ST(0)
ST(1)
-
-F
±0
+0 < +F < +1
+1
+F > +1
+
NaN
-
*
*
+
+
*
-
-
NaN
-F
*
*
**
+F
-0
-F
-
NaN
-0
*
*
*
+0
-0
-0
*
NaN
+0
*
*
*
-0
+0
+0
*
NaN
+F
*
*
**
-F
+0
+F
+
NaN
+
*
*
-
-
*
+
+
NaN
NaN
NaN
NaN
NaN
NaN
NaN
NaN
NaN
NaN
Opcode |
Instruction |
Description |
|---|---|---|
|
D9 F1 |
FYL2X |
Replace ST(1) with (ST(1) * log2ST(0)) and pop the register stack |
Computes (ST(1) * log2 (ST(0))), stores the result in resister ST(1), and pops the FPU register stack. The source operand in ST(0) must be a non-zero positive number.
The following table shows the results obtained when taking the log of various classes of numbers, assuming that neither overflow nor underflow occurs.
|
|
ST(0) |
|
|
|
|
|
|
|
| ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
ST(1) |
|
- |
-F |
±0 |
+0 < +F < +1 |
+1 |
+F > +1 |
+ |
NaN |
|
|
|
|
|
|
|
|
|
|
- |
* |
* |
+ |
+ |
* |
- |
- |
NaN |
|
|
|
|
|
|
| |
|
|
-F |
* |
* |
** |
+F |
-0 |
-F |
- |
NaN |
|
|
|
|
|
|
| |
|
|
-0 |
* |
* |
* |
+0 |
-0 |
-0 |
* |
NaN |
|
|
|
|
|
|
| |
|
|
+0 |
* |
* |
* |
-0 |
+0 |
+0 |
* |
NaN |
|
|
|
|
|
|
| |
|
|
+F |
* |
* |
** |
-F |
+0 |
+F |
+ |
NaN |
|
|
|
|
|
|
| |
|
|
+ |
* |
* |
- |
- |
* |
+ |
+ |
NaN |
|
|
|
|
|
|
| |
|
|
NaN |
NaN |
NaN |
NaN |
NaN |
NaN |
NaN |
NaN |
NaN |
|
|
|
|
|
|
| |
Note F Means finite-real number.
* Indicates floating-point invalid-operation (#IA) exception.
** Indicates floating-point zero-divide (#Z) exception.
If the divide-by-zero exception is masked and register
ST(0) contains ±0, the instruction returns with a sign that is the opposite of the sign of the source operand
in register ST(1).
The FYL2X instruction is designed with a built-in multiplication to optimize the calculation of logarithms with an arbitrary positive base (b):
logbx
(log2b)-1
* log2x
ST(1) ST(1) * log2ST(0);
PopRegisterStack;
C1 - Set to 0 if stack underflow occurred. Indicates rounding direction if the inexact-result exception (#P) is generated:
0 not roundup;
1 roundup.
C0, C2, C3 - Undefined.
#IS - Stack underflow occurred.
#IA - Either operand is an SNaN or unsupported format. Source operand in register ST(0) is a negative finite value (not -0).
#Z - Source operand in register ST(0) is ±0.
#D - Source operand is a denormal value.
#U - Result is too small for destination format.
#O - Result is too large for destination format.
#P - Value cannot be represented exactly in destination format.
#NM - EM or TS in CR0 is set.
#NM - EM or TS in CR0 is set.
#NM - EM or TS in CR0 is set.
For details, see Volume 2A and Volume 2B of the Intel(R) 64 and IA-32 Intel Architecture Software Developer's Manual. For the latest updates on the instruction set information, go to the web site.