User's Manual

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
I
nteger 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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