x86 Instruction Set Reference

# FCHS

# Change Sign

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

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) SourceST(0) Destination -inf+inf -F+F -0+0 +0-0 +F-F +inf-inf NaNNaN

NOTE: F Means finite floating-point value.

|-inf|+inf|-F|+F|-0|+0|+0|-0|+F|-F|+inf|-inf|NaN|NaN|NOTE: F Means finite floating-point value.| | |-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));
1
2

# 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

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