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3-286 Vol. 2A F2XM1—Compute 2x–1
INSTRUCTION SET REFERENCE, A-M
F2XM1—Compute 2
x
–1
Description
Computes the exponential value of 2 to the power of the source operand minus 1.
The source operand is located in register ST(0) and the result is also stored in ST(0).
The value of the source operand must lie in the range –1.0 to +1.0. If the source value
is outside this range, the result is undefined.
The following table shows the results obtained when computing the exponential
value of various classes of numbers, assuming that neither overflow nor underflow
occurs.
Values other than 2 can be exponentiated using the following formula:
x
y
← 2
(y
∗ log
2
x)
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Operation
ST(0) ← (2
ST(0)
− 1);
FPU Flags Affected
C1 Set to 0 if stack underflow occurred.
Set if result was rounded up; cleared otherwise.
C0, C2, C3 Undefined.
Floating-Point Exceptions
#IS Stack underflow occurred.
#IA Source operand is an SNaN value or unsupported format.
#D Source is a denormal value.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
D9 F0 F2XM1 Valid Valid Replace ST(0) with (2
ST(0)
– 1).
Table 3-21. Results Obtained from F2XM1
ST(0) SRC ST(0) DEST
−1.0 to −0 −0.5 to −0
−0 −0
+0 +0
+0 to +1.0 +0 to 1.0