FXAM—Examine ModR/M

Opcode	Instruction	64-Bit Mode	Compat/Leg Mode	Description
D9 E5	FXAM	Valid	Valid	Classify value or number in ST(0).

Description

Examines the contents of the ST(0) register and sets the condition code flags C0, C2, and C3 in the FPU status word to indicate the class of value or number in the register (see the table below).

Table 3-52. FXAM Results

Class	C3	C2	C0
Unsupported	0	0	0
NaN	0	0	1
Normal finite number	0	1	0
Infinity	0	1	1
Zero	1	0	0
Empty	1	0	1
Denormal number	1	1	0

The C1 flag is set to the sign of the value in ST(0), regardless of whether the register is empty or full.

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

Operation

C1 ← sign bit of ST; (* 0 for positive, 1 for negative *)
CASE (class of value or number in ST(0)) OF
    Unsupported:C3, C2, C0 ← 000;
    NaN:
                    C3, C2, C0 ← 001;
    Normal:
                    C3, C2, C0 ← 010;
    Infinity:
                    C3, C2, C0 ← 011;
    Zero:
                    C3, C2, C0 ← 100;
    Empty:
                    C3, C2, C0 ← 101;
    Denormal:
                    C3, C2, C0 ← 110;
ESAC;

FPU Flags Affected

C1	Sign of value in ST(0).
C0, C2, C3	See Table 3-52.

Floating-Point Exceptions

None.

Protected Mode Exceptions

#NM	CR0.EM[bit 2] or CR0.TS[bit 3] = 1.
#MF	If there is a pending x87 FPU exception.
#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.