Macro svc3 

macro_rules! svc3 {
    ($num:expr, $arg0:expr, $arg1:expr, $arg2:expr) => { ... };
}

Generate an SVC call with 3 parameters

Puts the first argument in the instruction, and the parameters in r0-r2. Gives you back the value left in r0 by the handler.

const SYSCALL_FOO: u32 = 0x100;
let result = svc3!(0x00, SYSCALL_FOO, 1, 2);