Fcomi/fcomip – Intel 253666-024US User Manual

Page 355

Vol. 2A 3-309

INSTRUCTION SET REFERENCE, A-M

FCOMI/FCOMIP/ FUCOMI/FUCOMIP—Compare Floating Point Values and Set EFLAGS

FCOMI/FCOMIP/

FUCOMI/FUCOMIP—Compare Floating Point Values and

Set EFLAGS

Description

Performs an unordered comparison of the contents of registers ST(0) and ST(i) and
sets the status flags ZF, PF, and CF in the EFLAGS register according to the results
(see the table below). The sign of zero is ignored for comparisons, 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 FUCOMI/FUCOMIP instructions
perform the same operations as the FCOMI/FCOMIP instructions. The only difference
is that the FUCOMI/FUCOMIP instructions raise the invalid-arithmetic-operand
exception (#IA) only when either or both operands are an SNaN or are in an unsup-
ported format; QNaNs cause the condition code flags to be set to unordered, but do
not cause an exception to be generated. The FCOMI/FCOMIP 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.

Opcode

Instruction

64-Bit

Mode

Compat/

Leg Mode

Description

DB F0+i

FCOMI ST, ST(i)

Valid

Compare ST(0) with ST(i) and set status

flags accordingly.

DF F0+i

FCOMIP ST, ST(i)

Valid

Compare ST(0) with ST(i), set status flags

accordingly, and pop register stack.

DB E8+i

FUCOMI ST, ST(i)

Valid

Compare ST(0) with ST(i), check for

ordered values, and set status flags

accordingly.

DF E8+i

FUCOMIP ST, ST(i) Valid

Valid

Compare ST(0) with ST(i), check for

ordered values, set status flags

accordingly, and pop register stack.

Table 3-27. FCOMI/FCOMIP/ FUCOMI/FUCOMIP Results

Comparison Results*

ST0 > ST(i)

ST0 < ST(i)

ST0 = ST(i)

Unordered**

NOTES:

* See the IA-32 Architecture Compatibility section below.
** Flags not set if unmasked invalid-arithmetic-operand (#IA) exception is generated.