0

I am new to IAR and Embedded Programming. I was debugging the following C code, and found that R0 gets to hold the address of counter1 through ??main_0, while R1 gets to hold address of counter2 through [PC,#0x20]. This is completely understandable, but I cannot get why it was assigned to R0 to use LDR Rd, -label while R1 used LDR Rd, [PC+Offset] and what is the difference between the two approaches?

I only knew about literal pools after searching but It didn't answer my question. In addition, where did ??main_0 get defined in the first place?

IAR Disassembler

int counter1=1;
int counter2=1;
int main()
{
    int *ptr;
    int *ptr2;
    ptr=&counter1;
    ptr2=&counter2;
 
    ++(*ptr);
    ++(*ptr2);
 
    ++counter2;
    return 0;
}
CherryDT
  • 25,571
  • 5
  • 49
  • 74
miserylord
  • 300
  • 2
  • 15
  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Oct 30 '21 at 10:17

1 Answers1

0

??main_0 is not "defined" as such, it's just an auto-generated label for the address used here so that when reading the disassembly you don't have to remember that address 0x8c is that counter pointer. In fact it would make sense to have the other counter pointer as ??main_1 and I'm not sure why it shows the bare [PC, #0x20] instead. As you can see on page 144/145 of the IAR assembly reference, those two forms are just different interpretations of the same machine code. If the disassembler decides to assign a label to an address, it can show the label form, otherwise the offset form.

The machine code of the first instruction is 48 07, which means LDR.N R0, [PC, #0x1C]. The interpretation as ??main_0 (and the assignment of a label ??main_0 to address 0x8c in the first place) is just something the disassembler decided to do. You cannot know what the original assembly source (if it even exists and the compiler didn't directly compile to machine code) looked like and whether it used a label there or not.

CherryDT
  • 25,571
  • 5
  • 49
  • 74