FCOS — Cosine

Opcode Instruction 64-Bit Mode Compat/Leg Mode Description
D9 FF FCOS Valid Valid Replace ST(0) with its approximate cosine.


Computes the approximate cosine of the source operand in register ST(0) and stores the result in ST(0). The source operand must be given in radians and must be within the range −263 to +263. The following table shows the results obtained when taking the cosine of various classes of numbers.

−∞ *
−F −1 to +1
−0 +1
+0 +1
+F − 1 to + 1
+∞ *
Table 3-23. FCOS Results

F Meansfinitefloating-pointvalue.

* Indicatesfloating-pointinvalid-arithmetic-operand(#IA)exception.

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. However, even within the range -263 to +263, inaccurate results can occur because the finite approximation of π used internally for argument reduction is not sufficient in all cases. Therefore, for accurate results it is safe to apply FCOS only to arguments reduced accurately in software, to a value smaller in absolute value than 3π/8. See the sections titled “Approximation of Pi” and “Transcendental Instruction Accuracy” 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.

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


IF |ST(0)| < 263
    C2 ← 0;
    ST(0) ← FCOS(ST(0)); // approximation of cosine
ELSE (* Source operand is out-of-range *)
    C2 ← 1;

FPU Flags Affected

C1 Set to 0 if stack underflow occurred.
Set if result was rounded up; cleared otherwise.
Undefined if C2 is 1.
Set to 1 if outside range (−263 < source operand < +263); otherwise, set to 0.
C0, 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.
#P Value cannot be represented exactly in destination format.

Protected Mode Exceptions

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

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

Same exceptions as in protected mode.