# Synchronization Hardware
## Hardware Support
- CS問題發生的原因是更改shared data的過程可能被中斷
- 如果在CS，disable interrupts
- 在多processor的機器上行不通
- ==clock interrupts==不能fire in any machine
- HW support解法：atomic指令
- atomic：原子的，不能被分開、中斷的單元
- 例子
1. TestAndSet(var)
2. Swap(a,b)

### Atomic TestAndSet()
執行==atomically==，回傳lock的值，一律將lock設為TRUE

boolean TestAndSet (bool &lock) {
    bool value = lock ;
    lock = TRUE ;
    return value ;
}

- Mutual exclusion? Yes
- Progress? Yes
- ==Bounded-Wait? No!==
### Atomic Swap()
- 想法：enter CS if lock==false
- key可表示是否想進去
- 多個procress也可以行得通
- ==搶到lock==的人才能進去
- Shared data: boolean lock; //initially lock = FALSE;
- Mutual exclusion? Yes
- Progress? Yes
- ==Bounded-Wait? No!==