Types of ROM. Read Only Memory (ROM)

Read-only memory (ROM)– A memory designed to store immutable information (programs, constants, table functions). In the process of solving problems, the ROM allows only reading information. As a typical example of the use of ROM, we can point out LSI ROM used in PCs to store BIOS (Basic Input Output System).

In the general case, a ROM storage device (an array of its storage cells) with a capacity of EPROM words, a length of r+ 1 digits each, usually a system of horizontal (address) EPROMS and r+ 1 vertical (discharge) conductors, which at the intersection points can be connected by coupling elements (Fig. 1.46). Communication elements (EC) are fuse-links or p-n-transitions. The presence of an element of connection between j-th horizontal and i th vertical conductors means that in i-th digit of memory cell number j one is written, the absence of ES means that zero is written here. Writing a word to cell number j ROM is produced by proper arrangement of communication elements between the bit conductors and the address wire number j. Reading a word from cell number j The ROM goes like this.

Rice. 1.46. ROM storage with a capacity of EPROM words, a length of r+ 1 digits each

Address code A = j is deciphered, and on the horizontal conductor the number j The drive is supplied with voltage from the power source. Those of the bit conductors that are connected to the selected address conductor by communication elements are energized U 1 level unit, the remaining discharge conductors remain energized U 0 level zero. Set of signals U 0 and U 1 on the bit conductors and forms the contents of the PL number j, namely the word at the address A.

Currently, ROMs are built from LSI ROMs that use semiconductor ES. LSI ROM is usually divided into three classes:

– mask (MPZU);

– programmable (PROM);

– reprogrammable (RPM).

Mask ROMs(ROM - from Read Only Memory) - ROM into which information is written from a photomask during the process of growing a crystal. For example, LSI ROM 555PE4 with a capacity of 2 kbytes is a character generator using the KOI-8 code. The advantage of mask ROMs is their high reliability, but the disadvantage is their low manufacturability.

Programmable ROMs(PROM - Programmable ROM) - ROM, information into which is written by the user using special devices– programmers. LSI data is manufactured with full set ES at all intersection points of address and bit conductors. This increases the manufacturability of such LSIs, and hence their mass production and use. Recording (programming) of information in EEPROM is carried out by the user at the place of their use. This is done by burning out the communication elements at those points where zeros should be written. Let's point out, for example, the TTLSH-BIS PROM 556RT5 with a capacity of 0.5 kbytes. The reliability of EPROM LSIs is lower than that of masked LSIs. Before programming, they must be tested for the presence of ES.

In MPOM and PROM it is impossible to change the contents of their PL. Flashable ROMs(RPM) allow multiple changes of the information stored in them. In fact, RPOM is RAM in which t Salary>> t Thurs. Replacing the contents of the ROM begins with erasing the information stored in it. ROMs with electrical (EEPROM) and ultraviolet (UVEPROM) erasure of information are available. For example, the KM1609RR2A LSI RPOM with electrical erasure with a capacity of 8 kbytes can be reprogrammed at least 104 times, stores information for at least 15,000 hours (about two years) in the on state and at least 10 years in the off state. LSI RPOM with ultraviolet erasure K573RF4A with a capacity of 8 kbytes allows for at least 25 rewrite cycles, stores information in the on state for at least 25,000 hours, and in the off state for at least 100,000 hours.

The main purpose of RPOMs is to use them instead of ROMs in development and debugging systems software, microprocessor systems and others when it is necessary to make changes to programs from time to time.

The operation of a ROM can be considered as a one-to-one conversion N-bit address code A V n-bit code of the word read from it, i.e. ROM is a code converter (digital machine without memory).

In Fig. Figure 1.47 shows a conventional image of a ROM in the diagrams.

Rice. 1.47. Conditional ROM image

The functional diagram of the ROM is shown in Fig. 1.48.

Rice. 1.48. Functional diagram ROM

According to the terminology adopted among storage device specialists, the input code is called an address, 2 n vertical buses - number lines, m outputs - by bits of the stored word. When any ROM enters the input binary code One of the number lines is always selected. In this case, at the output of those OR elements whose connection with a given number line is not destroyed, 1 appears. This means that 1 is written in this bit of the selected word (or number line). At the outputs of those bits whose connection with the selected number line is burned out, zeros will remain. The programming law can also be inverse.

