第四週 Chp:4 系統單元的組成元件

Cache

In addition to clock speed and architecture, a processor's cache and front side bus (FSB) speed also determine a computer's overall power. In brief, cache is a form of very fast memory integrated into the processor chip, and used to store up instructions (work for the processor) so that it has to slow down as little as possible between tasks. Cache is measured in megabytes (MB), with (for example) low-end Celeron processors having as little as 0.25MB of cache (256KB), and high-end Itaniums having up to 24MB. The simple message is, the more cache the better -- though high levels of cache still come at a very significant price.
除了時鐘速度和架構，處理器的高速緩存和前端總線（FSB）速度也決定了計算機的整體實力。簡而言之，緩存是一個非常快的內存集成到處理器芯片，用於存儲指令處理器的工作，因此，它有減緩任務之間盡可能少的形式。緩存是衡量兆字節（MB），（例如）低端賽揚少0.25MB緩存（256KB）處理器，高端的安騰有到24MB。簡單的消息是，更多的緩存更好 - 雖然仍處於一個非常重要的價格高層次的緩存。
From: explainingcomputers.com

(1)CPU cache

Small memories on or close to the CPU can operate faster than the much larger main memory. Most CPUs since the 1980s have used one or more caches, and modern high-end embedded, desktop and server microprocessors may have as many as half a dozen, each specialized for a specific function. Examples of caches with a specific function are the D-cache and I-cache (data cache and instruction cache).
小回憶或靠近CPU可以操作速度比大得多的主存儲器。自20世紀80年代的大部分的CPU使用一個或多個高速緩存，和現代高端嵌入式，桌面和服務器微處理器中，可能有多達一半的打，每個專門為特定的功能。具有特定功能的高速緩存的例子是在D-cache和我緩存（數據緩存和指令高速緩存）。

(2)Disk cache

While CPU caches are generally managed entirely by hardware, a variety of software manages other caches. The page cache in main memory, which is an example of disk cache, is managed by the operating system kernel.

While the hard drive's hardware disk buffer is sometimes misleadingly referred to as "disk cache", its main functions are write sequencing and read prefetching. Repeated cache hits are relatively rare, due to the small size of the buffer in comparison to the drive's capacity. However, high-end disk controllers often have their own on-board cache of hard disk data blocks.

Finally, fast local hard disk can also cache information held on even slower data storage devices, such as remote servers (web cache) or local tape drives or optical jukeboxes. Such a scheme is the main concept of hierarchical storage management. 

雖然CPU緩存一般是完全由硬件，各種軟件管理等緩存。這是一個磁盤高速緩存的例子，在主內存中的頁面緩存，管理由操作系統的內核。

而硬盤驅動器的硬件磁盤緩衝區誤導有時簡稱為“磁盤高速緩存”，其主要職能是寫序，讀預取。重複的緩存命中率是比較少見，由於緩衝區在驅動器的容量比較小大小。然而，高端磁盤控制器通常有自己的板上緩存硬盤中的數據塊。

最後，快速的本地硬盤，也可以緩存更慢的數據存儲設備，如遠程服務器（Web緩存）或本地磁帶驅動器或光盤庫，掌握的信息。這項計劃是分層存儲管理的主要概念。