Read Only Memory
From Saferpedia
ROM memory (Read Only Memory) is a storage environment used in computers and other electronic devices. Because data stored in ROM can't be edited this kind of memory is generally used to deliver and store hardware components' firmware.
Read Only Memory chips are integrated circuits programmed with specific data when they are manufactured, used in computers and most of electronic devices.
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Computer Memory Basics
Memory is represented by any form of electronic storage and is usually used to identify fast, temporary forms of storage. If your computer's CPU had to constantly access the hard drive to retrieve every piece of data it needs, it would operate very slowly. When the information is kept in memory, the CPU can access it much more quickly. Most forms of memory are intended to store data temporarily.
Types of ROM
There are five basic types of ROM:
Each type has unique characteristics, but they all have two things in common:
- Stored data are nonvolatile, meaning data are not deleted when the power supply is shut down;
- Stored data are either unchangeable or requires a special operation to change them (unlike RAM, which can be changed easily).
ROM chips contain a grid of columns and rows and use a diode to connect the lines if the value is 1. If the value is 0, then the lines are not connected at all.
PROM
Creating ROM chips from scratch is time-consuming and very expensive in small quantities. For this reason developers created a type of ROM known as Programmable Read Only Memory (PROM). Blank PROM chips can be bought inexpensively and coded by anyone with a special tool called a programmer.
PROM chips also have a grid of columns and rows like ROM do. The difference is that in PROM every intersection is connected by a fuse. A charge sent through a column will pass through the fuse in a cell to a grounded row indicating a value of 1. Since all the cells have a fuse, the initial state of a PROM will be 1. To change the value of a cell to 0, you use a programmer to send a specific amount of current to break the connection between the column and row by burning out the fuse. This process is known as burning the PROM.
PROM can only be programmed once because they are even more fragile than ROM. A shock of static electricity may burn the fuses, changing bits from 1 to 0. Blank PROM are inexpensive and great for prototyping the data before starting the ROM manufacturing process.
EPROM
Erasable Programmable Read Only Memory (EPROM) chips can be rewritten many times but erasing them requires a special tool that emits a certain frequency(253.7) of ultraviolet (UV) light. EPROM are configured using an EPROM programmer that provides voltage at specified levels depending on the type of EPROM used.
Attention! An EPROM eraser is not selective, it will erase it entirely and the EPROM must be removed from its device and placed under the UV light of the EPROM eraser for several minutes. An EPROM that is left under too long can become over-erased.
EEPROM and Flash memory
EPROM represented a big step up from PROM in terms of re-usability but they still require dedicated equipment and a labor-intensive process to remove and reinstall them each time a change is necessary. Also, changes cannot be made incrementally to an EPROM; the whole chip must be erased.
While using Electrically Erasable Programmable Read Only Memory (EEPROM):
- The chip does not have to removed to be rewritten;
- You can change only a specific portion of the chip;
- Changing the contents does not require additional dedicated equipment.
There is only one problem: EEPROM chips are too slow.
That's why manufacturers invented the Flash memory, a type of EEPROM that uses in-circuit wiring to erase by applying an electrical field to the entire chip or to predetermined sections of the chip called blocks. Flash memory works much faster than traditional EEPROM because it writes data in chunks, usually 512 bytes in size, instead of 1 byte at a time.
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