fxch

Exchange Register

FXCH ST(i)

Exchanges contents of ST(0) and ST(i).

Details

Exchanges the contents of ST(0) and ST(i), swapping the top of the x87 FPU stack with another stack register. This is a register-only operation with no impact on EFLAGS. The instruction is part of the x87 FPU instruction set and does not cause pipeline serialization.

Pseudocode Operation

temp ← ST(0);
ST(0) ← ST(i);
ST(i) ← temp;

Example

FXCH st(1)

Encoding

Binary Layout
D9
+0
 
Format Legacy
Opcode D9 C8+i
Extension x87 FPU

Operands

  • dest
    x87 FPU stack register ST(i)

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID 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;

Exceptions

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. FXCH—Exchange Register Contents Vol. 2A 3-407

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

Same exceptions as in protected mode. FXCH—Exchange Register Contents Vol. 2A 3-408