FXCH — Exchange Register Contents

Opcode	Instruction	64-Bit Mode	Compat/Leg Mode	Description
D9 C8+i	FXCH ST(i)	Valid	Valid	Exchange the contents of ST(0) and ST(i).
D9 C9	FXCH	Valid	Valid	Exchange the contents of ST(0) and ST(1).

Description ¶

Exchanges the contents of registers ST(0) and ST(i). If no source operand is specified, the contents of ST(0) and ST(1) are exchanged.

This instruction provides a simple means of moving values in the FPU register stack to the top of the stack [ST(0)], so that they can be operated on by those floating-point instructions that can only operate on values in ST(0). For example, the following instruction sequence takes the square root of the third register from the top of the register stack:

FXCH ST(3);

FSQRT;

FXCH ST(3);

This instruction’s operation is the same in non-64-bit modes and 64-bit mode.

Operation ¶

IF (Number-of-operands) is 1
    THEN
        temp ← ST(0);
        ST(0) ← SRC;
        SRC ← temp;
    ELSE
        temp ← ST(0);
        ST(0) ← ST(1);
        ST(1) ← temp;
FI;

FPU Flags Affected ¶

C1	Set to 0.
C0, C2, C3	Undefined.

Floating-Point Exceptions ¶

#IS	Stack underflow occurred.

Protected Mode Exceptions ¶

#NM	CR0.EM[bit 2] or CR0.TS[bit 3] = 1.
#MF	If there is a pending x87 FPU exception.
#UD	If the LOCK prefix is used.

Real-Address Mode Exceptions ¶

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions ¶

Same exceptions as in protected mode.

Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

64-Bit Mode Exceptions ¶

Same exceptions as in protected mode.