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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.  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.
'''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).
If the subtraction required a borrow, the carry flag is cleared (otherwise it is set).
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===== Cycle Penalties =====
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 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 [[direct page addressing]] modes, CMP takes an extra cycle when the low byte of the [[direct page register]] is nonzero
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=== External Links ===
=== External Links ===
* [[Eyes & Lichty]] page 445, on CMP: https://archive.org/details/0893037893ProgrammingThe65816/page/445
* [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/445 page 445] on CMP
* [[Labiak]] page 134 on CMP: https://archive.org/details/Programming_the_65816/page/n144
* [[Labiak]], [https://archive.org/details/Programming_the_65816/page/n144 page 134] on CMP
* [[MCS6500 Manual]] page 45 on CMP: https://archive.org/details/mos_microcomputers_programming_manual/page/n60
* 4.2.1 on [[MCS6500 Manual]], [https://archive.org/details/mos_microcomputers_programming_manual/page/n60 page 45] on CMP
* [[Carr]] page 255 on CMP: https://archive.org/details/6502UsersManual/page/n268
* [[Carr]], [https://archive.org/details/6502UsersManual/page/n268 page 255] on CMP
* [[Leventhal]] page 3-56 on CMP: https://archive.org/details/6502-assembly-language-programming/page/n105
* [[Leventhal]], [https://archive.org/details/6502-assembly-language-programming/page/n105 page 3-56] on CMP
* snes9x implementation of CMP: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L304
* 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
* undisbeliever on CMP: https://undisbeliever.net/snesdev/65816-opcodes.html#cmp-compare-accumulator-with-memory
* http://www.6502.org/tutorials/6502opcodes.html#CMP
* Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#CMP
* Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.1.2


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

Latest revision as of 17:52, 23 August 2024

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

See Also

External Links

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