fcom

Compare Real

FCOM m32fp/m64fp

Compares ST(0) with source.

Pseudocode Operation

src_value ← [memory];
if (ST(0) > src_value) { C0←0; C2←0; C3←0; }
else if (ST(0) < src_value) { C0←1; C2←0; C3←0; }
else if (ST(0) == src_value) { C0←0; C2←0; C3←1; }
else { C0←1; C2←1; C3←1; }

Encoding

Binary Layout

ModRM

Format Legacy

Opcode D8 /2

Extension x87 FPU

Operands

src
Memory/Reg

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	64/32-bit 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.

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. Table 3-23. 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

Operation

CASE (relation of operands) OF
ST > SRC:         C3, C2, C0 := 000;
ST < SRC:         C3, C2, C0 := 001;
ST = SRC:         C3, C2, C0 := 100;
ESAC;
IF ST(0) or SRC = NaN or unsupported format
THEN
#IA
IF FPUControlWord.IM = 1
THEN
C3, C2, C0 := 111;
FI;
FI;
IF Instruction = FCOMP
THEN
PopRegisterStack;
FI;
IF Instruction = FCOMPP
THEN
PopRegisterStack;
PopRegisterStack;
FI;

Exceptions

Protected Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector. #SS(0) If a memory operand effective address is outside the SS segment limit. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS If a memory operand effective address is outside the SS segment limit. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #UD If the LOCK prefix is used. FCOM/FCOMP/FCOMPP-Compare Floating-Point Values Vol. 2A 3-325

Virtual-8086 Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS(0) If a memory operand effective address is outside the SS segment limit. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made. #UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form. #GP(0) If the memory address is in a non-canonical form. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #MF If there is a pending x87 FPU exception. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used. FCOM/FCOMP/FCOMPP-Compare Floating-Point Values Vol. 2A 3-326