Writing U-mode Apps

userapp.aero (Aero code)
    ↓ aero compiler --emit-obj --target riscv64-unknown-none
userapp.o (RISC-V object)
    ↓ aero-ld + linker.ld → AeroOS-26R1-riscv64.elf
Kernel image (S-mode + U-mode linked together)
    ↓ QEMU -kernel → OpenSBI → kernel_start
scheduler_create2(userapp_main, 1)
    ↓ timer interrupt / sret
userapp_main() runs in U-mode!

extern "C" fn sys_write(s: str, n: i64) -> i64;
extern "C" fn sys_yield();

#[no_mangle]
fn myapp_main() {
    let mut count = 0;
    loop {
        sys_write("hello from myapp\n", 17);
        count = count + 1;
        sys_yield();
    }
}