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DEC: Difference between revisions
From SnesLab
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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]] | ||
|+ | |+ | ||
| | | | ||
Revision as of 04:21, 27 November 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.
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 on DEC: https://archive.org/details/0893037893ProgrammingThe65816/page/n477
- Labiak page 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
- Carr page on DEC: https://archive.org/details/6502UsersManual/page/n270
- Leventhal page 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