Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1436

FIST/FISTP—Store Integer
Opcode
DF /2
DB /2
DF /3
DB /3
DF /7
Description
The FIST instruction converts the value in the ST(0) register to a signed integer and
stores the result in the destination operand. Values can be stored in word- or
short-integer format. The destination operand specifies the address where the first byte
of the destination value is to be stored.
The FISTP instruction performs the same operation as the FIST instruction and then
pops the register stack. To pop the register stack, the processor marks the ST(0)
register as empty and increments the stack pointer (TOP) by 1. The FISTP instruction
can also stores values in long-integer format.
The following table shows the results obtained when storing various classes of numbers
in integer format.
ST(0)

 F <  1
 1 <  F <  0
 0
 0
+0 < +F < +1
+F > +1
+ 
NaN
Notes:
Fmeans finite-real number.
Imeans integer.
*indicates floating-point invalid-operation (#IA) exception.
**  0 or  1, depending on the rounding mode.
If the source value is a non-integral value, it is rounded to an integer value, according
to the rounding mode specified by the RC field of the FPU control word.
If the value being stored is too large for the destination format, is an , is a NaN, or is
in an unsupported format and if the invalid-arithmetic-operand exception (#IA) is
unmasked, an invalid-operation exception is generated and no value is stored in the
destination operand. If the invalid-operation exception is masked, the integer indefinite
value is stored in the destination operand.
4:134
Instruction Description
FIST m16int Store ST(0) in m16int
FIST m32int Store ST(0) in m32int
FISTP m16int Store ST(0) in m16int and pop register stack
FISTP m32int Store ST(0) in m32int and pop register stack
FISTP m64int Store ST(0) in m64int and pop register stack
DEST
*
 I
**
0
0
**
+I
*
*
Volume 4: Base IA-32 Instruction Reference