Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 8-29
PROGRAMMING WITH THE X87 FPU
2. Check ordered comparison result. Use the constants given in Table 8-8 in the
TEST instruction to test for a less than, equal to, or greater than result, then use
the corresponding conditional branch instruction to transfer program control to
the appropriate procedure or section of code.
If a program or procedure has been thoroughly tested and it incorporates periodic
checks for QNaN results, then it is not necessary to check for the unordered result
every time a comparison is made.
See Section 8.1.4, “Branching and Conditional Moves on Condition Codes,” for
another technique for branching on x87 FPU condition codes.
Some non-comparison x87 FPU instructions update the condition code flags in the
x87 FPU status word. To ensure that the status word is not altered inadvertently,
store it immediately following a comparison operation.
8.3.7 Trigonometric Instructions
The following instructions perform four common trigonometric functions:
FSIN Sine
FCOS Cosine
FSINCOS Sine and cosine
FPTAN Tangent
FPATAN Arctangent
These instructions operate on the top one or two registers of the x87 FPU register
stack and they return their results to the stack. The source operands for the FSIN,
FCOS, FSINCOS, and FPTAN instructions must be given in radians; the source
operand for the FPATAN instruction is given in rectangular coordinate units.
The FSINCOS instruction returns both the sine and the cosine of a source operand
value. It operates faster than executing the FSIN and FCOS instructions in succession.
The FPATAN instruction computes the arctangent of ST(1) divided by ST(0),
returning a result in radians. It is useful for converting rectangular coordinates to
polar coordinates.
Table 8-8. TEST Instruction Constants for Conditional Branching
Order Constant Branch
ST(0) > Source Operand 4500H JZ
ST(0) < Source Operand 0100H JNZ
ST(0) = Source Operand 4000H JNZ
Unordered 0400H JNZ