ARM System Calls: Difference between revisions

<pre>mov pc, lr</pre>

Most of the time you won't need to worry about returning from the interrupt, as GCC, set up to cross compile for ARM, lets you code the interrupt handlers in C:

 

==Creating System Calls==

</pre>

We can code the interrupt handler like this:

void __attribute__ ((interrupt ("SWI"))) swi_handler (void) {}

Parameters to functions on ARM, are passed in registers r0-r3, if follow the same convention for system calls, then our interrupt handler can take parameters:

void __attribute__ ((interrupt ("SWI"))) swi_handler (int r0, int r1, int r2, int r3) {}

You've probably noticed that <span style="font-family:monospace">swi</span> takes an integer argument. To get this argument in C, we have to do this:

uint8_t int_vector = 0;

asm volatile ("ldrb %0, [lr, #-2]" : "=r" (int_vector));

This loads the high 8 bits (16-23) of the argument into <span style="font-family:monospace">int_vector</span>, as loading the full 24-bits won't work using Thumb.