ARM System Calls: Difference between revisions

<pre>swi 420x420000</pre>

This is how you call the system on ARM. The instruction `<span style="font-family:monospace">swi`</span> jumps to a predefined address, which in turn jumps to the system call handler. The system call handler executes the specific function and return to the user code with:

<pre>subsmov pc, lr, #4</pre>

This is the ARM equivalent to the IDT, on the x86, and it is stored at address 0. The only entry, we need to worry about is the SWI Handler. To install our own SWI handler, we replace the <span style="font-family:monospace">b ."</span> instruction with a branch to our handler:

You have probably noticed from the first example (<span style="font-family:monospace">swi 42"0x420000</span>), that the "swi" instruction takes an integer as an argument. For it to work in both ARM and THUMB modes of the processor, the integer must be left shifted by 16. To get this argumentinteger fromin the C code, you do this:

uint32_tuint8_t int_vector = 0;

asm volatile ("ldrldrb %0, [lr, #-82]" : "=r" (int_vector));