1
|C1
|2 bytes
|2 bytes
|6 cycles
|6 cycles*
|+
|+
|direct page indirect indexed by Y
|direct page indirect indexed by Y
|D1
|D1
|2 bytes
|2 bytes
|5 cycles
|5 cycles*
|+
|+
|direct page indirect long indexed by Y
|direct page indirect long indexed by Y
|D7
|D7
|2 bytes
|2 bytes
|6 cycles
|6 cycles*
|+
|+
|stack relative
|[[Stack Relative]]
|C3
|C3
|2 bytes
|2 bytes
|4 cycles
|4 cycles*
|+
|+
|stack relative indirect indexed by Y
|stack relative indirect indexed by Y
|D3
|D3
|2 bytes
|2 bytes
|7 cycles
|7 cycles*
|}
|}


{| class="wikitable" style="float:right;clear:right;width:30%"
{| class="wikitable" style="float:right;clear:right;width:30%"
!colspan="9"|Flags Clobbered
!colspan="9"|Flags Affected
|+
|+
|N
|[[Negative Flag|N]]
|V
|[[Overflow Flag|V]]
|M
|[[M Flag|M]]
|X
|[[X Flag|X]]
|D
|[[Decimal Flag|D]]
|I
|[[I Flag|I]]
|Z
|[[Zero Flag|Z]]
|C
|[[Carry Flag|C]]
|+
|+
|
|
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|
|}
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'''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.
===== 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 ===
=== See Also ===
* [[CPX]]
* [[CPX]]
* [[CPY]]
* [[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


[[Category:ASM]]
[[Category:ASM]]
[[Category:Group One Instructions]]
[[Category:Group One Instructions]]
[[Category:Inherited from 6502]]
[[Category:Inherited from 6502]]

Latest revision as of 06:15, 6 December 2023

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.

Cycle Penalties

See Also

External Links

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