stack

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

---
register handler

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

---
helper functions


    #define CLINT0_MTIME     0x200bff8
    #define CLINT0_MTIMECMP  0x2004000

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

    void mtimecmp_set(unsigned long long time) {
        *(unsigned int*)(CLINT0_MTIMECMP + 0) = (unsigned int)time;
        *(unsigned int*)(CLINT0_MTIMECMP + 4) = (unsigned 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));


---
union

    union tmp {
      int  num;
      char chars[4];
    };

    union tmp u = {0};
    u.num = 0x41; // ascii, 'A'
    CRITICAL("%s\n", u.chars);

---
string 1

    u.num = (0x41 << 8); // ascii, 'A'

---
string 2

    u.num = (0x41 << 8) + 0x42; // ascii, 'A'


---
[skipped]
resilient to overflow

  static unsigned long long mtime_get() {
      unsigned int low, high;
      /* Q: Why having a loop? */
      do {
          high = *(unsigned int*)(CLINT0_MTIME + 4);
          low  = *(unsigned int*)(CLINT0_MTIME);
      }  while ( *(unsigned int*)(CLINT0_MTIME + 4) != high );

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

  /* set "mtimecmp" to "time" */
  static void mtimecmp_set(unsigned long long time) {
      /* Q: Why setting mtimecmp low to all 0xF? */
      *(unsigned int*)(CLINT0_MTIMECMP + 4) = 0xFFFFFFFF;
      *(unsigned int*)(CLINT0_MTIMECMP + 0) = (unsigned int)time;
      *(unsigned int*)(CLINT0_MTIMECMP + 4) = (unsigned int)(time >> 32);
  }