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3-406 Vol. 2A FTST—TEST
INSTRUCTION SET REFERENCE, A-M
FTST—TEST
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).
This instruction performs an “unordered comparison.” An unordered comparison also
checks the class of the numbers being compared (see “FXAM—ExamineModR/M” 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 gener-
ated.
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 if stack underflow occurred; otherwise, set to 0.
C0, C2, C3 See Table 3-45.
Floating-Point Exceptions
#IS Stack underflow occurred.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
D9 E4 FTST Valid Valid Compare ST(0) with 0.0.
Table 3-45. FTST Results
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