NEG

NEG dst replaces the operand with its two's-complement negation — exactly 0 - dst. It works on a register or memory operand.

mov eax, 5
neg eax           ; eax = -5  (0xFFFFFFFB)
neg eax           ; eax = 5   again

Flags

NEG sets the flags as if it had computed 0 - dst:

• CF = 0 only when the operand was 0, and 1 otherwise. (Because 0 - 0 produces no borrow, but 0 - anything does.) This makes neg a one-byte "is this value non-zero?" test that drops the answer into CF.
• ZF, SF, OF, AF, PF are set from the result in the usual way.
• neg of INT_MIN (0x80000000) yields itself and sets OF — the only value that has no positive counterpart.

Reverse-engineering notes

• The obvious source mapping is unary minus: -x.
• Absolute value is often compiled branchlessly as mov tmp, x; sar tmp, 31; xor x, tmp; sub x, tmp rather than with neg — but neg does appear in the simpler pattern test/js … neg. Recognise both.
• neg then sbb reg, reg (or adc) is an idiom that broadcasts "was it non-zero?" across a whole register (0 → 0, non-zero → all-ones), used to build branchless masks.
• Don't confuse NEG (arithmetic, 0 - x) with NOT (bitwise complement, ~x). They differ by one: -x == ~x + 1.

See also: Two's complement · AND / OR / NOT · ADC / SBB.