Intel 253666-024US - Manual (Page 411)

Vol. 2A 3-365
INSTRUCTION SET REFERENCE, A-M
FPTAN—Partial Tangent
FPTAN—Partial Tangent
Description
Computes the tangent of the source operand in register ST(0), stores the result in
ST(0), and pushes a 1.0 onto the FPU register stack. The source operand must be
given in radians and must be less than ±2
63 . The following table shows the
unmasked results obtained when computing the partial tangent of various classes of
numbers, assuming that underflow does not occur.
If the source operand is outside the acceptable range, the C2 flag in the FPU status
word is set, and the value in register ST(0) remains unchanged. The instruction does
not raise an exception when the source operand is out of range. It is up to the
program to check the C2 flag for out-of-range conditions. Source values outside the
range −2
63 to +2 63 can be reduced to the range of the instruction by subtracting an
appropriate integer multiple of 2π or by using the FPREM instruction with a divisor of
2π. See the section titled “Pi” in Chapter 8 of the Intel® 64 and IA-32 Architectures
Software Developer’s Manual, Volume 1, for a discussion of the proper value to use
for π in performing such reductions.
The value 1.0 is pushed onto the register stack after the tangent has been computed
to maintain compatibility with the Intel 8087 and Intel287 math coprocessors. This
operation also simplifies the calculation of other trigonometric functions. For
instance, the cotangent (which is the reciprocal of the tangent) can be computed by
executing a FDIVR instruction after the FPTAN instruction.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
D9 F2 FPTAN Valid Valid Replace ST(0) with its tangent and
push 1 onto the FPU stack.
Table 3-38. FPTAN Results
ST(0) SRC ST(0) DEST
− ∞ *
−F − F to +F
−0 − 0
+0 +0
+F − F to +F
+ ∞ *
NaN NaN
NOTES:
F Means finite floating-point value.
* Indicates floating-point invalid-arithmetic-operand (#IA) exception.