Computers and any electronics are complex devices, the principles of which are not always understood by most ordinary people. What is ROM and why is the device necessary? Most people will not be able to answer this question. Let's try to correct this misunderstanding.
What is ROM?
What are they and where are they used? Constant memories (ROM) are nonvolatile memory. Technologically they are implemented as a microcircuit. At the same time, we learned the abbreviation of the ROM decryption. Devices are designed for storing information entered by the user and installed programs. In a permanent memory device, you can find documents, melodies, pictures - i.e. Everything that should be stored for months or even years. The amount of memory, depending on the device used, can vary from a few kilobytes (on the simplest devices that have one silicon crystal, such as microcontrollers) to terabytes. The more the ROM volume - the more objects can be saved. The volume is directly proportional to the amount of data. If you compact the answer to the question of what a ROM is, you should answer: it's a data store that does not depend on the DC voltage.
Hard disks as basic permanent storage devices
On the question, what is ROM, has already been answered. Now we should talk about what they are. The main permanent storage device are hard disks. They are in every modern computer. They are used due to their wide possibilities of information accumulation. But there are a number of ROMs that use multiplexers (these are microcontrollers, initial loaders and other similar electronic mechanisms). With a detailed study it will be necessary not only to understand the value of the ROM. Deciphering other terms is also necessary, in order to delve into the topic.
Expansion and addition of the capabilities of ROM through flash technologies
If the standard amount of memory is not enough for the user, then you can take advantage of the additional expansion of the capabilities of the provided ROM in the field of data storage. This is done through modern technologies implemented in memory cards and USB flash drives. They are based on the principle of reusable use. In other words, data on them can be erased and recorded tens and hundreds of thousands of times.
What constitutes a permanent storage device
The ROM contains two parts, which are designated as ROM-A (for storing programs) and ROM-E (for issuing programs). A permanent type A storage device is a diode-transformer array that is stitched using address wires. This ROM section performs the main function. The filling depends on the material from which the ROM is made (perforating and magnetic tapes, punched cards, magnetic disks, drums, ferrite tips, dielectrics and their property of accumulation of electrostatic charges can be used).
Schematic structure of the ROM
This object of electronics is represented in the form of a device, which in appearance resembles the connection of a certain number of single-digit cells. The chip ROM, despite the potential complexity and, it would seem, significant opportunities, in size is small. When memorizing a specific bit, a seal is made to the case (when zero is written) or to a power source (when a unit is written). To increase the number of memory cells in permanent memory devices, the chips can be connected in parallel. So do the manufacturers to get a modern product, because a high-performance ROM chip allows them to be competitive in the market.
Memory volumes when used in different units of technology
The amount of memory varies depending on the type and purpose of the ROM. So in a simple home appliances like washing machines or refrigerators you can have enough installed microcontrollers (with their stock of several tens of kilobytes), and in rare cases something more complicated is installed. Use a large amount of ROM here is meaningless, because the amount of electronics is small, and the technology does not require complex calculations. For modern TVs, you need something more perfect. And the vertex of complexity is computer technology like computers and servers, ROMs for which, at least, can accommodate from several gigabytes (for released 15 years ago) to tens and hundreds of terabytes of information.
Masked ROM
In cases where the recording is carried out using a metallization process and a mask is used, such a read-only memory device is called masked. The addresses of memory cells in them are fed to 10 pins, and a specific chip is selected using a special CS signal. Programming of this type of ROM is carried out in factories, as a result, manufacturing in small and medium volumes is unprofitable and rather inconvenient. But in a large-scale production, they are the cheapest among all permanent storage devices, which provided them with popularity.
Schematically from the total mass differ in that in the memory matrix the conductor junctions are replaced by fusible bridges made of polycrystalline silicon. At the production stage, all jumpers are created, and the computer believes that logical units are recorded everywhere. But during the preparatory programming an increased voltage is applied, with the help of which logical units are left. When low voltages are applied, the jumpers evaporate and the computer reads that there is a logic zero. By this principle, programmable read-only memories operate.
Programmable read-only memories
EPROMs proved to be quite convenient in the process of manufacturing, so that they could be used for medium- and small-scale production. But such devices have their limitations - so, you can write down the program only once (due to the fact that the jumpers evaporate once and for all). Because of this impossibility to use a permanent storage device repeatedly, if it is written incorrectly, it must be discarded. As a result, the cost of all manufactured equipment increases. Due to the imperfection of the production cycle, this problem was very much occupied by the minds of the developers of memory devices. The way out of this situation was the development of ROM, which can be programmed anew many times.
ROM with ultraviolet or electric erasure
And they received such devices as "permanent memory with ultraviolet or electric erasure". They are created on the basis of the memory matrix, in which memory cells have a special structure. So, each cell is a MOSFET, in which the gate is made of polycrystalline silicon. It looks like the previous version, right? But the peculiarity of these ROMs is that silicon is additionally surrounded by a dielectric with wonderful insulating properties, silicon dioxide. The principle of operation here is based on the content of the induction charge, which can be stored for dozens of years. There are features for erasure. So, for the ultraviolet ROM device, ultraviolet rays coming from outside (ultraviolet lamp, etc.) need to hit. Obviously, from the point of view of simplicity, the operation of permanent memories with electric erasure is optimal, since for their activation it is necessary simply to apply voltage. The principle of electric erasure has been successfully implemented in such ROMs as flash drives, which can be seen by many.
But such a ROM scheme, with the exception of building a cell, does not differ structurally from a conventional masked permanent storage device. Sometimes such devices are also called reprogrammable. But with all the advantages, there are certain limits to the speed of erasing the information: this action usually takes about 10-30 minutes.
Despite the possibility of overwriting, reprogrammable devices have limitations on use. So, electronics with ultraviolet erasure can survive from 10 to 100 cycles of rewriting. Then the destructive effect of radiation becomes so noticeable that they cease to function. You can see the use of such elements as storage for BIOS programs, in video and sound cards, for additional ports. But the principle of electric erasure is optimal with respect to rewriting. So, the number of rewrites in ordinary devices is from 100 000 to 500 000! There are separate ROM-devices that can work and more, but to most users they do not need.