Computer Fundamentals 简明教程
Computer - Read Only Memory
ROM 代表 Read Only Memory 。我们只能从中读取但不能写入的内存。此类内存是非易失性的。信息在制造过程中永久存储在这些存储器中。ROM 存储启动计算机所需的操作说明。此操作称为 bootstrap 。ROM 芯片不仅用于计算机，还用于其他电子产品，如洗衣机和微波炉。
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. A ROM stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.
现在让我们讨论一下各种类型的 ROM 及其特性。
Let us now discuss the various types of ROMs and their characteristics.
MROM (Masked ROM)
最早的 ROM 是硬连线设备，其中包含预先编程的一组数据或说明。这类 ROM 被称为屏蔽 ROM，价格低廉。
The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kind of ROMs are known as masked ROMs, which are inexpensive.
PROM (Programmable Read Only Memory)
PROM 是一种只读存储器，用户只能修改一次。用户购买一个空白的 PROM，并使用 PROM 程序输入所需的内容。在 PROM 芯片内部，有一些小保险丝会在编程过程中被烧断。它只能编程一次，并且不可擦除。
PROM is 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 program. Inside the PROM chip, there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.
EPROM (Erasable and Programmable Read Only Memory)
通过将 EPROM 暴露于紫外线下长达 40 分钟的时间，可以擦除 EPROM。通常，EPROM 擦除器可以实现此功能。在编程期间，电荷被困在绝缘栅极区域中。该电荷保持超过 10 年，因为电荷没有泄漏路径。要清除该电荷，紫外线会穿过石英晶体窗口（盖）。这种紫外线照射会消散电荷。在正常使用中，石英盖用贴纸密封。
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. During programming, an electrical charge is trapped in an insulated gate region. The charge is retained for more than 10 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.
EEPROM (Electrically Erasable and Programmable Read Only Memory)
EEPROM 可通过电气方式编程和擦除。它可以被擦除和重新编程大约一万次。擦除和编程都需要大约 4 到 10 毫秒（毫秒）。在 EEPROM 中，任何位置都可以被选择性地擦除和编程。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，但很慢。
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). 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 reprogramming is flexible but slow.