Copy and paste into main.cpp:

#include "mbed.h"

extern "C" int asm_func(int number);

int number = 42;

int main() {
   printf("Before: %d \n\r", number);
   number = asm_func(number);
   printf("\n\rAfter: %d\n\r", number);
   while(1) {}
}


Copy and paste into asm_func.s:

    AREA asm_demo, CODE, READONLY
 ;  Export asm_func function location so that C compiler can find it and link
    EXPORT asm_func
asm_func
 ;  Value is in R0
    PUSH    {R2}                     ;Save incoming register value
    AND     R2, R2, #0               ;Set R2 to 0
    ADD     R2, R0, R0               ;Set R2 TO R0 + R0
    SUB     R2, R2, #1               ;Decrement R2
    MOV     R0, R2                   ;Put result back in R0
    POP     {R2}                     ;Restore incoming register
    BX      LR                       ;Return
    ALIGN
    END