register handler

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

---
helper functions


    #define CLINT0_MTIME     0x200bff8
    #define CLINT0_MTIMECMP  0x2004000

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

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

---
set timer

    mtimecmp_set(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));


---
resilient to timer overflow

    long long mtime_get() {
        int low, high;
        do {
            high = *(int*)(CLINT0_MTIME + 4);
            low  = *(int*)(CLINT0_MTIME);
        }  while ( *(int*)(CLINT0_MTIME + 4) != high );

        return (((long long)high) << 32) | low;
    }

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

---
See which interrupts

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