# 作業系統筆記(台科資管 賴教授)
* **After I/O starts, control returns to user** program without waiting for I/O completion
* **System call** – request to the OS to allow user to wait for I/O completion
* **Device-status table** contains entry for each I/O device indicating its type, address, and state
* OS indexes into I/O device table to determine device status and to modify table entry to include interrupt

* Only one interrupt is generated per block, rather than the one interrupt per byte

* Caching – copying information into faster storage system; main memory can be viewed as a cache for secondary storage

* **Multiprocessors systems** growing in use and importance
Also known as **parallel systems**, **tightly-coupled systems(vs. loosely-coupled 透過網路的分散式運算系統)**
Advantages include:
* Increased throughput
* Economy of scale(規模效益)
* Increased reliability – graceful degradation(?降解) or fault tolerance
* Two types:
* Asymmetric Multiprocessing(不對稱，有一個manager cpu，通常是超級電腦)
* Symmetric Multiprocessing(無manager cpu)