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
	.	.	.	.	.		.

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.

Syntax

DEC
DEC A
DEA
DEC addr
DEC dp
DEC addr, X
DEC dp, X

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: 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