Computer Abstractions and Techno

1.4 Under the Covers

这里的重点概念是计算机的组成：

The five classic components of a computer are input, output, memory, datapath, and control, with the last two sometimes combined and called the processor. Figure 1.5 shows the standard organization of a computer. This organization is independent of hardware technology: you can place every piece of every computer, past and present, into one of these five categories. To help you keep all this in perspective, the five components of a computer are shown on the front page of each of the following chapters, with the portion of interest to that chapter highlighted.

几个概念的定义如下：

input device A mechanism through which the computer is fed information, such as a keyboard.

output device A mechanism that conveys the result of a computation to a user, such as a display, or to another computer.

memory The storage area in which programs are kept when they are running and that contains the data needed by the running programs.

datapath The component of the processor that performs arithmetic operations.

control The component of the processor that commands the datapath, memory, and I/O devices according to the instructions of the program.

其中，数据通路（datapath）又称为运算器。重要的I/O设备包括键盘、鼠标、屏幕等等。

而存储按照分层来看，除了日常理解的内存：

dynamic random access memory (DRAM) Memory built as an integrated circuit; it provides random access to any location. Access times are 50 nanoseconds and cost per gigabyte in 2012 was $5 to $10.

还有就是缓存：

Cache memory consists of a small, fast memory that acts as a buffer for the DRAM memory. (The nontechnical definition of cache is a safe place for hiding things.) Cache is built using a different memory technology, static random access memory (SRAM). SRAM is faster but less dense, and hence more expensive, than DRAM (see Chapter 5). SRAM and DRAM are two layers of the memory hierarchy.

而这一类存储，又称之为易失性存储：

volatile memory Storage, such as DRAM, that retains data only if it is receiving power.

对应地当然就是有非易失性存储：

nonvolatile memory A form of memory that retains data even in the absence of a power source and that is used to store programs between runs. A DVD disk is nonvolatile.

非易失性存储，除了硬盘（磁盘），还有闪存（flash memory）。

为了区分，也称为：

To distinguish between the volatile memory used to hold data and programs while they are running and this nonvolatile memory used to store data and programs between runs, the term main memory or primary memory is used for the former, and secondary memory for the latter.

这一节的最后，ISA这个重要的概念，作为抽象的一个重要示范：

Both hardware and software consist of hierarchical layers using abstraction, with each lower layer hiding details from the level above. One key interface between the levels of abstraction is the instruction set architecture—the interface between the hardware and low-level software. This abstract interface enables many implementations of varying cost and performance to run identical software.

这里的implementations指的是：

implementation Hardware that obeys the architecture abstraction.

为了调用ISA：

application binary interface (ABI) The user portion of the instruction set plus the operating system interfaces used by application programmers. It defines a standard for binary portability across computers.