fabs

Absolute Value

FABS

Replaces ST(0) with its absolute value.

Details

Replaces the value in ST(0) with its absolute value by clearing the sign bit. The operation does not modify the x87 status flags and does not raise exceptions. This is a fast unary operation that operates solely on the register stack.

Pseudocode Operation

ST(0) ← |ST(0)|;

Example

FABS

Encoding

Binary Layout
D9
+0
E1
+1
 
Format Legacy
Opcode D9 E1
Extension x87 FPU

Operands

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID 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 3-19. Results Obtained from FABS ST(0) SRC ST(0) DEST − ∞ + ∞ − F + F − 0 + 0 + 0 + 0 + F + F + ∞ + ∞ NaN NaN

Operation

ST(0) := |ST(0)|;

Exceptions

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. FABS—Absolute Value Vol. 2A 3-309 FABS—Absolute Value Vol. 2A 3-310