Date of last update of the file 23.10.2009

Read only memory (ROM)

Very often, various applications require the storage of information that does not change during the operation of the device. This information such as programs in microcontrollers, boot loaders (BIOS) in computers, tables of coefficients of digital filters in, and, tables of sines and cosines in NCO and DDS. Almost always, this information is not required at the same time, so the simplest devices for storing permanent information (ROM) can be built on multiplexers. Sometimes in the translated literature, read-only memory is called ROM (read only memory). A diagram of such a read-only memory (ROM) is shown in Figure 1.

Figure 1. Schematic of read-only memory (ROM) built on a multiplexer

In this circuit, a read-only memory is built with eight one-bit cells. Memorizing a specific bit in a one-bit cell is done by sealing the wire to the power source (writing one) or sealing the wire to the body (writing zero). In schematic diagrams, such a device is designated as shown in Figure 2.

Figure 2. Designation of read-only memory on schematic diagrams

In order to increase the capacity of the ROM memory cell, these microcircuits can be connected in parallel (the outputs and the recorded information naturally remain independent). The scheme of parallel connection of one-bit ROMs is shown in Figure 3.

Figure 3. Scheme of multi-bit ROM (ROM)

In real ROMs, information is recorded using the last operation of the microcircuit production - metallization. Metallization is carried out using a mask, which is why such ROMs are called mask ROMs... Another difference between real microcircuits and the simplified model given above is the use, in addition to the multiplexer, also. Such a solution makes it possible to transform a one-dimensional storage structure into a two-dimensional one and, thereby, significantly reduce the volume of the circuit required for the operation of the ROM circuit. This situation is illustrated by the following figure:

Figure 4. Diagram of masked read only memory (ROM)

Masked ROMs are shown in schematic diagrams as shown in Figure 5. The addresses of memory cells in this microcircuit are fed to pins A0 ... A9. The microcircuit is selected by the CS signal. Using this signal, you can increase the amount of ROM (an example of using the CS signal is given in the discussion). Reading the microcircuit is performed by the RD signal.

Figure 5.Mask ROM (ROM) on schematic diagrams

The masked ROM is programmed at the manufacturing plant, which is very inconvenient for small and medium production series, not to mention the device development stage. Naturally, for large-scale production, mask ROMs are the cheapest type of ROM, and therefore are widely used at the present time. Microcircuits have been developed for small and medium series of radio equipment production, which can be programmed in special devices - programmers. In these ROMs, the permanent connection of the conductors in the memory matrix is ​​replaced by fusible links made of polycrystalline silicon. During the production of ROM, all jumpers are made, which is equivalent to writing logical units to all memory cells of the ROM. In the process of programming the ROM, increased power is supplied to the power pins and outputs of the microcircuit. In this case, if the supply voltage (logical unit) is applied to the ROM output, then the current will not flow through the jumper and the jumper will remain intact. If a low voltage level is applied to the ROM output (connected to the case), then a current will flow through the memory matrix jumper, which will evaporate it, and upon subsequent reading of information from this ROM cell, a logical zero will be read.

Such microcircuits are called programmable ROM (EPROM) or PROM and are depicted on schematic diagrams as shown in Figure 6. As an example of a PROM, one can name the chips 155PE3, 556PT4, 556PT8 and others.

Figure 6. Conventional graphic designation of programmable read only memory (PROM) on schematic diagrams

Programmable ROMs have proven to be very convenient for small and medium batch production. However, when developing electronic devices, it is often necessary to change the program written in ROM. In this case, the EPROM cannot be reused, therefore, once the written ROM has to be discarded with an erroneous or intermediate program, which naturally increases the cost of hardware development. To eliminate this drawback, another type of ROM was developed, which could be erased and re-programmed.

UV erasable ROM is built on the basis of a memory matrix built on memory cells, the internal structure of which is shown in the following figure:

Figure 7. Memory cell of ROM with ultraviolet and electric erasure

The cell is a MOSFET with a polysilicon gate. Then, during the manufacture of the microcircuit, this gate is oxidized and as a result it will be surrounded by silicon oxide - a dielectric with excellent insulating properties. In the described cell, with the ROM completely erased, there is no charge in the floating gate, and therefore the transistor does not conduct current. When programming the ROM, a high voltage is applied to the second gate, located above the floating gate, and charges are induced into the floating gate due to the tunneling effect. After removing the programming voltage, the induced charge remains on the floating gate, and therefore the transistor remains in a conducting state. The charge on the floating gate of such a cell can be stored for tens of years.

