x86 Instruction Set Reference

FCHS

Change Sign

Opcode	Mnemonic	Description
`D9 E0`	`FCHS`	Complements sign of ST(0)

Description

Complements the sign bit of ST(0). This operation changes a positive value into a negative value of equal magnitude or vice versa. The following table shows the results obtained when changing the sign of various classes of numbers.

FCHS Results
ST(0) Source	ST(0) Destination
-inf	+inf
-F	+F
-0	+0
+0	-0
+F	-F
+inf	-inf
NaN	NaN
NOTE: F Means finite floating-point value.

Operation
SignBit(ST(0)) = ~SignBit(ST(0));

FPU flags affected
C1 Set to 0 if stack underflow occurred; otherwise, set to 0. C0, C2, C3 Undefined.

Floating-Point Exceptions

#IS Stack underflow occurred.

Protected Mode Exceptions

#NM EM or TS in CR0 is set.

Real-Address Mode Exceptions

#NM EM or TS in CR0 is set.

Virtual-8086 Mode Exceptions

#NM EM or TS in CR0 is set.

Instruction	Latency	Throughput	Execution Unit
`CPUID`	0F3n/0F2n	0F3n/0F2n	0F2n
`FCHS`	3/2	1/1	FP_MISC