ROM Memory (Read-Only Memory)

Read-only memory, also known as ROM (read-only memory), is a storage medium used in computers and electronic devices, which allows only the reading of information and not its writing, regardless of the presence Or not from an energy source.

The data stored in the ROM cannot be modified, or at least not quickly or easily. It is mainly used in its strictest sense, refers only to English ROM mask, MROM (the oldest type of solid state ROM), which is manufactured with stored data permanently and therefore its content cannot be modified in any way. However, the most modern ROMs, such as EPROM and Flash EEPROM, can effectively be deleted and reprogrammed several times, even though they are described as “read-only Memory ” (ROM).

The reason for continuing to call them is that the rescheduling process is generally rare, relatively slow, and writing is often not allowed in random places in memory. Despite the simplicity of the ROM, the reprogrammed devices are more flexible and economical, so the old ROM masks are not usually found in hardware produced from 2007.


Developed by Toshiba, the designers explicitly broke with the practices of the past, stating that it focused on “being a replacement for hard disks”, rather than having the traditional use of the ROMPROM Memory as a non-volatile primary form of storage. In 2007, NAND has advanced considerably in its goal, offering comparable performance to hard disks, better tolerance to blows, extreme miniaturization (such as USB Ejemplomemorias and MicroSD memory cards), and much lower power consumption.

There are several types of memory:

· RAM (Random access memory): This is the same as main memory. When used by itself, the term RAM refers to read and write memory; That is, you can both write data in RAM and read them from RAM. This is in contrast to the ROM, which allows you to just read the read data. Most of the RAM is volatile, which means it requires a constant flow of electricity to keep its content. As soon as the power supply is interrupted, all data that was in RAM is lost.

· ROM (unalterable memory): Computers almost always contain a small amount of read-only memory that saves the instructions for starting the computer. In the ROM memory you cannot write.

· Prom (programmable unchangeable memory): An prom is a memory chip in which you can save a program. But once the PROM has been used, you cannot reuse it to save something else. Like the ROMs, the Proms are permanent.

· EPROM (erasable programmable memory): An EPROM is a special type of PROM which can be erased by exposing it to ultraviolet light.

· EEPROM (electrically erasable programmable memory): A EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.

PROM Memory

PROM is in English of programmable read-only memory, which means “programmable read-only memory”. It is a digital memory where the value of each bit depends on the state of a fuse (or antifuse), which can be burned only once. For this reason the memory can be programmed (the data can be written) only once through a special device, a programmer PROM. These memories are used to record permanent data in smaller amounts to the ROMs, or when the data should change in many or all cases.

Small proms have been used as function generators, usually in conjunction with a multiplexer. Sometimes they preferred the ROMs because they are bipolar, habitulamente Schottky, getting higher speeds.


A common PROM is found with all the bits in value 1 as the factory default value; The burn of each fuse changes the value of the corresponding bit to 0. The programming is done by applying high voltage pulses that are not during normal operations (12 to 21 volts). The term read-only refers to that, unlike other memories, the data cannot be changed (at least by the end-user).


The memory PROM was invented in 1956 by Wen Tsing Chow, working for the “Weapon Division”, of the American Bosch arma Corporation in Garden City, new York. The invention was conceived at the request of the United States Air Force, to obtain a more secure and flexible way to store the constants of the objectives in the digital computer of the MBI Atlas E/F.

The associated patent and technology were kept secret for several years while the E/F Atlas was the main missile in the United States. The term “burning”, referring to the process of recording an PROM, is also found in the original patent, because as part of the original implementation the internal diodes with an excess of current had to be literally burned to produce the discontinuity of the Circuit. The first Proms programming machines were also developed by engineers from the Arma division under the direction of Mr. Chow and were located the weapon laboratory of Garden City, and at the head of the Air Strategic Air Force command.

EPROM Memory

EPROM is the acronym of Eraseable programmable Read-Only Memory (erasable programmable ROM). It is a type of non-volatile ROM memory chip invented by engineer Dov Frohman. It consists of FAMOS cells (floating gate Avalanche-injection Metal-oxide semiconductor) or “Floating gate transistors “, each of which comes from factory without load, so they are read as 1 (so, an unrecorded EPROM is read as FF in all its Cells).

EPROM memories are programmed using an electronic device, such as ElCromemco Bytesaver, which provides voltages higher than those normally used in electronic circuits. Cells receiving load are then read as a 0.

ROM Memory Types

The ROM memory, by the abbreviation of Read only memory, in Castilian means read-only memory.

This memory is the one used by electronic equipment, as in the case of computers. The information stored in this memory does not allow its modification by the user, hence its denomination.

There are currently the following classes of ROM memory:

PROM: By the acronym of Programmable Read Only memory, in Spanish programmable ROM, it is identified by being digital. In this kind of memory each of the bits is determined by a fuse, which is only possible to burn it only once. This generates that, by means of an PROM programmer, they will be programmed for only once. The PROM memory is used in situations where information needs to be transformed into all or most opportunities. It is also sought especially in that information that wants to be stored in a lasting way that does not exceed those of the ROM.

EPROM: Its acronym for Erase programmable read-only Memory, in Castilian, erasable programmable read-only ROM. This kind of ROM memory is a chip without volatility and consists of floating door transistors or FAMOS cells that are produced from the factory without any load. This memory can be programmed using an electronic device in which these voltages exceed the employees in electronic circuits. From this, the cells begin to be read as 1, before this is done as 0. This memory offers the possibility of being erased only if it is exposed to ultraviolet lights. At the time that the EPROM is programmed, it becomes non-volatile, that is, that the stored information remains there in a timeless way. However, it can be eliminated and reprogrammed with the use of high voltage levels. While they continue to be employed, they reveal some drawbacks, among them that the process of erasing the chip is always complete, ie it is not possible to choose a particular direction. On the other hand, to reprogram or delete them, they must move from their circuit and this process takes at least twenty minutes. This type of disadvantages have been surpassed by flash memories and EEPROM, so that the eproms are going into disuse in certain designs and applications.

EEPROM: Its acronym in English electrically erase programmable Read Only Memory, which means in Castilian programmable and electrically erasable ROM. This memory, as its denomination indicates it can be programmed, erased and reprogrammed electrically without the need for exposure to ultraviolet rays, for example as in EPROMs, which implies that they are non-volatile. At the same time of possessing the floating, like the aforementioned, it has a layer of oxide located in the drainage of the MOSFET cell, which facilitates that the memory manages to be erased electrically. To do this, no specific programmers or ultraviolet rays are required, it is possible to carry it out on the circuit itself. At the same time you can rewrite and delete bytes individually, and are more affordable and faster to reprogram than previous ones. The disadvantages it has in relation to the aforementioned are the density and its high costs.