Intel 253666-024US - Manual (Page 459)

Vol. 2A 3-413
INSTRUCTION SET REFERENCE, A-M
FXCH—Exchange Register Contents
FXCH—Exchange Register Contents
Description
Exchanges the contents of registers ST(0) and ST(i). If no source operand is speci-
fied, 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 if stack underflow occurred; otherwise, set to 1.
C0, C2, C3 Undefined.
Floating-Point Exceptions
#IS Stack underflow occurred.
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).