如何在MSP430上的汇编器中写入外部变量

Question

I am currently working with the TI MSP430 and wrote the assembler code shown below. I want to write the value '1' to the variable var, but indirectly via var_ptr, which holds the address of var. After reading about addressing modes in the User-Guide I thought this should work by using & in front of the pointer variable.

///< For testing
.extern var;
.extern var_ptr;

///< A function for testing different commands
.global testfunc
  .type testfunc, @function
testfunc:
  mov #1, &var_ptr

  ret

Those two external variables are defined in another c file.

uint16_t    var = 0;
uint16_t*   var_ptr = 0;

I am printing the content of both values before and after the function call.

var_ptr = &var;
DEBUG_PRINT(("var: %u, var_ptr: %u\n", var, var_ptr));
testfunc();
DEBUG_PRINT(("var: %u, var_ptr: %u\n", var, var_ptr));

Results:

mov #1, &var_ptr

var: 0, var_ptr: 9630<\n>
var: 0, var_ptr: 1<\n>
-------------------------------------------
mov #1, var_ptr

var: 0, var_ptr: 9630<\n>
var: 0, var_ptr: 1<\n>

Independently of using &, the value '1' is always written directly to the variable var_ptr, but not to var. What is the correct way to write to the variable var using var_ptr?

EDIT: A great explanation about addressing modes can be found here .

Answer 1

In MSP430 assembly syntax, &ADDR and ADDR only differ in that the former specifies a PC-relative address while the latter specifies an absolute address. The difference is mostly relevant for position independent code. To implement what you want, you need to perform two moves:

mov var_ptr, r4 // load content of var_ptr into r4
mov #1, @r4     // write #1 to where r4 points