ARM System Calls

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System Calls

swi 42

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:

subs pc, lr, #4

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 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 have probably noticed from the first example "swi 42", that the "swi" instruction takes an integer as an argument. To get this argument from the C code, you do:

int int_vector = 0;
asm volatile ("ldr %0, [lr, #-8]" : "=r" (int_vector));
int_vector &= 0xFFFFFF;

We only need the first 24-bits, because the last 8-bits are those of the "swi" instruction, 0xEF.