Computes the 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 -2^63 to +2^63. The following table shows the results obtained when taking the cosine of various classes of numbers.

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-ofrange 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 2 * pi or by using the FPREM instruction with a divisor of 2 * pi. See the section titled "Pi" in Chapter 8 of the IA-32 Intel Architecture Software Developer's Manual, Volume 1, for a discussion of the proper value to use for pi in performing such reductions.

if(GetAbsoluteValue(ST(0)) < (1 << 63)) {
	C2 = 0;
	ST(0) = Cosine(ST(0));
}
else C2 = 1; //source operand is out-of-range

C1 Set to 0 if stack underflow occurred. Set if result was rounded up; cleared otherwise. Undefined if C2 is 1. C2 Set to 1 if outside range (-2^63 < source operand < +2^63); otherwise, set to 0. C0, C3 Undefined.