DEC: Difference between revisions

Latest revision as of 23:24, 27 September 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

Although the NMOS 6502 does have DEC, it does not work on the accumulator. When porting code to the 65c816, utilizing DEC more often instead of SBC can make code smaller and faster.

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

@@ Line 75: / Line 75: @@
 * 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
+Although the NMOS 6502 does have DEC, it does not work on the accumulator.  When porting code to the 65c816, utilizing DEC more often instead of SBC can make code smaller and faster.
 === See Also ===