Intel 80286, 80287 manual Programming Numeric Applications

	PROGRAMMING NUMERIC APPLICATIONS
	Table 2-1. Data Transfer Instructions
	Real Transfers
FLD	Load real
FST	Store real
FSTP	Store real and pop
FXCH	Exchange registers
	Integer Transfers
FILD	Integer load
FIST	Integer store
FISTP	Integer store and pop
	Packed Decimal Transfers
FBLD	Packed decimal (BCD) load
FBSTP	Packed decimal (BCD) store and pop

If, however, the stack top is tagged special (it contains 00, a NaN, or a denormal) then the stack top's significand is not rounded but is chopped (on the right) to fit the destination. Neither is the exponent converted, but it also is chopped on the right and transferred "as is." This preserves the value's identi- fication as co or a NaN (exponent all ones) or a denormal (exponent all zeros) so that it can be properly loaded and tagged later in the program if desired.

FSTP destination

FSTP (store real and pop) operates identically to FST except that the stack is popped following the transfer. This is done by tagging the top stack element empty and then incrementing ST. FSTP permits storing to a temporary real memory variable, whereas FST does not. Coding FSTP ST(O) is equivalent to popping the stack with no data transfer.

FXCH//des tina tion

FXCH (exchange registers) swaps the contents of the destination and the stack top registers. If the destination is not coded explicitly, ST(l) is used. Many 80287 instructions operate only on the stack top; FXCH provides a simple means of effectively using these instructions on lower stack elements. For example, the following sequence takes the square root of the third register from the top:

FXCH ST(3)

FSGRT

FXCH ST(3)

FILD source

FILD (integer load) converts the source memory operand from its binary integer format (word, short, or long) to temporary real and loads (pushes) the result onto the stack. The (new) stack top is tagged zero if all bits in the source were zero, and is tagged valid otherwise.

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