Intel 253666-024US User Manual

Page 454

3-408 Vol. 2A

FUCOM/FUCOMP/FUCOMPP—Unordered Compare Floating Point Values

INSTRUCTION SET REFERENCE, A-M

FUCOM/FUCOMP/FUCOMPP—Unordered Compare Floating Point Values

Description

Performs an unordered comparison of the contents of register ST(0) and ST(i) and
sets condition code flags C0, C2, and C3 in the FPU status word according to the
results (see the table below). If no operand is specified, the contents of registers
ST(0) and ST(1) are compared. The sign of zero is ignored, so that –0.0 is equal to
+0.0.

An unordered comparison checks the class of the numbers being compared (see
“FXAM—ExamineModR/M” in this chapter). The FUCOM/FUCOMP/FUCOMPP instruc-
tions perform the same operations as the FCOM/FCOMP/FCOMPP instructions. The
only difference is that the FUCOM/FUCOMP/FUCOMPP instructions raise the invalid-
arithmetic-operand exception (#IA) only when either or both operands are an SNaN
or are in an unsupported format; QNaNs cause the condition code flags to be set to
unordered, but do not cause an exception to be generated. The
FCOM/FCOMP/FCOMPP instructions raise an invalid-operation exception when either
or both of the operands are a NaN value of any kind or are in an unsupported format.
As with the FCOM/FCOMP/FCOMPP instructions, if the operation results in an invalid-
arithmetic-operand exception being raised, the condition code flags are set only if the
exception is masked.

Opcode

Instruction

64-Bit

Mode

Compat/

Leg Mode

Description

DD E0+i

FUCOM ST(i)

Valid

Compare ST(0) with ST(i).

DD E1

FUCOM

Valid

Compare ST(0) with ST(1).

DD E8+i

FUCOMP ST(i)

Valid

Compare ST(0) with ST(i) and pop

DD E9

FUCOMP

Valid

Compare ST(0) with ST(1) and pop

DA E9

FUCOMPP

Valid

Compare ST(0) with ST(1) and pop

Table 3-46. FUCOM/FUCOMP/FUCOMPP Results

Comparison Results*

ST0 > ST(i)

ST0 < ST(i)

ST0 = ST(i)

Unordered

NOTES:

* Flags not set if unmasked invalid-arithmetic-operand (#IA) exception is generated.