FCHS — Change Sign

Opcode	Instruction	64-Bit Mode	Compat/Leg Mode	Description
D9 E0	FCHS	Valid	Valid	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.

ST(0) SRC	ST(0) DEST
−∞	+∞
−F	+F
−0	+0
+0	−0
+F	−F
+∞	−∞
NaN	NaN

Table 3-20. FCHS Results

* Fmeansfinitefloating-pointvalue.

This instruction’s operation is the same in non-64-bit modes and 64-bit mode.

Operation ¶

SignBit(ST(0)) ← NOT (SignBit(ST(0)));

FPU Flags Affected ¶

C1	Set to 0.
C0, C2, C3	Undefined.

Floating-Point Exceptions ¶

#IS	Stack underflow occurred.

Protected Mode Exceptions ¶

#NM	CR0.EM[bit 2] or CR0.TS[bit 3] = 1.
#UD	If the LOCK prefix is used.

Real-Address Mode Exceptions ¶

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions ¶

Same exceptions as in protected mode.

Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

64-Bit Mode Exceptions ¶

Same exceptions as in protected mode.