Thus, ROM is a functional unit with n entrances and m outputs storing 2 n m-bit words, which when working digital device do not change. When a ROM address is applied to the input, the word corresponding to it appears at the output. At logical design permanent storage is considered either as a memory with a fixed set of words, or as a code converter.

In the diagrams (see Fig. 1.47), ROM is designated as ROM. Read-only memory devices usually have an E enable input. When the E input level is active, the ROM performs its functions. If there is no resolution, the outputs of the microcircuit are inactive. There can be several enabling inputs, then the microcircuit is unlocked when the signals at these inputs match. In ROM, the E signal is often called reading CT (read), selecting a VM chip, selecting a VC crystal (chip select - CS).

ROM chips are expandable. To increase the number of bits of stored words, all inputs of the microcircuits are connected in parallel (Fig. 1.49, A), and from the increased total number of outputs, the output word is removed according to the increased bit depth.

To increase the number of stored words themselves (Fig. 1.49, b) the address inputs of the microcircuits are switched on in parallel and are considered as the low-order bits of the new, extended address. The added high-order bits of the new address are sent to the decoder, which selects one of the microcircuits using the E inputs. With a small number of microcircuits, decoding of the most significant bits can be done on the conjunction of the enabling inputs of the ROMs themselves. The outputs of the same bits must be combined using OR functions as the number of stored words increases. Special OR elements are not required if the outputs of the ROM chips are made either according to an open collector circuit for combining using the wiring OR method, or according to a three-state buffer circuit, allowing direct physical combining of the outputs.

The outputs of ROM chips are usually inverse, and input E is often inverted. Increasing the ROM may require the introduction of buffer amplifiers to increase the load capacity of some signal sources, taking into account the additional delays introduced by these amplifiers, but in general, with a relatively small volumes memory, which is typical for many control centers (for example, automation devices), increasing ROM usually does not give rise to fundamental problems.

Rice. 1.49. Increasing the number of bits of stored words when microcircuit inputs are connected in parallel and increasing the number of stored words when microcircuit address inputs are connected in parallel

Any electronics are complex devices, the principle of operation of which is not clear to every average person. What is ROM and why is it necessary? this device? Most users today cannot answer this question. Let's try to fix this situation.

What is ROM?

What are ROMs and where can they be used? Read-only storage devices are the so-called non-volatile memory. Purely technically, these devices are implemented in the form of microcircuits. At the same time, we learned what the abbreviation ROM stands for. Such chips are designed to store information entered by the user, as well as installed programs. In ROM you can find everything from documents to pictures. Information on this chip is stored for several months or even years.

Depending on the device used, memory sizes can vary from several kilobytes to very simple devices, which have just one silicon chip, up to terabytes. The larger the permanent storage capacity, the more objects it can store. The volume of the chip is directly proportional to the amount of data. If we try to more succinctly answer the question of what ROM is, we can say the following: it is a storage of information that does not depend on constant voltage.

Usage hard drives as ROM

So, we have already answered the question of what ROM is. Now let's talk about what ROMs can be. The main storage device in any computer is HDD. Today they are in every computer. This item used due to its wide data storage capabilities. At the same time, there are also a number of ROMs that use multiplexers in their device. These are special microcontrollers, bootloaders and other electronic mechanisms. Upon closer examination, you need to not only understand the meaning of the ROM abbreviation. To understand the topic, you need to decipher other terms.

Addition and expansion of ROM capabilities through the use of flash technologies

If the user does not have enough standard memory capacity, then you can try to take advantage of the expanded information storage capabilities provided by the ROM. This is done by using modern technologies, which are implemented in USB drives and memory cards. These technologies are based on the principle of reusable use. To put it simply, information on such media can be erased and recorded again. A similar operation can be performed tens and hundreds of thousands of times.

What does ROM consist of?