The described read-only memory does not differ from the previously described masked ROM. The only difference is that the cell described above is used instead of a fusible link. This type of ROM is called Reprogrammable Read Only Memory (EPROM) or EPROM. In the EPROM, the erasure of previously recorded information is carried out by ultraviolet radiation. In order for this light to pass unhindered to the semiconductor crystal, a quartz glass window is built into the ROM microcircuit case.

Figure 8. External view of erasable read-only memory (EPROM)

When the EPROM microcircuit is irradiated, the insulating properties of silicon oxide are lost, the accumulated charge from the floating gate flows into the semiconductor volume, and the transistor of the memory cell goes into a closed state. The erasing time of the RPZU chip ranges from 10 ... 30 minutes.

Very often, various applications require the storage of information that does not change during the operation of the device. This information such as programs in microcontrollers, boot loaders (BIOS) in computers, tables of coefficients of digital filters in signal processors, DDC and DUC, tables of sines and cosines in NCO and DDS. Almost always, this information is not required at the same time, so the simplest devices for storing permanent information (ROM) can be built on multiplexers. Sometimes in the translated literature, read-only memory is called ROM (read only memory). A diagram of such a read-only memory (ROM) is shown in Figure 3.1.

Figure 3.1. A read only memory (ROM) circuit based on a multiplexer.

In this circuit, a read-only memory is built with eight one-bit cells. Memorizing a specific bit in a one-bit cell is done by sealing the wire to the power source (writing one) or sealing the wire to the body (writing zero). In schematic diagrams, such a device is designated as shown in Figure 3.2.

Figure 3.2. Permanent storage designation on schematic diagrams.

In order to increase the capacity of the ROM memory cell, these microcircuits can be connected in parallel (the outputs and the recorded information naturally remain independent). The scheme of parallel connection of one-bit ROMs is shown in Figure 3.3.

Figure 3.3 Scheme of multi-bit ROM (ROM).

In real ROMs, information is recorded using the last operation of the microcircuit production - metallization. Metallization is carried out using a mask, therefore such ROMs are called mask ROMs. Another difference between real microcircuits and the simplified model given above is the use of a demultiplexer in addition to the multiplexer. This solution makes it possible to transform a one-dimensional storage structure into a two-dimensional one and, thereby, significantly reduce the volume of the decoder circuit required for the operation of the ROM circuit. This situation is illustrated by the following figure:

Figure 3.4. A masked read only memory (ROM) circuit.

Masked ROMs are shown in schematic diagrams as shown in Figure 3.5. The addresses of the memory cells in this microcircuit are fed to the pins A0 ... A9. The microcircuit is selected by the CS signal. Using this signal, you can increase the amount of ROM (an example of using the CS signal is given in the discussion of RAM). Reading the microcircuit is performed by the RD signal.

Figure 3.5. Conventional graphic designation of masked ROM (ROM) on schematic diagrams.

The masked ROM is programmed at the manufacturing plant, which is very inconvenient for small and medium production series, not to mention the device development stage. Naturally, for large-scale production, mask ROMs are the cheapest type of ROM, and therefore are widely used at the present time. Microcircuits have been developed for small and medium series of radio equipment production, which can be programmed in special devices - programmers. In these ROMs, the permanent connection of the conductors in the memory matrix is ​​replaced by fusible links made of polycrystalline silicon. During the production of ROM, all jumpers are made, which is equivalent to writing logical units to all memory cells of the ROM. In the process of programming the ROM, increased power is supplied to the power pins and outputs of the microcircuit. In this case, if the supply voltage (logical unit) is applied to the ROM output, then the current will not flow through the jumper and the jumper will remain intact. If a low voltage level is applied to the ROM output (connected to the case), then a current will flow through the memory matrix jumper, which will evaporate it, and upon subsequent reading of information from this ROM cell, a logical zero will be read.

Such microcircuits are called programmable ROM (EPROM) or PROM and are shown on schematic diagrams as shown in Figure 3.6. As an example of an EPROM, one can name the chips 155PE3, 556PT4, 556PT8 and others.

Figure 3.6. Conventional graphic designation of programmable read-only memory (PROM) on schematic diagrams.

Programmable ROMs have proven to be very convenient for small and medium batch production. However, when developing electronic devices, it is often necessary to change the program written in ROM. In this case, the EPROM cannot be reused, therefore, once the written ROM has to be discarded with an erroneous or intermediate program, which naturally increases the cost of hardware development. To eliminate this drawback, another type of ROM was developed, which could be erased and re-programmed.

