只读存储器（ROM）

As the name implies, a read-only memory(ROM) is a memory unit that performs the read operation only; it does not have a write capability. This implies that the binary information stored in a ROM is made permanent during the hardware production of the unit and cannot be altered by writing different words into it.

顾名思义， 只读存储器(ROM)是仅执行读取操作的存储单元； 它没有写功能。 这意味着存储在ROM中的二进制信息在该单元的硬件生产期间被永久化，并且不能通过在其中写入不同的字来更改。

Whereas a RAM is a general-purpose device whose contents can be altered during the computational process, a ROM is restricted to reading words that are permanently stored within the unit. The binary information to be stored, specified by the designer, is then embedded in the unit to form the required interconnection pattern. ROMs come with special internal electronic fuses that can be programmed for a specific configuration. Once the pattern is established, it stays within the unit even when power is turned off and on again.

RAM是一种通用设备，其内容可以在计算过程中更改，而ROM被限制为读取永久存储在单元中的字。 然后，由设计人员指定要存储的二进制信息，然后将其嵌入到单元中以形成所需的互连模式。 ROM带有特殊的内部电子保险丝，可以针对特定配置进行编程 。 一旦建立了模式，即使关闭电源然后再打开，它也会保留在设备内。

An m x n ROM is an array of binary cells organized into m words of n bits each. As shown in the block diagram below, a ROM has k address input lines to select one of 2^k = m words of memory, and n input lines, one for each bit of the word. An integrated circuit ROM may also have one or more enable inputs for expanding a number of packages into a ROM with larger capacity.

mxn ROM是二进制单元的阵列，这些单元被组织为m个字，每个n位。 如下方框图所示，ROM具有k条地址输入线以选择2 ^k = m个存储字中的一个，以及n条输入线，每个字对应一位。 集成电路ROM也可以具有一个或多个使能输入，用于将多个封装扩展成具有更大容量的ROM。

The ROM does not need a read-control line since at any given time, the output lines automatically provide the n bits of the word selected by the address value. Because the outputs are a function of only the present inputs (the address lines), a ROM is classified as a combinational circuit. In fact, a ROM is constructed internally with decoders and a set of OR gates. There is no need for providing storage capabilities as in RAM, since the values of the bits in the ROM are permanently fixed.

ROM不需要读取控制线，因为在任何给定时间，输出线都会自动提供由地址值选择的字的n位。 由于输出仅是当前输入(地址线)的函数，因此ROM被归类为组合电路。 实际上，ROM内部是由解码器和一组“或”门构成的。 无需像RAM中那样提供存储功能，因为ROM中的位值是永久固定的。

ROMs find a wide range of applications in the design of digital systems. As such, it can implement any combinational circuit with k inputs and n outputs. When employed in a computer system as a memory unit, the ROM is used for storing fixed programs that are not to be altered and for tables of constants that are not subject to change. ROM is also employed in the design of control units for digital computers. As such, they are used to store coded information that represents the sequence of internal control variables needed for enabling the various operations in the computer. A control unit that utilizes a ROM to store binary control information is called a microprogrammed control unit.

ROM在数字系统设计中有广泛的应用。 这样，它可以实现具有k个输入和n个输出的任何组合电路。 当在计算机系统中用作存储单元时，ROM用于存储固定不变的程序和不变的常数表。 ROM还用于数字计算机的控制单元设计中。 这样，它们用于存储编码信息，该编码信息表示启用计算机中的各种操作所需的内部控制变量的顺序。 利用ROM存储二进制控制信息的控制单元称为微程序控制单元。

ROM：不同类型的ROM (ROM: Different Types of ROM)

The required paths in a ROM may be programmed in three different ways.

ROM中所需的路径可以用三种不同的方式进行编程。

1. The first, mask programming, is done by the semiconductor company during the last fabrication process of the unit. This procedure is costly because the vendor charges the customer a special fee for custom masking the particular ROM. For this reason, mask programming is economical only if a large quantity of the same ROM configuration is to be ordered.

   首先， 掩模编程是由半导体公司在单元的最后制造过程中完成的。 此过程成本很高，因为供应商向客户收取特殊费用以自定义屏蔽特定ROM。 因此，仅当订购大量相同的ROM配置时，掩模编程才是经济的。

2. For small quantities it is more economical to use a second type of ROM called a Programmable Read Only Memory(PROM). The hardware procedure for programming ROMs or PROMs is irreversible, and once programmed, the fixed pattern is permanent and cannot be altered. Once a bit pattern has been established, the unit must be discarded if the bit pattern is to be changed.

   对于小批量，使用第二种ROM(称为可编程只读存储器(PROM))更为经济。 对ROM或PROM进行编程的硬件过程是不可逆的 ，并且一旦进行编程，固定模式将是永久性的，无法更改。 建立位模式后，如果要更改位模式，则必须丢弃该单元。

3. A third type of ROM available is called Erasable PROM or EPROM. The EPROM can be restructured to the initial value even though its fuses have been blown previously. Certain PROMs can be erased with electrical signals instead of ultraviolet light. These PROMs are called Electrically Erasable PROM or EEPROM. Flash memory is a form of EEPROM in which a block of bytes can be erased in a very short duration.

   可用的第三种类型的ROM称为可擦除PROM或EPROM 。 即使EPROM的保险丝已经烧断，也可以将其重组为初始值。 某些PROM可以用电信号代替紫外线来擦除。 这些PROM称为电可擦除PROM或EEPROM 。 闪存是EEPROM的一种形式，其中一个字节块可以在很短的时间内擦除。

Example applications of EEPROM devices are:

EEPROM设备的示例应用是：

• Storing current time and date in a machine.

  在机器中存储当前时间和日期。

• Storing port statuses.

  存储端口状态。

Example of Flash memory device applications are:

闪存设备应用程序的示例包括：

• Storing messages in a mobile phone.

  在手机中存储消息。

• Storing photographs in a digital camera.

  将照片存储在数码相机中。

翻译自: https://www.studytonight.com/computer-architecture/read-only-memory