The ROM consists of two parts, which are designated as ROM-A and ROM-E. ROM-A is used to store programs, and ROM-E is used to issue programs. Type A ROM is a diode-transformer matrix, which is flashed using address wires. This section of the ROM performs the main function. The filling will depend on the material used in the manufacture of the ROM. Magnetic tapes can be used for this. magnetic disks, punched cards, drums, ferrite tips, dielectrics with their property of accumulating electrostatic charges.

ROM: schematic structure

This electronics object is usually depicted as a device that resembles the connection of a number of single-bit cells. Despite its potential complexity, the ROM chip is very small in size. When storing a certain bit of information, it is sealed to the case (recording a zero) or to the power source (recording a one). To increase the capacity of memory cells, circuits in permanent storage devices can be connected in parallel. This is exactly what manufacturers do in order to obtain a modern product. After all, when using ROMs with high technical characteristics the device will be competitive in the market.

Amount of memory used in various units of equipment

The amount of memory may depend on the type and purpose of the ROM. In simple household appliances like refrigerators or washing machines The installed microcontrollers will be quite sufficient. Something more complex is installed in rare cases. There is no point in using more ROM here. The amount of electronics is quite small. In addition, the equipment is not required to perform complex calculations. For modern TVs something more complex may be required. The pinnacle of complexity of ROM circuits is Computer Engineering like servers and personal computers. In this technique, ROMs contain from several gigabytes to hundreds of terabytes of information.

Mask ROM

If the recording is done when the recording is done using the metallization process and a mask is used, then such a ROM will be called a mask ROM. In them, the addresses of memory cells are supplied to ten pins. A specific chip is selected using a special CS signal. ROMs of this type are programmed at factories. Therefore, producing them in medium and small volumes is inconvenient and unprofitable. However, in large-scale production, such devices will be the cheapest of the ROMs.

This is what made it popular of this type devices. From the point of view of the circuit design, such ROMs differ from the general mass in that the connections in the memory matrix are replaced with fusible jumpers, which are made of polycrystalline silicon. At the production stage, all jumpers are created. The computer believes that logical ones are written everywhere. However, during pre-programming, increased voltage is applied.

Using it, logical units are left. The jumpers evaporate when low voltages are applied. The computer believes that a logical zero is written there. The same principle is used in programmable read only memory devices. Programmable ROMs or PROMs have proven to be quite convenient from a technological manufacturing point of view. They can be used in both medium and small-scale production. However, these devices also have their limitations. You can only record a program once, after which the jumpers disappear forever.

Due to the inability to reuse the ROM. If you make a mistake, you have to throw it away. As a result, the cost of all manufactured equipment increases. Due to imperfections in the production cycle. This problem occupied the minds of developers for quite a long time. As a way out of this situation, it was decided to develop a ROM that can be programmed many times.

Electrically or ultraviolet erasable ROM

Such devices are created on the basis of a memory matrix, in which memory cells have a special structure. Each cell here is a MOS transistor, the gate of which is made of polycrystalline silicon. Somewhat reminiscent of the previous option. The peculiarity of these ROMs is that the silicon in in this case it is additionally surrounded by a dielectric, which has insulating properties. Silicon dioxide is used as a dielectric.

Here the operating principle is based on the content of the inductive charge. It can be stored for decades. There are some issues with erasing here. For example, an ultraviolet ROM device requires exposure to UV rays from the outside, for example, from an ultraviolet lamp. Of course, from the point of view of ease of operation, the electrical erase ROM design will be the best option. In this case, to activate you just need to apply voltage. This principle of electrical erasure has been successfully implemented in devices such as flash drives. However, such a ROM circuit is structurally no different from a conventional mask ROM with the exception of the cell structure.

Such devices are sometimes also called reprogrammable. However, with all the advantages of devices of this type, there are certain limits to the speed of erasing information. Typically, this operation takes from 10 to 30 minutes to complete. Despite the ability to rewrite, reprogrammable devices have limitations on their use. UV erasable electronics can survive 10 to 100 write cycles. After this, the destructive influence of ultraviolet radiation will become so noticeable that the device will cease to function.