The UV erasable ROM is built on the basis of a memory matrix built on memory cells, the internal structure of which is shown in the following figure:

Figure 3.7. Memory cell of ROM with ultraviolet and electric erasure.

The cell is a MOSFET with a polysilicon gate. Then, during the manufacture of the microcircuit, this gate is oxidized and as a result it will be surrounded by silicon oxide - a dielectric with excellent insulating properties. In the described cell, with the ROM completely erased, there is no charge in the floating gate, and therefore the transistor does not conduct current. When programming the ROM, a high voltage is applied to the second gate, located above the floating gate, and charges are induced into the floating gate due to the tunneling effect. After removing the programming voltage, the induced charge remains on the floating gate, and therefore the transistor remains in a conducting state. The charge on the floating gate of such a cell can be stored for tens of years.

The block diagram of the described read-only memory does not differ from the previously described masked ROM. The only difference is that the cell described above is used instead of a fusible link. This type of ROM is called Reprogrammable Read Only Memory (EPROM) or EPROM. In the EPROM, the erasure of previously recorded information is carried out by ultraviolet radiation. In order for this light to pass unhindered to the semiconductor crystal, a quartz glass window is built into the ROM microcircuit case.

When the EPROM microcircuit is irradiated, the insulating properties of silicon oxide are lost, the accumulated charge from the floating gate flows into the semiconductor volume, and the transistor of the memory cell goes into a closed state. The erasing time of the RPZU chip ranges from 10 to 30 minutes.

The number of write cycles - erasing EPROM chips is in the range from 10 to 100 times, after which the EPROM chip fails. This is due to the destructive effect of ultraviolet radiation on silicon oxide. As an example of EPROM microcircuits, one can name the 573 series of Russian-made microcircuits, and foreign-made 27cXXX microcircuits. The EPROM most often stores BIOS programs for general-purpose computers. EPROMs are depicted on schematic diagrams as shown in Figure 3.8.

Figure 3.8. Conditional graphic designation of EPROM on schematic diagrams.

Since cases with a quartz window are very expensive, as well as a small number of write-erase cycles, they have led to the search for ways to erase information from an EPROM electrically. On this path, many difficulties were encountered, which have practically been resolved by now. Nowadays, microcircuits with electric data erasure are quite widespread. As a memory cell, they use the same cells as in the EPROM, but they are erased by electrical potential, so the number of write-erase cycles for these microcircuits reaches 1,000,000 times. The erasing time of a memory cell in such ROMs is reduced to 10 ms. The control circuit for electrically erasable programmable ROMs turned out to be complex, therefore, there were two directions for the development of these microcircuits:

1. EEPROM (EEPROM) - electrically erasable programmable read-only memory

Electrically erasable EPROMs (EEPROM) are more expensive and smaller in volume, but they allow you to rewrite each memory cell separately. As a result, these microcircuits have the maximum number of write - erase cycles. The field of application of electrically erasable ROMs is storing data that should not be erased when the power is turned off. Such microcircuits include domestic microcircuits 573PP3, 558PP3 and foreign EEPROM microcircuits of the 28cXX series. Electrically Erasable ROMs are indicated on the schematic diagrams as shown in Figure 3.9.

Figure 9. Conventional graphic designation of electrically erasable read-only memory (EEPROM) on schematic diagrams.

Recently, there has been a tendency to reduce the size of the EEPROM by reducing the number of external outputs of microcircuits. For this, the address and data are transmitted to and from the microcircuit via the serial port. In this case, two types of serial ports are used - SPI port and I2C port (microcircuits 93cXX and 24cXX series, respectively). The foreign series 24cXX corresponds to the domestic series of 558PPX microcircuits.

FLASH - ROMs differ from EEPROMs in that erasure is performed not for each cell separately, but for the entire microcircuit as a whole or the block of the memory matrix of this microcircuit, as was done in the EPROM.

Figure 3.10. Conditional graphic designation of FLASH memory on schematic diagrams.

When accessing read-only memory, you first need to set the address of the memory cell on the address bus, and then perform a read operation from the microcircuit. This timing diagram is shown in Figure 3.11.

Figure 3.11. Timing diagrams of signals for reading information from ROM.

In Figure 3.11, the arrows show the sequence in which the control signals should be generated. In this figure, RD is the read signal, A is the signals for selecting the cell address (since individual bits in the address bus can take on different values, the paths of transition to both the single and zero state are shown), D is the output information read from selected ROM cell.

