Partial Tangent


Hex Mnemonic Encoding Long Mode Legacy Mode Description
D9 F2 FPTAN None Valid Valid Replace ST(0) with its tangent and push 1 onto the FPU stack.


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 ±263. The following table shows the unmasked results obtained when computing the partial tangent of various classes of numbers, assuming that underflow does not occur.

FPTAN Results
-∞ *
- F - F to + F
- 0 -0
+ 0 + 0
+ F - F to + F
+∞ *

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 -263 to +263 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 theIntel®64 and IA-32 ArchitecturesSoftware 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.

This instruction's operation is the same in non-64-bit modes and 64-bit mode.

Pseudo Code

IF ST(0) < 263
	C2 = 0;
	ST(0) = tan(ST(0));
	TOP = TOP - 1;
	ST(0) = 1.0;
	(* Source operand is out-of-range *)
	C2 = 1;

FPU Flags Affected

C1: Set to 0 if stack underflow occurred; set to 1 if stack overflow occurred. Set if result was rounded up; cleared otherwise. C2 Set to 1 if outside range (-263 < source operand <+263); otherwise, set to 0. C0, C3 Undefined.


Floating-Point Exceptions

Exception Description
#P Value cannot be represented exactly in destination format.
#U Result is too small for destination format.
#D Source operand is a denormal value.
#IA Source operand is an SNaN value, ∞, or unsupported format.
#IS Stack underflow or overflow occurred.

64-Bit Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#MF If there is a pending x87 FPU exception.
#NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1.