There are five basic types of ROM:
ROM
The ROM is a read-only memory. This means that you can only read its contents, but you cannot write over it and you cannot change or alter it in any way. A computer needs some sort of instructions when it is first turned on. These instructions are stored in the ROM. The ROM is also used to store certain information not needing to be changed, and it is nonvolatile, meaning that the information it stores is not erased when you turn the computer off.
Like the RAM, the ROM also has a spreadsheet layout. However, unlike the RAM, the ROM does not have capacitors in the cells where the columns and rows meet. Instead, it uses a diode. A diode allows an electrical current to flow in one direction. If a diode is present in a certain cell, then an electrical charge will be conducted through the cell to the ground, and the bit in the cell will be a 1. However, if there is no diode, then the cell will store a 0. ROMs are very cheap and reliable, but unfortunately, they are read-only.
PROM
PROM is an acronym for programming random-access memory. PROM is a ROM chip in which the user can write in the contents of the chip him or herself. When a PROM chip is first bought, all of the cells contain a 1. However, the 1s can be changed to 0s. In every memory cell there is a fuse, which allows for a charge to be sent through the column of the cell. When this fuse is in tact, there is a 1. However, the fuse can be broken. With a broken fuse, there will be no electrical current, and the 1 will then be changed to a 0.
The downside of the PROM is that it can
only be programmed once. If you make a mistake programming the chip, you will
have to start all over.
EPROM
EPROM stands for erasable and programmable ROM. It is like the PROM in that its contents can be written in by the user, but it also has the additional feature of being erasable. At the intersection of the columns and rows of the EPROM there are two transistors separated by an oxide layer. One transistor, known as the floating gate, has a link to the row of the cell. If this link is in tact, then a 1 is stored. However, the electrons in a floating gate can be altered so that they move to the other side of the oxide layer. This forms a barrier between the two transistors, and allows for a 0 to be stored.
To erase an EPROM, UV light is used. When a chip is exposed to UV light, it becomes erased, and a new pattern of 1s and 0s can be written in.
EEPROM
The EEPROM is similar to the EPROM, except that you don't have to erase the entire chip. In addition, you don't need any special equipment, like UV light to erase this chip.
However, the EEPROMs are very slow. You can only erase them one byte at a time.
Flash Memory
Flash memories are a type of EEPROM.
However, they are faster than EEPROMs because data can be written in chunks
(512 bytes at a time, to be exact).
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