FCOM/FCOMP/FCOMPP—Compare Floating Point Values

INSTRUCTION SET REFERENCE, A-L

3-324 Vol. 2A

FCOM/FCOMP/FCOMPP—Compare Floating Point Values

Description

Compares the contents of register ST(0) and source value and sets condition code flags C0, C2, and C3 in the FPU 
status word according to the results (see the table below). The source operand can be a data register or a memory 
location. If no source operand is given, the value in ST(0) is compared with the value in ST(1). The sign of zero is 
ignored, so that –0.0 is equal to +0.0. 

This instruction checks the class of the numbers being compared (see “FXAM—Examine Floating-Point” in this 
chapter). If either operand is a NaN or is in an unsupported format, an invalid-arithmetic-operand exception (#IA) 
is raised and, if the exception is masked, the condition flags are set to “unordered.” If the invalid-arithmetic-
operand exception is unmasked, the condition code flags are not set.
The FCOMP instruction pops the register stack following the comparison operation and the FCOMPP instruction pops 
the register stack twice 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.
The FCOM instructions perform the same operation as the FUCOM instructions. The only difference is how they 
handle QNaN operands. The FCOM instructions raise an invalid-arithmetic-operand exception (#IA) when either or 
both of the operands is a NaN value or is in an unsupported format. The FUCOM instructions perform the same 
operation as the FCOM instructions, except that they do not generate an invalid-arithmetic-operand exception for 
QNaNs.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.

Opcode

Instruction

64-Bit 

Mode

Compat/

Leg Mode

Description

D8 /2

FCOM m32fp

Valid

Valid

Compare ST(0) with m32fp.

DC /2

FCOM m64fp

Valid

Valid

Compare ST(0) with m64fp.

D8 D0+i

FCOM ST(i)

Valid

Valid

Compare ST(0) with ST(i).

D8 D1

FCOM

Valid

Valid

Compare ST(0) with ST(1).

D8 /3

FCOMP m32fp

Valid

Valid

Compare ST(0) with m32fp and pop register stack.

DC /3

FCOMP m64fp

Valid

Valid

Compare ST(0) with m64fp and pop register stack.

D8 D8+i

FCOMP ST(i)

Valid

Valid

Compare ST(0) with ST(i) and pop register stack.

D8 D9

FCOMP

Valid

Valid

Compare ST(0) with ST(1) and pop register stack.

DE D9

FCOMPP

Valid

Valid

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

twice.

Table 3-21.  FCOM/FCOMP/FCOMPP Results

Condition

C3

C2

C0

ST(0) > SRC

0

0

0

ST(0) < SRC

0

0

1

ST(0) = SRC

1

0

0

Unordered*

1

1

1

NOTES:

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