ARM: FUNCTION POINTER VALUES DIFFER IN THUMB MODE

Information in this article applies to:

• ARM Tools -- All versions

QUESTION

In the following ARM Thumb test program:

void (*ptr1)(void);

void main(void){
  while(1){
    ptr1=main;
  }
}

The linker map file shows main() function located at 0x15C, but ptr1 is set to 0x15D. What's wrong?  Why are the addresses different?

ANSWER

There is nothing wrong.  ARM mode instructions are located on 4-byte boundaries.  Thumb mode instructions are located on 2-byte boundaries.  In the ARM architecture, bit 0 of a function pointer indicates the mode (ARM or Thumb) of the called function, rather than a memory location.  When bit 0 = 1, Thumb mode is selected.

MORE INFORMATION

• Refer to Thumb Code Example in the CARM User's Guide.

Last Reviewed: Friday, October 21, 2005