NEG replaces a byte or word operand with its two's complement, computing 0 − operand. As signed numbers this flips the sign: 5 becomes −5 (FBh) and −5 becomes 5. A memory operand needs a size when it has none: NEG WORD PTR [BX].

The result is the same as inverting every bit and adding 1, that is NOT followed by INC, but NEG does it in one step and sets the flags.

Flags

The flags are those of the subtraction 0 − operand. SF, ZF and PF describe the result. CF = 1 for any non-zero operand and CF = 0 only when the operand was 0 (zero is the one value that needs no borrow). OF = 1 when the operand is the most negative value, 80h or 8000h: −128 has no positive byte counterpart, so NEG leaves it unchanged and reports overflow.

Printing a signed number

To print a value that may be negative, test its sign, print a minus and negate it, then print it as an unsigned number. The example prints −4.

MOV     AL, 3
SUB     AL, 7    ; AL = FCh = -4, SF = 1
JNS     POS1     ; not negative: print as is
NEG     AL       ; AL = 4
MOV     BL, AL
MOV     DL, '-'
MOV     AH, 02h
INT     21h
MOV     AL, BL
POS1:
ADD     AL, '0'  ; digit -> character
MOV     DL, AL
MOV     AH, 02h  ; DOS: print DL
INT     21h

Other uses

NEG turns a subtraction around: SUB AX, BX followed by NEG AX gives BX − AX. To negate a 32-bit value in DX:AX, negate both halves and fix the high word with the borrow from the low one:

NEG     DX
NEG     AX     ; CF = 1 unless AX was 0
SBB     DX, 0  ; DX:AX = -(DX:AX)