ARM Compiler toolchain v4.1 for µVision Using the Compiler

Using the Inline and Embedded Assemblers of the ARM Compiler
7.32 Embedded assembler syntax in C and C++
An embedded assembly language function definition is marked by the __asm function qualifier
in C and C++, or the asm function qualifier in C++, and can be used on:
• member functions
• non-member functions
• template functions
• template class member functions.
Functions declared with __asm or asm can have arguments, and return a type. They are called
from C and C++ in the same way as normal C and C++ functions. The syntax of an embedded
assembly language function is:
__asm return-type function-name(parameter-list)
{
// ARM/Thumb/Thumb-2 assembler code
instruction{;comment is optional}
...
instruction
}
Note
Argument names are permitted in the parameter list, but they cannot be used in the body of the
embedded assembly function. For example, the following function uses integer i in the body of
the function, but this is not valid in assembly:
__asm int f(int i)
{
ADD i, i, #1 // error
}
You can use, for example, r0 instead of i.
Example 7-1 shows a string copy routine as a not very optimal embedded assembler routine.
Example 7-1 String copy with embedded assembler
#include
__asm void my_strcpy(const char *src, char *dst)
{
loop
LDRB r2, [r0], #1
STRB r2, [r1], #1
CMP r2, #0
BNE loop
BX lr
}
int main(void)
{
const char *a = "Hello world!";
char b[20];
my_strcpy (a, b);
printf("Original string: '%s'\n", a);
printf("Copied string: '%s'\n", b);
return 0;
}
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