x86doc › FABS - Absolute Value

FABS—Absolute Value

Opcode	Instruction	64-Bit Mode	Compat/Leg Mode	Description
D9 E1	FABS	Valid	Valid	Replace ST with its absolute value.

Description

Clears the sign bit of ST(0) to create the absolute value of the operand. The following table shows the results obtained when creating the absolute value of various classes of numbers.

Table 3-17. Results Obtained from FABS

ST(0) SRC

ST(0) DEST

− ∞	+ ∞
− F	+ F
− 0	+ 0
+ 0	+ 0
+ F	+ F
+ ∞	+ ∞
NaN	NaN

NOTES:

F Means finite floating-point value.

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

Operation

ST(0) := |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.