Macro svc1 

macro_rules! svc1 {
    ($num:expr, $arg0:expr) => { ... };
}

Generate an SVC call with 1 parameters

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

const SYSCALL_FOO: u32 = 0x100;
let result = svc1!(0x00, SYSCALL_FOO);