Digital-electronics 简明教程

RAM and ROM

在上一章中，我们讨论了存储设备及其特性。阅读本章以了解两种最重要的存储器类型的特性，即 RAM（随机存取存储器）和 ROM（只读存储器），这些存储器用于计算机、笔记本电脑、智能手机等数字系统。

In the last chapter, we had a discussion on memory devices and their characteristics. Read this chapter to understand the characteristics of the two most important types of memories named, RAM (Random Access Memory) and ROM (Read Only Memory) which are used in digital systems like computer, laptops, smartphones, etc.

What is RAM?

RAM 构成了 CPU 的内部存储器，用于存储数据、程序和程序结果。它是可读可写的存储器。它称为随机存取存储器 (RAM)。

A RAM constitutes the internal memory of the CPU for storing data, program and program result. It is read/write memory. It is called Random Access Memory (RAM).

由于 RAM 中的访问时间与地址无关，即存储器内的每个存储位置都像其他位置一样容易到达，而且花费的时间相同。我们可以随机且极其快速地进入存储器，但它也可能非常昂贵。

Since the access time in RAM is independent of the address to the word that is, each storage location inside the memory is as easy to reach as other location & takes the same amount of time. We can reach into the memory at random & extremely fast but can also be quite expensive.

RAM 是一种易失性存储器，即在关掉计算机或断电时，存储在其中的数据就会丢失。因此，计算机通常使用备用不间断电源系统 (UPS)。RAM 很小，无论是其物理尺寸还是可以容纳的数据量。

RAM is a volatile memory i.e., data stored in it is lost when we switch off the computer or if there is a power failure. Hence, a backup uninterruptible power system (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold.

Types of RAM

RAM 或随机存取存储器分为以下两种类型 −

RAM or Random Access Memory is classified into the following two types −

  1. Static RAM (SRAM)

  2. Dynamic RAM (DRAM)

让我们详细讨论这两种类型的 RAM。

Let’s discuss about these two types of RAMs in detail.

Static RAM (SRAM)

“静态”一词表示只要有电力供应，内存就能保留其内容。然而，由于易失性，在电源关闭后数据丢失。SRAM 芯片使用由 6 个晶体管和电容器组成的矩阵。晶体管不需要电力来防止泄漏，因此 SRAM 不必经常刷新。

The word "static" indicates that the memory retains its contents as long as power remains applied. However, data is lost when the power gets down due to volatile nature. SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis.

由于矩阵中存在额外的空间，SRAM 使用的芯片比 DRAM 用于相同数量的存储空间的芯片多，因此制造成本更高。静态 RAM 主要用作高速缓存，需要非常快且小。

Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher. Static RAM is mainly used as cache memory needs to be very fast and small.

以下是 SRAM 的一些重要特性 -

The following are some important characteristics of SRAM −

  1. Being a type of RAM, the SRAM is also a volatile memory. Thus, it requires a continuous power supply to maintain its stored data. If power supply is removed or switched off, the data stored in the SRAM will delete.

  2. SRAM is a high-speed random access memory. SRAM does not need to be refreshed to maintain its stored data.

  3. SRAM is made up of semiconductor components called flip-flops which store data. SRAM has lower storage density. This is mainly because of its complex memory cell structure. This also results in larger physical size.

  4. SRAM is mostly employed in digital systems in which high-speed data access is important. For example, it is used as CPU cache memory, high-speed buffers, and registers in microprocessors and microcontrollers.

  5. SRAM is relatively more expensive. This is mainly because of its lower storage density and higher manufacturing cost.

Dynamic RAM (DRAM)

DRAM 与 SRAM 不同，必须对其进行连续刷新才能维护数据。这是通过将存储器放置在刷新电路上来实现的，该电路每秒重写数据数百次。DRAM 用于大多数系统存储器，因为它便宜且小。

DRAM, unlike SRAM, must be continually refreshed in order for it to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small.

所有 DRAM 都由存储器单元构成。这些单元由一个电容器和一个晶体管组成。

All DRAMs are made up of memory cells. These cells are composed of one capacitor and one transistor.

DRAM（动态随机存取存储器）的重要特性如下 -

The important characteristics of DRAM (Dynamic Random Access Memory) are listed below −

  1. Since DRAM is also a random access memory, hence it is also a volatile memory and thus requires a continuous power supply to retain its stored data. The data stored in DRAM is lost, when power supply to is turned off.

  2. In DRAM, the memory cells are made up of capacitors and transistors. Where each memory cell can store a 1-bit of data in the form electric charge in a capacitor.

  3. In DRAM, to prevent losing stored data due to leakage in capacitor, a refresh circuit is required for periodic refresh cycles. This is the primary reason the term "dynamic" is used in DRAM.

  4. For DRAM, the access time is typically of the order in nanoseconds (ns). DRAM is less expensive than SRAM.

这些就是有关 RAM（随机存取存储器）及其类型的全部内容。现在让我们讨论另一种称为 ROM 的存储设备类型。

This is all about RAM (Random Access Memory) and its types. Let us now discuss about another type of memory device called ROM.

What is ROM?

ROM 代表只读存储，即只能读取但无法写入的存储器。这种类型的存储器是非易失性的。信息在制造过程中永久存储在这样的存储器中。

ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture.

ROM 存储计算机第一次通电时启动计算机所需的指令，此操作称为引导。ROM 芯片不仅用于计算机，还用于洗衣机和微波炉等其他电子设备。

A ROM, stores such instruction as are required to start computer when electricity is first turned on, this operation is referred to as bootstrap. ROM chip are not only used in the computer but also in other electronic items like washing machine and microwave oven.

