FXCH
Usage: FXCH src Modifies flags: See below.
Exchanges the contents of registers ST(0) and ST(i). If no 'src' operand is specified, the contents of ST(0) and ST(1) are exchanged.
Floating-Point Exchange
FXCH ; D9 C9 [8086,FPU]
FXCH fpureg ; D9 C8+r [8086,FPU]
FXCH fpureg,ST0 ; D9 C8+r [8086,FPU]
FXCH ST0,fpureg ; D9 C8+r [8086,FPU]
FXCH exchanges ST0 with a given FPU register.
The no-operand form exchanges ST0 with ST1.
FPU Flags Affected C1 Set to 0 if stack underflow occurred; otherwise, set to 0.
C0, C2, C3 Undefined.
EXAMPLE:
fxch st6