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DEC: Difference between revisions
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|'''Speed''' | |'''Speed''' | ||
|+ | |+ | ||
| | |[[Accumulator Addressing|Accumulator]] | ||
|3A | |3A | ||
|1 byte | |1 byte | ||
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|6 cycles* | |6 cycles* | ||
|+ | |+ | ||
| | |[[Direct Page Addressing | Direct Page]] | ||
|C6 | |C6 | ||
|2 bytes | |2 bytes | ||
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!colspan="9"|Flags Affected | !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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|} | |} | ||
'''DEC''' (Decrement) is a 65x instruction that decrements the value in the location specified by the operand. | '''DEC''' (Decrement) is a 65x instruction that decrements the value in the location specified by the operand. The size of the [[accumulator]] determines whether this is an 8 or 16 bit operation. | ||
DEC ignores the [[decimal mode flag]]. | |||
===== Cycle Penalties ===== | ===== Cycle Penalties ===== | ||
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* [[DEX]] | * [[DEX]] | ||
* [[DEY]] | * [[DEY]] | ||
* [[DEC (Super FX)]] | |||
=== External Links === | === External Links === | ||
* [[Eyes & Lichty]] page on DEC: https://archive.org/details/0893037893ProgrammingThe65816/page/n477 | * [[Eyes & Lichty]] page 451 on DEC: https://archive.org/details/0893037893ProgrammingThe65816/page/n477 | ||
* [[Labiak]] page on DEC: https://archive.org/details/Programming_the_65816/page/n148 | * [[Labiak]] page 138 on DEC: https://archive.org/details/Programming_the_65816/page/n148 | ||
* [[MCS6500 Manual]] page on DEC: https://archive.org/details/mos_microcomputers_programming_manual/page/n176 | * [[MCS6500 Manual]] page 155 on DEC: https://archive.org/details/mos_microcomputers_programming_manual/page/n176 | ||
* [[Carr]] page on DEC: https://archive.org/details/6502UsersManual/page/n270 | * [[Carr]] page 257 on DEC: https://archive.org/details/6502UsersManual/page/n270 | ||
* [[Leventhal]] page on DEC: https://archive.org/details/6502-assembly-language-programming/page/n109 | * [[Leventhal]] page 3-60 on DEC: https://archive.org/details/6502-assembly-language-programming/page/n109 | ||
* snes9x implementation of DEC: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L482 | * snes9x implementation of DEC: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L482 | ||
* undisbeliever on DEC: https://undisbeliever.net/snesdev/65816-opcodes.html#dec-decrement | * undisbeliever on DEC: https://undisbeliever.net/snesdev/65816-opcodes.html#dec-decrement | ||
Latest revision as of 01:47, 19 December 2023
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Accumulator | 3A | 1 byte | 2 cycles | ||||
| Absolute | CE | 3 bytes | 6 cycles* | ||||
| Direct Page | C6 | 2 bytes | 5 cycles* | ||||
| absolute indexed X | DE | 3 bytes | 7 cycles* | ||||
| direct page indexed X | D6 | 2 bytes | 6 cycles* | ||||
| Flags Affected | ||||||||
|---|---|---|---|---|---|---|---|---|
| N | V | M | X | D | I | Z | C | |
| . | . | . | . | . | . | |||
DEC (Decrement) is a 65x instruction that decrements the value in the location specified by the operand. The size of the accumulator determines whether this is an 8 or 16 bit operation.
DEC ignores the decimal mode flag.
Cycle Penalties
- Except in accumulator addressing, DEC takes two extra cycles when the accumulator is 16 bits wide.
- In direct page addressing modes, DEC takes an extra cycle when the low byte of the direct page register is nonzero
See Also
External Links
- Eyes & Lichty page 451 on DEC: https://archive.org/details/0893037893ProgrammingThe65816/page/n477
- Labiak page 138 on DEC: https://archive.org/details/Programming_the_65816/page/n148
- MCS6500 Manual page 155 on DEC: https://archive.org/details/mos_microcomputers_programming_manual/page/n176
- Carr page 257 on DEC: https://archive.org/details/6502UsersManual/page/n270
- Leventhal page 3-60 on DEC: https://archive.org/details/6502-assembly-language-programming/page/n109
- snes9x implementation of DEC: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L482
- undisbeliever on DEC: https://undisbeliever.net/snesdev/65816-opcodes.html#dec-decrement