Filter
Top Products
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 Valid Compare ST(0) with ST(i) and set status
flags accordingly.
DF F0+i FCOMIP ST, ST(i) Valid Valid Compare ST(0) with ST(i), set status flags
accordingly, and pop register stack.
DB E8+i FUCOMI ST, ST(i) Valid 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* ZF PF CF
ST0 > ST(i) 0 0 0
ST0 < ST(i) 0 0 1
ST0 = ST(i) 1 0 0
Unordered** 1 1 1
NOTES:
* See the IA-32 Architecture Compatibility section below.
** Flags not set if unmasked invalid-arithmetic-operand (#IA) exception is generated.