storage over a channel (such as a coaxial cable). The message is interpreted, and the processor initiates action to retrieve the appropriate program and data from secondary storage [3].The program and data are “loaded”, or moves, to primary storage from secondary storage. This is a nondestructive read process. That is, the program and data that are read reside in both primary storage (temporarily) and secondary storage (permanently). The data are manipulated according to program instructions, and a report is written from primary storage to a printer.

图 1-1描述了输入/输出设备与主存储器间的读和写过程。在图中，VDT发出一个输入请求，请求是以消息模式通过通道(如同轴电缆)发送到主存储器。这个查询被解释，处理器发起操作从辅助存储器中调用合适的程序和数据。程序和数据从辅助存储器传送到主存储器中，这是一个非破坏性的读取过程，也就是说，程序和数据同时存在于主存(临时保存)和辅助存储器(永久保存)中。根据程序指令的指示，处理器对数据进行操作，并从主存传送一份报告到打印机。

A program instruction or a piece of data is stored in a specific primary storage location called an address. Addresses permit program instructions and data to be located, accessed, and processed. The content of each address is constantly changing as different programs are executed and new data are processed.

程序指令和数据是存储在主存中一个特殊的位置，称为地址空间。通过地址空间可以实现计算机对程序指令和数据的定位、访问和处理。地址空间的内容是经常变化的，这是由于计算机一直在执行不同的程序和数据。

Another name for primary storage is random-access memory, or RAM. A special type of primary storage, called read-only memory (ROM), cannot be altered by the programmer. The contents of ROM are “hard-wired” (designed into the logic of the memory chip) by the manufacturer and can be “read only”. When you turn on a microcomputer system, a program in ROM automatically readies the computer system for use. Then the ROM program produces the initial display screen prompt.

主存储器也称为随机存取存储器，或RAM。还有一种特殊的主存储器，称为只读存储器(ROM)，这种存储器不能被程序更改存放的内容。ROM的内容是被生产商通过硬件电路写入的，并且不能被重写。当你启动计算机，ROM中的一个程序会自动就绪等待计算机系统的调用，然后在显示器中显示开机提示。

A variation of ROM is programmable read-only memory (PROM). PROM is ROM into which you, the user, can load “read-only” programs and data. Once a program is loaded to PROM, it is seldom, if ever, changed [4]. However, if you need to be able to revise the contents of PROM, there is EPROM, erasable PROM. Before a write operation, all the storage cells must be erased to the same initial state.

可编程只读存储器(PROM)是另一种ROM，它可以载入只读的程序和数据，一旦载入，将不再改变。然而，若果你需要去修正PROM的内容，可以使用可擦可编程只读存储器。 EPROM在进行一次写操作之前，所有的存储单元必须被还原为同一初始状态。

A more attractive form of read-mostly memory is electrically erasable programmable read-only memory (EEPROM). It can be written into at any time without erasing prior contents; only the byte or bytes addressed are updated.

一种更吸引人的可改写只读存储器是电可擦除可编程只读存储器 (EEPROM)。它可以在任何时候写入，而且不会擦除以前的内容；只会更新被寻址的字节。

The EEPROM combines the advantage of nonvolatility with the flexibility of being updatable in place [6], using ordinary bus control, address, and data lines.

电可擦可编程存储器把非易失性优点和可更新、需要更新的地方的灵活性结合起来，修改时使用普通的总线控制线、地址线和数据线。