arm gcc assembly in C

arm gcc assembly in C

• General form of an inline assembler

  asm(code : output operand list : input operand list : clobber list);
  

• Rotating bits example

  asm("mov %[result], %[value], ror #1" : [result] "=r" (y) : [value] "r" (x));
  

• ex2) space, newlines and C comments can be used to increase readability

  asm("mov    %[result], %[value], ror #1"
  
      : [result]"=r" (y) /* Rotation result. */
      : [value]"r"   (x) /* Rotated value. */
      : /* No clobbers */
  );
  

• ex3) old form

  asm("mov %0, %1, ror #1" : "=r" (result) : "r" (value));
  

• ex4) you can write more than one assembler instruction in a single inline asm statement. To make it more readable, you can put each instruction on a separate line.

  asm volatile(
  "mov     r0, r0\n\t"
  "mov     r0, r0\n\t"
  "mov     r0, r0\n\t"
  "mov     r0, r0"
  );
  

• To prevent code opmization by compiler, you need to use ‘volatie’ keyword.

• Constraint characters may be prepended by a single constraint modifier. Constraints without a modifier specify read-only operands. Modifiers are:

  Modifier	Specifies
  =	Write-only operand, usually used for all output operands
  +	Read-write operand, must be listed as an output operand
  &	A register that should be used for output only

• clobber list. asm code에 의해서 변경되는 것을 표시(?)

     asm volatile("mrs r12, cpsr\n\t"
     "orr r12, r12, #0xC0\n\t"
     "msr cpsr_c, r12\n\t" :: : "r12", "cc", "memory");
     c *= b; /* This is safe. */
     asm volatile("mrs r12, cpsr\n"
     "bic r12, r12, #0xC0\n"
     "msr cpsr_c, r12" ::: "r12", "cc", "memory");
  

• ex5) mov.c

  #include <stdio.h>
  
  int asm_mov_ex(int x, int y);
  
  __asm__ (".global asm_mov_ex	\n\
  		asm_mov_ex:	\n\
  		mov r0, r1	\n\
  		mov pc, lr	\n\
  		");
  
  int main(void)
  {
  	int mov;
  	printf("\n+-------------------+\n");
  	printf("|ARM Instruction MOV|\n");
  	printf("\n+-------------------+\n");
  
  	mov = asm_mov_ex(3, 4);
  	printf("mov = %d\n\n", mov);
  
  	return 0;
  }
  

• ex6) lsl.c

  #include <stdio.h>
  
  int asm_lsl_ex(int x, int y)
  {
  	int a;
  
  __asm__ volatile( "lsl	%[param1], %[param2], %[param3]	\n\t"
  	:[param1] "=r" (a): [param2] "r" (x), [param3] "r"(y):
  	);
  
  	return a;
  
  }
  
  int main(void)
  {
  	int mov;
  	printf("\n+-------------------+\n");
  	printf("|ARM Instruction LSL|\n");
  	printf("\n+-------------------+\n");
  
  	mov = asm_lsl_ex(2, 3);
  	printf("mov = %d\n\n", mov);
  
  	return 0;
  }
  

• Reference
  • http://www.ethernut.de/en/documents/arm-inline-asm.html