FTST

TEST

Opcode		Mode	Leg Mode	Description
D9 E4				Compare ST(0) with 0.0.

Description

Compares the value in the ST(0) register with 0.0 and sets the condition code flags C0, C2, and C3 in the FPU status word according to the results (see table below).

Condition	C3	C2	C0
ST(0) > 0.0	0	0	0
ST(0) < 0.0	0	0	1
ST(0) = 0.0	1	0	0
Unordered	1	1	1

Table 3-40. FTST Results

This instruction performs an “unordered comparison.” An unordered comparison also checks the class of the numbers being compared (see “FXAM—Examine Floating-Point” in this chapter). If the value in register ST(0) is a NaN or is in an undefined format, the condition flags are set to “unordered” and the invalid operation exception is generated.

The sign of zero is ignored, so that (– 0.0 := +0.0).

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

Operation

CASE (relation of operands) OF
    Not comparable:
        C3, C2, C0 := 111;
    ST(0) > 0.0:
        C3, C2, C0 := 000;
    ST(0) < 0.0:
        C3, C2, C0 := 001;
    ST(0) = 0.0:
        C3, C2, C0 := 100;
ESAC;

FPU Flags Affected

C1	Set to 0.
C0, C2, C3	See Table 3-40.

Floating-Point Exceptions

#​IS	Stack underflow occurred.
#​IA	The source operand is a NaN value or is in an unsupported format.
#​D	The source operand is a denormal value.

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.

This UNOFFICIAL, mechanically-separated, non-verified reference is provided for convenience, but it may be incomplete or broken in various obvious or non-obvious ways. Refer to Intel® 64 and IA-32 Architectures Software Developer’s Manual for anything serious.