ARM System Calls

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System Calls

swi 0x420000

This is how you call the system on ARM. The instruction swi 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:

mov pc, lr

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:

void swi_handler () __attribute__((interrupt));

Creating System Calls

interrupt_vector_table:
    b . @ Reset Handler
    b . @ Undefined
    b . @ SWI Handler
    b . @ Prefetch Abort
    b . @ Data Abort
    b . @ IRQ
    b . @ FIQ

This is the ARM equivalent to the IDT, on the x86, and it is stored by default at address 0. The only entry, we need to worry about is the SWI Handler. To install our own SWI handler, we replace the b . instruction with a branch to our handler:

interrupt_vector_table:
    b . @ Reset Handler
    b . @ Undefined
    b swi_handler @ Our new SWI Handler
    b . @ Prefetch Abort
    b . @ Data Abort
    b . @ IRQ
    b . @ FIQ

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 swi 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 int_vector, as loading the full 24-bits won't work using Thumb.