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CMP: Difference between revisions
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'''CMP''' (Compare) is a 65x instruction that compares the value of the [[accumulator]]. In [[immediate addressing]] only, CMP is a total of 3 bytes long when the accumulator is 16 bits wide. | '''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 ===== | ===== Cycle Penalties ===== | ||
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* [[Eyes & Lichty]] page 445, on CMP: https://archive.org/details/0893037893ProgrammingThe65816/page/n471 | * [[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 | * [[Labiak]] page 134 on CMP: https://archive.org/details/Programming_the_65816/page/n144 | ||
* [[MCS6500 Manual]] page on CMP: https://archive.org/details/mos_microcomputers_programming_manual/page/n60 | * [[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 | * [[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 | * [[Leventhal]] page 3-56 on CMP: https://archive.org/details/6502-assembly-language-programming/page/n105 | ||
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
- 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
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