4. Perform the addition operation in two's complement, representing the given terms in binary form:

1) + 45 2) - 45

- 20 + 20

Solution:

1) x 1 = 45 = 0.101101 pr

x 2 = - 20 = 1.010100 pr = 1.101011 arr = 1.101100 add

+ 1,101100

Answer: 0.011001 pr = 25 10

2) x 1 = - 45 = 1.101101 pr

x 2 = 20 = 0.010100 pr

+ 0,010100

Answer: 1.100111 add = 1.011000 arr = 1.011001 pr = - 25 10

Question number 5.

Complete the following tasks:

1) write the logical function in SNDF;

2) minimize the logical function using Karnaugh maps;

Personal computers have four hierarchical memory levels:

microprocessor memory;

main memory;

register cache memory;

external memory.

Microprocessor memory is discussed above. The main memory is designed to store and quickly exchange information with other computer devices. Memory functions:

receiving information from other devices;

memorization of information;

delivery of information upon request to other devices of the machine.

The main memory contains two types of storage devices:

ROM - read only memory;

RAM is random access memory.

ROM is designed to store permanent program and reference information. Data in ROM is entered during manufacture. The information stored in the ROM can only be read, not changed.

The ROM contains:

processor control program;

computer startup and shutdown program;

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

control programs for display, keyboard, printer, external memory;

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

ROM is a non-volatile memory, when the power is turned off, the information is saved in it.

RAM is intended for operational recording, storage and reading of information (programs and data) directly involved in the information and computing process performed by a computer in the current period of time.

The main advantages of RAM are its high performance and the ability to access each memory cell separately (direct address memory access). All memory cells are combined into groups of 8 bits (1 byte), each such group has an address at which it can be accessed.

RAM is a volatile memory, when the power is turned off, the information in it is erased.

In modern computers, the amount of memory is usually 8-128 MB. Memory capacity is an important characteristic of a computer; it affects the speed and performance of programs.

In addition to ROM and RAM, the system board also contains non-volatile CMOS memory, which is constantly powered by its own battery. It stores computer configuration parameters that are checked each time the system is turned off. This is a semi-permanent memory. To change the configuration parameters of the computer, the BIOS contains the computer configuration program - SETUP.

To speed up access to RAM, a special super-fast cache memory is used, which is located, as it were, "between" the microprocessor and the RAM, it stores copies of the most frequently used sections of RAM. Cache registers are inaccessible to the user.

The cache memory stores data that the microprocessor received and will use in the next clock cycles of its work. Quick access to this data allows you to reduce the execution time of the next program commands.

Microprocessors, starting from MP 80486, have their own built-in cache memory. Pentium and Antium Pro microprocessors have separate caches for data and separately for instructions. For all microprocessors, additional cache memory located on the motherboard outside the microprocessor can be used, the capacity of which can be up to several MB. External memory refers to external computer devices and is used for long-term storage of any information that may be required to solve problems. In particular, all computer software is stored in external memory.

External storage devices - external storage devices - are very diverse. They can be classified by the type of media, by the type of construction, by the principle of writing and reading information, by the access method, etc.

The most common external storage devices are:

hard disk drives (HDD);

floppy disk drives (floppy disk drives);

optical disc drives (CD-ROM).

Less commonly, cassette tape storage devices - streamers - are used as external memory devices of a personal computer.

Disk drives are devices that read and write from magnetic or optical media. The purpose of these drives is to store large amounts of information, record and issue the stored information upon request to the random access memory.

HDD and HDD differ only structurally, the amount of stored information and the time of searching, recording and reading information.

As a storage medium for magnetic disks, magnetic materials with special properties are used, which make it possible to fix two magnetic states - two directions of magnetization. Binary digits 0 and 1 are assigned to each of these states. Information on magnetic disks is written and read by magnetic heads along concentric circles - tracks (tracks). The number of tracks on a disc and their information capacity depend on the type of disc, drive design, quality of magnetic heads and magnetic coating. Each track is divided into sectors. One sector usually contains 512 bytes of data. The exchange of data between the magnetic disk drive and the random access memory is carried out sequentially with an integer number of sectors. For a hard magnetic disk, the concept of a cylinder is also used - a set of tracks located at the same distance from the center of the disk.

Disks are classified as direct-access machine storage media. This means that the computer can refer to the track, on which the section with the required information begins, or where it is necessary to write new information, directly, wherever the read and write head of the drive is.

All disks - both magnetic and optical - are characterized by their diameter (form factor). Of the floppy magnetic disks, the most widespread are disks with a diameter of 3.5 (89 mm). The capacities of these drives are 1.2 and 1.44 MB.