Types of ROM

以下是只读存储器 (ROM) 的一些重要类型 -

The following are some important types of Read Only Memory (ROM) −

  1. MROM

  2. PROM

  3. EPROM

  4. EEPROM

让我们详细讨论这些不同类型的 ROM 及其重要特性。

Let’s discuss these different types of ROMs in detail along with their important characteristics.

MROM (Masked ROM)

最早的 ROM 是硬连线的设备，包含预编程的数据或指令集。这些类型的 ROM 称为掩模 ROM。这是廉价的 ROM。

The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kinds of ROMs are known as Masked ROMs. It is inexpensive ROM.

因为它是一种类型的 ROM，因此它也是非易失性存储器。MROM 在制造时进行编程，并且以后不能修改或更改其数据。

Since it is a type of ROM, thus it is also a non-volatile memory. The MROMs are programmed at the time of manufacturing and its data cannot be modified or changed at a later point of time.

以下是 MROM 的一些重要特征 -

The following are some important characteristics of MROM −

  1. MROM is a non-volatile memory. Hence, it can retain its data even when power supply is turned off or removed.

  2. MROM is mainly used for storing permanent software and instructions like firmware, bootloader code, and other system data essential for system operations.

  3. In MROM, data and programs are written during the manufacturing process. Once it is programmed, the stored data cannot be modified or changed. Thus, it is a one-time programmable memory.

  4. Another important characteristic of MROM, it is a read-only memory. Thus, it supports read only operations.

  5. MROM is an inexpensive type of read only memory.

  6. The major disadvantage of the MROM is its limited flexibility that means once it is programmed, the stored data cannot be changed or deleted.

PROM (Programmable Read Only Memory)

PROM 是一种只能由用户修改一次的只读存储器。用户购买空白 PROM，并使用 PROM 编程器输入所需内容。

PROM is a read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM programmer.

在 PROM 芯片内部，在编程期间会烧毁小保险丝。它只能编程一次，并且不可擦除。

Inside the PROM chip, there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.

以下是可编程只读存储器的重要特征 -

Here are the important characteristics of the programmable read only memory −

  1. PROM also retains its stored data, when the power supply is turned off.

  2. PROM is a programmable memory, but it can be programmed by the user only once. Then, its stored data cannot be changed, deleted, or rewritten.

  3. In PROM, the memory cells are made by using either fuse-based technology, in which storing the data involves blowing of tiny fuses.

  4. PROM is also a read only memory, thus it supports read operations only.

  5. Like MROM, PROM also offers limited flexibility, as the data cannot be changed or erased, once it is programmed.

EPROM (Erasable and Programmable Read Only Memory)

EPROM 可以通过将其暴露在紫外线下长达 40 分钟来擦除。通常，EPROM 擦除器可以实现此功能。

The EPROM can be erased by exposing it to ultra-violet light for a duration of up to 40 minutes. Usually, an EPROM eraser achieves this function.

在编程期间，电荷被困在绝缘栅极区域。该电荷保留的时间超过十年，因为该电荷没有泄漏路径。为了擦除此电荷，紫外线会通过石英晶体窗口（盖子）。这种紫外线照射会消散电荷。在正常使用中，石英盖使用标签密封。

During programming, an electrical charge is trapped in an insulated gate region. The charge is retained for more than ten years because the charge has no leakage path. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). This exposure to ultra-violet light dissipates the charge. During normal use, the quartz lid is sealed with a sticker.

以下是 EPROM 的一些主要特征 −

Some of the key characteristics of EPROM are highlighted below −

  1. EPROM provides a permanent storage for data even in the absence of power supply.

  2. EPROM is an electrically programmable memory. Hence, it can be programmed by applying specific voltage levels to its write circuit.

  3. EPROM can be erased multiple times by exposing it to UV rays for around 20 to 30 minutes. Then, it can be reprogrammed again.

  4. EPROM chips have a quartz window on its top. This is provided to penetrate the UV rays to erase the stored data.

  5. EPROM provides the high storage density. Hence, it can hold large amounts of data in a relatively small physical space.

  6. EPROM is slower to write, that can affect overall performance of the system.

EEPROM (Electrically Erasable and Programmable Read Only Memory)

EEPROM 是通过电进行编程和擦除的。它可以被擦除和重新编程大约一万次。擦除和编程都大约需要 4 到 10 毫秒（毫秒）。

The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (millisecond).

在 EEPROM 中，可以有选择地擦除和编程任何位置。可以一次擦除一个字节的 EEPROM，而不是擦除整个芯片。因此，重新编程的过程是灵活的，但是很慢。

In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow.

电可擦除可编程只读存储器的重要特征如下 −

The important characteristics of electrically erasable programmable read only memory are highlighted below −

  1. Being a ROM, EEPROM stores data permanently.

  2. EEPROM can be erased and reprogrammed electrically and does not require any special equipment or UV rays for erasing the stored data.

  3. EEPROM can be erased and reprogrammed multiple times.

  4. EEPROM also provides random data access capabilities. This feature allows for efficient and fast data manipulation and management.

  5. EEPROM consumes very less power. Hence, it is better suited to use in battery-powered devices where energy efficiency is important.

  6. EEPROM is a cost effective read only memory device designed use in modern digital systems.

Conclusion

在本章中，我们解释了 RAM 和 ROM 的基础知识以及它们的不同类型和特征。

In this chapter, we explained the basics RAM and ROM along with their different types and characteristics.