Function syscall 

pub unsafe fn syscall(
    a0: Syscall,
    a1: usize,
    a2: usize,
    a3: usize,
    a4: usize,
    a5: usize,
    a6: usize,
    a7: usize,
) -> Result<(usize, usize, usize, usize, usize, usize, usize, usize), Error>

Perform a type-checked syscall and check the return value.

Safety: The safety of this function depends on the syscall.