Assembly 1: single cmd

    printf("hello...\n");
    asm("lw ra,12(sp)");
    asm("ret");
    printf("...world!\n");

---
Assembly 2: with output

  void *sp;
  asm("mv %0, sp" : "=r"(sp));

---
Assembly 3: with input

  void *sp = (void*)0x803fffc0;
  asm("mv sp,%0" :: "r"(sp));

---
register handler

    asm("csrw mtvec, %0" ::"r"(handler));

---
helper functions

    #define CLINT0_MTIME     0x200bff8
    #define CLINT0_MTIMECMP  0x2004000

    unsigned long long helper_mtime_get() {
        int low  = *(int*)(CLINT0_MTIME);
        int high = *(int*)(CLINT0_MTIME + 4);
        return (((long long)high) << 32) | low;
    }

    void helper_mtimecmp_set(unsigned long long time) {
        *(int*)(CLINT0_MTIMECMP + 0) = (int)time;
        *(int*)(CLINT0_MTIMECMP + 4) = (int)(time >> 32);
    }

---
set timer

    helper_mtimecmp_set(helper_mtime_get() + QUANTUM);

---
enable timer interrupt

    int mstatus, mie;
    asm("csrr %0, mstatus" : "=r"(mstatus));
    asm("csrw mstatus, %0" ::"r"(mstatus | 0x8));
    asm("csrr %0, mie" : "=r"(mie));
    asm("csrw mie, %0" ::"r"(mie | 0x80));

---
See which interrupts

    int mcause;
    asm("csrr %0,mcause" : "=r"(mcause));
    printf("cause: %d\n", mcause);

---