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DEC: Difference between revisions
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{| class="wikitable" style="float:right;clear:right;width:40%" | |||
{| class="wikitable" style="float:right;clear:right;width: | |||
!colspan="8"|Basic Info | !colspan="8"|Basic Info | ||
|+ | |+ | ||
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|'''Speed''' | |'''Speed''' | ||
|+ | |+ | ||
| | |[[Accumulator Addressing|Accumulator]] | ||
|3A | |3A | ||
|1 byte | |1 byte | ||
|2 cycles | |2 cycles | ||
|+ | |+ | ||
| | |[[Absolute]] | ||
|CE | |CE | ||
|3 bytes | |3 bytes | ||
|6 cycles | |6 cycles* | ||
|+ | |+ | ||
| | |[[Direct Page Addressing | Direct Page]] | ||
|C6 | |C6 | ||
|2 bytes | |2 bytes | ||
|5 cycles | |5 cycles* | ||
|+ | |+ | ||
|absolute indexed X | |absolute indexed X | ||
|DE | |DE | ||
|3 bytes | |3 bytes | ||
|7 cycles | |7 cycles* | ||
|+ | |+ | ||
|direct page indexed X | |direct page indexed X | ||
|D6 | |D6 | ||
|2 bytes | |2 bytes | ||
|6 cycles | |6 cycles* | ||
|} | |} | ||
{| class="wikitable" style="float:right;clear:right;width:30%" | {| class="wikitable" style="float:right;clear:right;width:30%" | ||
!colspan="9"|Flags | !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. 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 === | === See Also === | ||
* [[INC]] | * [[INC]] | ||
* [[DEX]] | |||
* [[DEY]] | |||
* [[DEC (Super FX)]] | |||
=== 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 | |||
[[Category:ASM]] | [[Category:ASM]] | ||
[[Category:Group Two Instructions]] | [[Category:Group Two Instructions]] | ||
[[Category:Read-Modify-Write Instructions]] | |||
[[Category:Inherited from 6502]] |
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