AMD Am186 Series, Instruction Set

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Instruction Set 4-163
NEG Two’s Complement Negation NEG
What It Does
NEG changes the sign of an integer.
Syntax
Description
NEG replaces the value of a register or memory operand with its two’s complement. The
operand is subtracted from zero and the result is placed in the operand.
NEG sets CF if the operand is not zero. If the operand is zero, it is not changed and NEG
clears CF.
Operation It Performs
Flag Settings After Instruction
Form Opcode Description
Clocks
Am186 Am188
NEG
r/m8
F6
/3
Perform a two’s complement negation of r/m byte 3/10 3/10
NEG r/m16 F7 /3 Perform a two’s complement negation of r/m word 3/10 3/14
NEG integer
if ( integer == 0)
/* clear carry flag */
CF = 0;
else
/* set carry flag
CF = 1;
/* change sign of integer */
integer = 0 - integer;
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Processor Status
Flags Register
reserved
OF DF IF TF SF ZF AF PF CF
? = undefined; – = unchanged
OF=1 if result larger than destination operand
OF=0 otherwise
AF=1 if carry or borrow to low nibble
AF=0 otherwise
PF=1 if low byte of result has even number of set bits
PF=0 otherwise
CF=1 if operand is not equal to 0
CF=0 if operand is equal to 0
– – – res res res
SF=1 if result is 0 or positive
SF=0 if result is negative
ZF=1 if result equal to 0
ZF=0 if result not equal to 0