Datasheet

Floating-point Support
ARM DUI 0067D Copyright © 1999-2001 ARM Limited. All rights reserved. 5-3
5.2 The software floating-point library, fplib
When programs are compiled to use a floating-point coprocessor, they perform basic
floating-point arithmetic (for example addition and multiplication) by means of
floating-point machine instructions for the target coprocessor. When programs are
compiled to use software floating-point, there is no floating-point instruction set
available, and so the ARM libraries have to provide a set of procedure calls to do
floating-point arithmetic. These are the software floating-point library, fplib.
These routines have names like
_dadd (add two double s) and _fdiv (divide two float s).
The complete list is given in:
• Table 5-1 on page 5-4
• Table 5-2 on page 5-5
• Table 5-3 on page 5-6
• Table 5-4 on page 5-7.
User programs can call these routines directly. Even in environments with a
coprocessor, the routines are provided, though they are typically only a few instructions
long (as all they do is to execute the appropriate coprocessor instruction).
All the fplib routines are called using a software floating-point variant of the calling
standard. This means that floating-point arguments are passed and returned in integer
registers. In the rest of the program, if the program is compiled for a coprocessor,
floating-point data is passed in its floating-point registers.
So, for example,
_dadd takes a double in registers r0 and r1, and another double in
registers r2 and r3, and returns the sum in r0 and r1.
Note
For a double in registers r0 and r1, the register that holds the high 32 bits of the double
depends on whether your program is little-endian or big-endian.
C programs are not required to handle the register allocation.
All the fplib routines are declared in the header file
rt_fp.h . You can include this file if
you want to call an fplib routine directly.
A complete list of the fplib routines is provided on the following pages.