Decrement

Decrement by one a register or a memory location

Tags:

6502 instruction

The decrement instructions add one to either a register or a memory location.

Unlike subtracting 1 with ADC, these instructions does not use the Carry flag in any way. You can test for wraparound only by testing after every decrement to see if the result is zero or negative.

The d flag does not affect these instructions. The decrement is always in binary mode.

For all processors, the decrement is an 8-bit operation unless m=0 on the 65816 in which case the decrement is 16-bit.

DEC - Decrement

Decrement by 1 the contents of the memory location or accumulator.

DEX - Decrement Index Register X

Decrement by 1 the X index register.

DEY - Decrement Index Register Y

Decrement by 1 the Y index register.

Flags Affected

Flags

n

-

-

-

-

-

z

-

n

Set if most significant bit of the result is set

z

Set if result is zero

Instructions

Syntax	Opcode	Available on:			# of	# of	Addressing Mode
Syntax	(hex)	6502	65C02	65816	bytes	cycles	Addressing Mode
DEC A	3A		x	x	1	2	Accumulator
DEC addr	CE	x	x	x	3	6¹	Absolute
DEC dp	C6	x	x	x	2	5^{1, 2}	Direct Page
DEC addr,X	DE	x	x	x	3	7^{1, 3}	Absolute Indexed X
DEC dp,X	D6	x	x	x	2	6^{1, 2}	Direct Page Indexed X
DEX	CA	x	x	x	1	2	Implied
DEY	88	x	x	x	1	2	Implied

Notes:

65816: Add 2 cycles if m=0 (16-bit memory/accumulator)
65816: Add 1 cycle if low byte of Direct Page register is not 0
65C02: Subtract 1 cycle if no page boundary is crossed

Last modified November 5, 2021: Add instruction categories (6b74ff9)