DEC

This instruction subtracts one from the destination operand and stores the result in the same operand.

The flags are modified as follows:

• If the subtraction produces a result that cannot fit in the destination operand, then CF=1. Otherwise, CF=0.
• If the result is zero, then ZF=1. Otherwise, ZF=0.
• If the most significant bit of the result is 1, then SF=1. Otherwise, SF=0.
• If the operand is negative and the result is positive, then OF=1. Otherwise, OF=0.

Usage

DEC dest

dest can be a register or a memory address (see operand types).

Encoding

• Register
  0100011w, 00000rrr
• Memory (direct)
  0100011w, 11000000, addr-low, addr-high
• Memory (indirect)
  0100011w, 11010000
• Memory (indirect with offset)
  0100011w, 11100000, disp-low, disp-high

Where w is the operand size bit. w=0 indicates 8-bit operands, and w=1 indicates 16-bit operands.

rrr encodes a register according to the following table:

rrr	w=0	w=1
000	AL	AX
001	CL	CX
010	DL	DX
011	BL	BX
100	AH	SP
101	CH	--
110	DH	--
111	BH	--