A.5.110 FXCH: 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.