DEC

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

• INC
• DEX
• DEY
• DEC (SPC700)
• DEC (Super FX)

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