Such elements can be used to store BIOS programs in the video and sound cards for additional ports. Regarding the possibility of rewriting, the principle of electrical erasure will be optimal. The number of rewrites in such devices ranges from 100 to 500 thousand. Of course, you can find devices that can do more, however ordinary users such supernatural possibilities are absolutely useless.

Good day.

If you're looking to fill the knowledge gap regarding what a ROM is, you've come to the right place. In our blog you can read comprehensive information about this in a language accessible to the common user.

Decoding and explanation

The ROM letters are capitalized in the wording "read only memory". It can also be equally called “ROM”. The English abbreviation stands for Read Only Memory, and is translated as read-only memory.

These two names reveal the essence of the subject of our conversation. This is a non-volatile type of memory that can only be read. What does it mean?

• Firstly, it stores immutable data laid down by the developer during the manufacture of the equipment, that is, those without which its operation is impossible.
• Secondly, the term “non-volatile” indicates that when the system is rebooted, the data does not disappear from it, unlike what happens with RAM.

You can only erase information from such a device special methods, for example, ultraviolet rays.

Examples

Read-only memory in a computer is a specific location on motherboard, which stores:

• Test utilities that check the correct operation of the hardware every time you start the PC.
• Drivers for controlling main peripheral devices (keyboard, monitor, disk drive). In turn, those slots on the motherboard whose functions do not include turning on the computer do not store their utilities in ROM. After all, space is limited.
• Progu bootstrap(BIOS), which launches the boot loader when the computer is turned on operating system. Although the current BIOS can turn on a PC not only from optical and magnetic disks, but also from USB drives.

IN mobile gadgets permanent memory stores standard applications, themes, pictures and ringtones. If desired, the space for additional multimedia information can be expanded using rewritable SD cards. However, if the device is used only for calls, there is no need to expand the memory.

In general, now ROM is found in any household appliances, car players and other electronic devices.

Physical execution

So that you can get to know each other better permanent memory, I’ll tell you more about its configuration and properties:

• Physically it is a microcircuit with a reading crystal, if included in a computer, for example. But there are also independent data arrays (CD, gramophone record, barcode, etc.).
• ROM consists of two parts “A” and “E”. The first is a diode-transformer matrix, stitched using address wires. Used to store programs. The second is intended for issuing them.
• Schematically it consists of several single-digit cells. When a specific bit of data is written, a seal is made to the case (zero) or to the power supply (one). IN modern devices circuits are connected in parallel to increase the capacity of cells.
• Memory capacity varies from a few kilobytes to terabytes, depending on which device it is applied to.

Kinds

There are several types of ROM, but in order not to waste your time, I will name only two main modifications:

• The first letter adds the word “programmable”. This means that the user can flash the device himself once.

• Two more letters in front hide the wording “electrically erasable”. Such ROMs can be rewritten as much as you like. Flash memory belongs to this type.

In principle, this is all I wanted to convey to you today.

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All read only memory devices (ROM) can be divided into the following groups:

● programmable at manufacture (designated as ROM or ROM);

● with one-time programming, allowing the user to electrically change the state of the memory matrix once given program(designated as PROM or PROM);

● reprogrammable (reprogrammable), with the possibility of multiple electrical reprogramming, with electrical or ultraviolet erasure of information (referred to as RPROM or RPROM).

To provide the ability to combine outputs when expanding memory, all ROMs have tri-state outputs or open collector outputs.

(xtypo_quote) In EEPROM, the drive is built on storage cells with fusible links made of nichrome or other refractory materials. The recording process consists of selectively burning out fusible links. (/xtypo_quote)
In ROM, storage cells are built on the basis of MOS technologies. Various are used physical phenomena charge storage at the boundary between two different dielectric media or a conducting and dielectric media.

In the first case, the dielectric under the gate of the MOS transistor is made of two layers: silicon nitride and silicon dioxide (SiN 4 - SiO 2). It was found that in the complex structure of SiN 4 - SiO 2, when changing electrical voltage Charge hysteresis occurs at the interface between the two layers, which makes it possible to create memory cells.

