The compiler enables you to access registers of an ARM architecture-based processor using named register variables.

Named register variables are declared by combining the register keyword with the __asm keyword. The __asm keyword takes one parameter, a character string, that names the register. For example, the declaration:

register int R0 __asm("r0");

declares R0 as a named register variable for the register r0. See Named register variables in the Compiler Reference Guide for more information on the registers of ARM architecture-based processors that can be accessed using named register variables.

You can declare named register variables as global variables. You can declare some, but not all, named register variables as local variables. In general, do not declare Vector Floating-Point (VFP) registers and core registers as local variables. Do not declare caller-save registers, such as R0, as local variables. (Caller-save registers are registers that the caller must save the values of, if it wants the values after the subroutine completes.) Your program might still compile if you declare these locally, but you risk unexpected runtime behavior if you do this.

A typical use of named register variables is to access bits in the Application Program Status Register (APSR) (see The Application Program Status Register (APSR) in the Assembler Guide). Example 3-3 shows the use of named register variables to set the saturation flag Q in the APSR.

#ifndef __BIG_ENDIAN // bitfield layout of APSR is sensitive to endianness
typedef union
{
    struct
    {
        int mode:5;
        int T:1;
        int F:1;
        int I:1;
        int _dnm:19;
        int Q:1;
        int V:1;
        int C:1;
        int Z:1;
        int N:1;
    } b;
    unsigned int word;
} PSR;
#else /* __BIG_ENDIAN */
typedef union
{
    struct 
    {
        int N:1;
        int Z:1;
        int C:1;
        int V:1;
        int Q:1;
        int _dnm:19;
        int I:1;
        int F:1;
        int T:1;
        int mode:5;
    } b;
    unsigned int word;
} PSR;
#endif /* __BIG_ENDIAN */
register PSR apsr __asm("apsr");
void set_Q(void)
{
    apsr.b.Q = 1;
}