Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3, Manual

Page: 1467 / 1898

Volume 4: Base IA-32 Instruction Reference 4:165
FSCALE—Scale
Description
Multiplies the destination operand by 2 to the power of the source operand and stores
the result in the destination operand. This instruction provides rapid multiplication or
division by integral powers of 2. The destination operand is a real value that is located
in register ST(0). The source operand is the nearest integer value that is smaller than
the value in the ST(1) register (that is, the value in register ST(1) is truncate toward 0
to its nearest integer value to form the source operand). The actual scaling operation is
performed by adding the source operand (integer value) to the exponent of the value in
register ST(0). The following table shows the results obtained when scaling various
classes of numbers, assuming that neither overflow nor underflow occurs.
Notes:
Fmeans finite-real number.
Nmeans integer.
In most cases, only the exponent is changed and the mantissa (significand) remains
unchanged. However, when the value being scaled in ST(0) is a denormal value, the
mantissa is also changed and the result may turn out to be a normalized number.
Similarly, if overflow or underflow results from a scale operation, the resulting mantissa
will differ from the source’s mantissa.
The FSCALE instruction can also be used to reverse the action of the FXTRACT
instruction, as shown in the following example:
FXTRACT;
FSCALE;
FSTP ST(1);
In this example, the FXTRACT instruction extracts the significand and exponent from
the value in ST(0) and stores them in ST(0) and ST(1) respectively. The FSCALE then
scales the significand in ST(0) by the exponent in ST(1), recreating the original value
before the FXTRACT operation was performed. The FSTP ST(1) instruction returns the
recreated value to the FPU register where it originally resided.
Opcode Instruction Description
D9 FD FSCALE Scale ST(0) by ST(1).
ST(1)

N 0 +N
   
ST(0)

F

F

F

F

0

0

0

0
+0 +0 +0 +0
+F +F +F +F
+

+

+

+

NaN NaN NaN NaN