bern_arch/cortex_m/
syscall.rs

1//! ARM Cortex-M implementation of [`ISyscall`].
2
3use crate::arch::Arch;
4use crate::syscall::ISyscall;
5use core::arch::{asm, global_asm, naked_asm};
6
7impl ISyscall for Arch {
8    #[inline(always)]
9    fn syscall(service: u8, arg0: usize, arg1: usize, arg2: usize) -> usize {
10        // we need to move the arguments to the correct registers, because the
11        // function is inlined
12        let ret;
13        unsafe {
14            asm!(
15                "push {{r4}}",
16                "svc 0",
17                "mov r0, r4",
18                "pop {{r4}}",
19                in("r0") service,
20                in("r1") arg0,
21                in("r2") arg1,
22                in("r3") arg2,
23                lateout("r0") ret,
24            )
25        }
26        ret
27    }
28}
29
30/// Extract and prepare system call for Rust handler.
31/// r0 is used to store the service id, r1-r3 can contain call parameter.
32///
33/// The system call service id (`svc xy`) is not passed on, we have to
34/// retrieve it from code memory. Thus we load the stack pointer from the
35/// callee and read the link register. The link register is pointing to
36/// instruction just after the system call, the system call service id is
37/// placed two bytes before that.
38///
39/// The exception link register tells SVC which privilege mode the callee used
40/// | EXC_RETURN (lr) | Privilege Mode     | Stack |
41/// |-----------------|--------------------|------ |
42/// | 0xFFFFFFF1      | Handler Mode       | MSP   |
43/// | 0xFFFFFFF9      | Thread Mode        | MSP   |
44/// | 0xFFFFFFFD      | Thread Mode        | PSP   |
45/// | 0xFFFFFFE1      | Handler Mode (FPU) | MSP   |
46/// | 0xFFFFFFE9      | Thread Mode (FPU)  | MSP   |
47/// | 0xFFFFFFED      | Thread Mode (FPU)  | PSP   |
48
49#[no_mangle]
50#[unsafe(naked)]
51unsafe extern "C" fn SVCall() {
52    naked_asm!(
53        "push {{lr}}",
54        "bl syscall_handler",
55        "mov r4, r0", // let's use r4 as return value, because r0 is popped from stack
56        "pop {{lr}}",
57        "bx lr",
58    );
59}
bern_arch/cortex_m/syscall.rs

bern_arch/cortex_m/
syscall.rs
