An EPROM, or erasable programmable read-only memory, is a type of computer memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in electronic circuits. Once programmed, an EPROM can be erased only by exposing it to strong ultraviolet light. EPROMs are easily recognizable by the transparent window in the top of the package, through which the silicon chip can be seen, and which emits UV light during erasing.
A 32KB (256Kbit) EPROM.
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A 32KB (256Kbit) EPROM.
As the quartz window is expensive to make, OTP (one-time programmable) chips were introduced; the only difference is that the EPROM chip is packed in an opaque package, so it can not be erased after programming. OTP versions are manufactured for both EPROMs themselves and EPROM-based microcontrollers. However, OTP EPROM (whether separate or part of a larger chip) is being increasingly replaced by EEPROM for small amounts where the cell cost isn't too important and flash for larger amounts.
A programmed EPROM retains its data for about ten to twenty years and can be read an unlimited number of times. The erasing window must be kept covered with a foil label to prevent accidental erasure by sunlight. Old PC BIOS chips were often EPROMs, and the erasing window was often covered with a label containing the BIOS publisher's name, the BIOS revision, and a copyright notice.
Some microcontrollers, often those from before the era of EEPROMs and flash memory, use EPROM to store their program. This is not very useful for development, as using one-time programmable devices would be horribly wasteful for debugging and windowed versions are expensive. Leaving the die of such a chip exposed to light can also change behavior in ways that may be disastrous when moving from a windowed part used for development to a non-windowed part for production. For example a device may power up with data memory random with the die covered but with it in a predictable state with the die exposed to light.
The EPROM was invented by engineer Dov Frohman.
This 8749 Microcontroller stores its program in internal EPROM.
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This 8749 Microcontroller stores its program in internal EPROM.
EPROMs come in several sizes both in physical packaging as well and storage capacity
EPROM Type Size — bits Size — bytes Length (hex) Last address (hex)
1702, 1702A 2 Kibit 256 100 000FF
2704 4 Kibit 512 200 001FF
2708 8 Kibit 1 KiB 400 003FF
2716, 27C16 16 Kibit 2 KiB 800 007FF
2732, 27C32 32 Kibit 4 KiB 1000 00FFF
2764, 27C64 64 Kibit 8 KiB 2000 01FFF
27128, 27C128 128 Kibit 16 KiB 4000 03FFF
27256, 27C256 256 Kibit 32 KiB 8000 07FFF
27512, 27C512 512 Kibit 64 KiB 10000 0FFFF
27C010, 27C100 1 Mibit 128 KiB 20000 1FFFF
27C020 2 Mibit 256 KiB 40000 3FFFF
27C040 4 Mibit 512 KiB 80000 7FFFF
27C080 8 Mibit 1 MiB 100000 FFFFF
(also called an E2PROM) or Electrically Erasable Programmable Read-Only Memory, is a non-volatile storage chip used in computers and other devices to store small amounts of volatile (configuration) data. When larger amounts of more static data are to be stored (such as in USB flash drives) other memory types like flash memory are more economic. SEEPROM, meaning Serial EEPROM, is an EEPROM chip that uses a serial interface to the circuit board.
Random access memory
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"RAM" redirects here. For other uses of the acronym, see Ram.
A four-megabyte RAM card for the VAX 8600 computer (circa 1986). The RAM chips are located in the rectangular areas to the left and right.
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A four-megabyte RAM card for the VAX 8600 computer (circa 1986). The RAM chips are located in the rectangular areas to the left and right.
Random-access memory (usually known by its acronym, RAM) refers to data storage formats and equipment that allow the stored data to be accessed in any order — that is, at random, not just in sequence. In contrast, other types of memory devices (such as magnetic tapes, disks, and drums) can access data on the storage medium only in a predetermined order due to constraints in their mechanical design.
Generally, RAM in a computer is considered main memory (or primary storage): the working area used for displaying and manipulating data. This type of RAM is usually in the form of integrated circuits (IC). These are commonly called memory sticks or RAM sticks because they are manufactured as small circuit boards with plastic packaging and are about the size of a few sticks of gum. Most personal computers have slots for adding and replacing memory chips.
RAM is typically erased when a computer is shut down, though some RAM chips maintain data indefinitely without electrical power. Technically, RAM devices are not limited to memory chips, and random-access memory as a storage format is not limited to use as working memory. In a broad sense, modern storage devices for long-term or secondary storage, including magnetic media and laser-readable CDs and DVDs, are forms of random-access memory.
Look up RAM, random access memory in Wiktionary, the free dictionary.
Most RAM can be both written to and read from, so "RAM" is often used interchangeably with "read-write memory." In this sense, RAM is the opposite of read-only memory (ROM). Strictly speaking, however, "RAM" and "ROM" are not mutually exclusive designations because "RAM" refers only to the method of accessing stored data, not whether data can be written.
Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor. As real-world capacitors are not ideal and hence leak electrons, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory. Its advantage over SRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to six transistors in SRAM. This allows DRAM to reach very high density. Since DRAM loses its data when the power supply is removed, it is in the class of