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3-310 Vol. 2A FCOMI/FCOMIP/ FUCOMI/FUCOMIP—Compare Floating Point Values and Set EFLAGS
INSTRUCTION SET REFERENCE, A-M
If the operation results in an invalid-arithmetic-operand exception being raised, the
status flags in the EFLAGS register are set only if the exception is masked.
The FCOMI/FCOMIP and FUCOMI/FUCOMIP instructions clear the OF flag in the
EFLAGS register (regardless of whether an invalid-operation exception is detected).
The FCOMIP and FUCOMIP instructions also pop the register stack following the
comparison operation. To pop the register stack, the processor marks the ST(0)
register as empty and increments the stack pointer (TOP) by 1.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
IA-32 Architecture Compatibility
The FCOMI/FCOMIP/FUCOMI/FUCOMIP instructions were introduced to the IA-32
Architecture in the P6 family processors and are not available in earlier IA-32 proces-
sors.
Operation
CASE (relation of operands) OF
ST(0)
> ST(i): ZF, PF, CF ← 000;
ST(0)
< ST(i): ZF, PF, CF ← 001;
ST(0)
= ST(i): ZF, PF, CF ← 100;
ESAC;
IF Instruction is FCOMI or FCOMIP
THEN
IF ST(0) or ST(i)
= NaN or unsupported format
THEN
#IA
IF FPUControlWord.IM
= 1
THEN
ZF, PF, CF ← 111;
FI;
FI;
FI;
IF Instruction is FUCOMI or FUCOMIP
THEN
IF ST(0) or ST(i)
= QNaN, but not SNaN or unsupported format
THEN
ZF, PF, CF ← 111;
ELSE (* ST(0) or ST(i) is SNaN or unsupported format *)
#IA;
IF FPUControlWord.IM
= 1
THEN