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DEC: Difference between revisions
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If you need to subtract two or more from the [[accumulator]], consider [[SBC]] instead. | If you need to subtract two or more from the [[accumulator]], consider [[SBC]] instead. | ||
==== Cycle Penalties ==== | |||
* Except in [[accumulator addressing]], DEC takes two extra cycles when the accumulator is 16 bits wide. | * 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 | * In [[direct page addressing]] modes, DEC takes an extra cycle when the low byte of the [[direct page register]] is nonzero | ||
Revision as of 17:51, 23 August 2024
| 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 by X | DE | 3 bytes | 7 cycles* | ||||
| Direct Page Indexed by X | D6 | 2 bytes | 6 cycles* | ||||
| Flags Affected | ||||||||
|---|---|---|---|---|---|---|---|---|
| N | V | M | X | D | I | Z | C | |
| N | . | . | . | . | . | Z | . | |
DEC (Decrement) is a 65x instruction that decrements the value in the location specified by the operand by one. The size of the accumulator determines whether this is an 8 or 16 bit operation. An alternate mnemonic when the operand is the accumulator is "DEA."
DEC ignores the decimal mode flag and the carry flag.
Syntax
DEC DEC A DEA DEC addr DEC dp DEC addr, X DEC dp, X
If you need to subtract two or more from the accumulator, consider SBC instead.
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
- Labiak, page 138 on DEC
- 10.8 on MCS6500 Manual, page 155 on DEC
- Carr, page 257 on DEC
- Leventhal, page 3-60 on DEC
- 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
- Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#DEC
- Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.1.3