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DEC -- Decrement by 1

Opcode    Instruction        Clocks   Description

FE /1     DEC r/m8           2/6      Decrement r/m byte by 1
FF /1     DEC r/m16          2/6      Decrement r/m word by 1
          DEC r/m32          2/6      Decrement r/m dword by 1
48+rw     DEC r16            2        Decrement word register by 1
48+rw     DEC r32            2        Decrement dword register by 1

Operation

DEST := DEST - 1;

Description

DEC subtracts 1 from the operand. DEC does not change the carry flag. To affect the carry flag, use the SUB instruction with an immediate operand of 1.

Flags Affected

OF, SF, ZF, AF, and PF as described in Appendix C.

Protected Mode Exceptions

#GP(0) if the result is a nonwritable segment; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

up: Chapter 17 -- 80386 Instruction Set
prev: DAS Decimal Adjust AL after Subtraction
next: DIV Unsigned Divide