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80386 Programmer's Reference Manual -- Opcode DEC

DEC -- Decrement by 1

Opcode	Instruction	Clocks	Description	Example
FE /1	decb r/m8	2/6	Decrement r/m byte by 1	`decb %dl decb (%ebx,1) decb m8(%ebx,1) decb m8(%ebx,%ebp,1)`
FF /1	decw r/m16	2/6	Decrement r/m word by 1 Decrement r/m dword by 1	`decw %cx decw (%ebx,1) decw (%ebx,2) decw (%ebx,%ebp,1)`
48+rw	decw r16	2	Decrement word register by 1	`decw %bx`
48+rw	decl r32	2	Decrement dword register by 1	`decl %ebx`

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

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