FUCOM/FUCOMP/FUCOMPP—Unordered Compare Floating Point Values

INSTRUCTION SET REFERENCE, A-L

3-402 Vol. 2A

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—Examine Floating-Point” 
in this chapter). The FUCOM/FUCOMP/FUCOMPP instructions 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 unsup-
ported 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.
The FUCOMP instruction pops the register stack following the comparison operation and the FUCOMPP 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.
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

DD E0+i

FUCOM ST(i)

Valid

Valid

Compare ST(0) with ST(i).

DD E1

FUCOM

Valid

Valid

Compare ST(0) with ST(1).

DD E8+i

FUCOMP ST(i)

Valid

Valid

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

DD E9

FUCOMP

Valid

Valid

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

DA E9

FUCOMPP

Valid

Valid

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

Table 3-41.  FUCOM/FUCOMP/FUCOMPP Results

Comparison Results*

C3

C2

C0

ST0 

> ST(i)

0

0

0

ST0 

< ST(i)

0

0

1

ST0 

= ST(i)

1

0

0

Unordered

1

1

1

NOTES:

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