In the second case, the basis of the memory cell is an avalanche injection MOSFET transistor with a floating gate (AFL MOS). The simplified structure of such a transistor is shown in Fig. 3.77.
In an avalanche injection transistor with a floating gate, at a sufficiently high drain voltage, a reversible avalanche breakdown of the dielectric occurs, and charge carriers are injected into the floating gate region. Since the floating gate is surrounded by a dielectric, the leakage current is small and information storage is ensured for a long period of time (tens of years). When voltage is applied to the main gate, the charge is dissolved due to the tunnel effect, i.e. erasing information.

Here are some characteristics of the ROM (Table 3.1).

Industry produces a large number of ROM chips. Let's take two ROM chips as an example (Fig. 3.78).

The following designations are used in the diagrams: A i - address inputs; D i — information outputs; CS—chip selection; CE - exit permission.

The K573RF5 chip is a reprogrammable ROM (RPM) with ultraviolet erasure, having a 2Kx8 structure. In terms of input and output, this microcircuit is compatible with TTL structures. The K556RT5 chip is a one-time programmable ROM, made on the basis of TTLSH structures, input and output compatible with TTL structures, having a 512-bit x8 structure.

ROM- fast, non-volatile memory, which is intended only for reading. Information is entered into it once (usually at the factory) and stored permanently (when the computer is turned on and off). ROM stores information that is constantly needed on the computer. A set of programs located in ROM forms the basic input/output system BIOS (Basic Input Output System). BIOS (Basic Input Output System - basic system input-output) - a set of programs designed for automatic testing of devices after turning on the computer power and loading the operating system into RAM.

The ROM contains:

Test programs that check the correct operation of its units every time the computer is turned on;

Programs for controlling basic peripheral devices - disk drive, monitor, keyboard;

Information about where the operating system is located on the disk.

ROM types:

ROM with mask programming, this is a memory in which information is written once and for all during the manufacturing process of semiconductor integrated circuits. Read-only storage devices are used only in cases where we're talking about about mass production, because Manufacturing masks for integrated circuits for private use is quite expensive.

PROM(programmable read-only memory).

ROM programming is a one-time operation, i.e. information once recorded in the PROM cannot be subsequently changed.

EPROM(erasable programmable read-only memory). When working with it, the user can program it and then erase the recorded information.

EIPZU(electrically variable read only memory). Its programming and modification are carried out using electrical means. Unlike EPROM, no special external devices are required to erase information stored in EPROM.

Visually, RAM and ROM can be imagined as an array of cells into which individual bytes of information are written. Each cell has its own number, and the numbering starts from zero. The cell number is the byte address.

CPU when working with RAM, it must indicate the address of the byte that it wants to read from memory or write to memory. Of course, you can only read data from ROM. The processor writes data read from RAM or ROM into its internal memory, which is structured similarly to RAM, but operates much faster and has a capacity of no more than tens of bytes.

The processor can only process data that is in its internal memory, RAM or ROM. All of these types of memory devices are called internal memory devices, and they are usually located directly on the computer's motherboard ( inner memory processor is located in the processor itself).

Cache memory. Data exchange within the processor is much faster than data exchange between the processor and RAM. Therefore, in order to reduce the number of calls to random access memory, so-called super-RAM or cache memory is created inside the processor. When the processor needs data, it first accesses the cache memory, and only when there is no necessary data there does it access the RAM. The larger the cache, the more likely it is that the data you need is there. Therefore, high-performance processors have larger cache sizes.

There are L1 caches(runs on the same chip with the processor and has a volume of the order of several tens of kilobytes), second level (performed on a separate chip, but within the boundaries of the processor, with a volume of one hundred or more KB) and the third level (performed on separate high-speed chips located on the motherboard and with a volume of one or more MB).

During operation, the processor processes data located in its registers, RAM and external processor ports. Some of the data is interpreted as data itself, some of the data is interpreted as address data, and some is interpreted as commands. The set of various instructions that a processor can execute on data forms the processor instruction system. The larger the processor's instruction set, the more complex its architecture, the longer the commands are written in bytes, and the longer the average instruction execution time.