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Intel Pentium CPU Instruction Set Reference

DEC instruction - Decrement by 1

Code Mnemonic Description
FE /1 DEC r/m8 Decrement r/m8 by 1
FF /1 DEC r/m16 Decrement r/m16 by 1
FF /1 DEC r/m32 Decrement r/m32 by 1
48 + rw DEC r16 Decrement r16 by 1
48 + rd DEC r32 Decrement r32 by 1


Subtracts 1 from the destination operand, while preserving the state of the CF flag. The destination operand can be a register or a memory location. This instruction allows a loop counter to be updated without disturbing the CF flag. (To perform a decrement operation that updates the CF flag, use a SUB instruction with an immediate operand of 1.)

Operands Bytes Clocks
r8 2 1 UV
r16 1 1 UV
r32 1 1 UV
mem 2 + d(0, 2) 3 UV


ID unaffected DF unaffected
VIP unaffected IF unaffected
VIF unaffected TF unaffected
AC unaffected SF sets according to the result
VM unaffected ZF sets according to the result
RF unaffected AF sets according to the result
NT unaffected PF sets according to the result
IOPL unaffected CF unaffected
OF sets according to the result