Hard disk drives are called "hard drives". This term originated from the slang name for the first hard drive model, which had 30 tracks of 30 sectors each, which coincidentally coincided with the caliber of the Winchester hunting rifle. Hard disk drive capacity is measured in MB and GB.

Recently, new magnetic disk drives - ZIP-disk - portable devices with a capacity of 230-280 MB have appeared.

In recent years, optical disk drives (CD-ROMs) have become the most widespread. Small size, high capacity and reliability make these drives more and more popular. Capacities for optical drives are 640 MB and up.

Optical discs are divided into non-rewritable laser-optical discs, rewritable laser-optical discs, and rewritable magneto-optical discs. Non-rewritable discs are supplied by the manufacturers with the information already recorded on them. Recording information on them is possible only in laboratory conditions, outside the computer.

In addition to its main characteristic - information capacity, disk drives are also characterized by two time indicators:

access time;

the speed of reading consecutive bytes.

Good day.

If you want to fill the knowledge gap as to what a ROM is, then you've come to the right place. In our blog, you can read capacious information about this in a language accessible to an ordinary user.

Decoding and explanation

ROM letters are capitalized in read only memory. It can also be called “ROM” with equal rights. The English abbreviation stands for Read Only Memory, and translates 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 unchangeable data laid down by the developer during the manufacture of equipment, that is, those without which its operation is impossible.
• Secondly, the term "non-volatile" indicates that when the system is rebooted, data from it does not disappear anywhere, in contrast to the way it happens with RAM.

It is possible to erase information from such a device only by special methods, for example, ultraviolet rays.

Examples of

Permanent memory in a computer is a specific place on the motherboard that stores:

• Test utilities that verify that the hardware is working properly every time the PC is started.
• Drivers for controlling the main peripheral devices (keyboard, monitor, disk drive). In turn, those slots on the motherboard, the function of which does not include turning on the computer, do not store their utilities in ROM. After all, the place is limited.
• Bootstrap run (BIOS), which, when the computer is turned on, starts the operating system loader. Although the current BIOS can include 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 melodies. If desired, the space for additional multimedia information is expanded with rewritable SD cards. However, if the device is used only for calls, there is no need to expand the memory.

In general, nowadays ROM is found in all household appliances, car players and other devices with electronics.

Physical execution

To get you better acquainted with persistent memory, I'll tell you more about its configuration and properties:

• Physically, it is a microcircuit with a readout crystal, if included with 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, which is stitched with address wires. Serves for storing programs. The second is for issuing them.
• Schematically consists of several one-bit cells. When writing a certain bit of data, it is sealed to the body (zero) or to the power source (one). In modern devices, circuits are connected in parallel to increase the capacity of the cells.
• The amount of memory varies from a few kilobytes to terabytes, depending on which device it is applied to.

Views

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 independently flash the device once.

• Two more letters in front of them hide the wording "electrically erasable" (electrically erasable). Such ROMs can be rewritten as much as you like. Flash memory belongs to this type.

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

I will be glad if you subscribe to updates and visit more often.

Read only memory (ROM) are designed for permanent, non-volatile storage of information.

By recording method ROM classified as follows:

1. once programmed by a mask at the manufacturer;
2. once programmable by the user using special devices called programmers - EPROM ;
3. reprogrammable, or reprogrammable ROM - RPZU.

Mask ROMs

Programming mask ROMs occurs during the LSI manufacturing process. Usually on a semiconductor crystal, all storage elements (ZE), and then, at the final technological operations, using a photomask of the switching layer, connections between the address, data lines and the actual storage element are realized. This template (mask) is made in accordance with the wishes of the customer according to the order cards. The list of possible options for order cards is given in the technical specifications for the IC ROM... Such ROM are made on the basis of matrixes of diodes, bipolar or MOS transistors.

Diode Array Masked ROMs

The scheme of such ROM is shown in Fig. 12.1. Here, the horizontal lines are address lines, and the vertical lines are data lines; in this case, 8-bit binary numbers are removed from them. In this diagram, ZE is the conditional intersection of the address line and the data line. The selection of the entire line of ZE is made when a logical zero is applied to the address line LA i from the corresponding output of the decoder. A logical 0 is written to the selected ZE in the presence of a diode at the intersection of the line D i and LA i, because in this case, the circuit closes: + 5 V, diode, ground on the address line. So, in this ROM when address 11 2 is given, an active zero signal appears on the address line LA 3, it will have a logic level of 0, on the data bus D 7 D 0 information will appear 01100011 2.

MOSFET masked ROMs

An example of a circuit of this ROM is shown in Fig. 12.2. Information is recorded by connecting or not connecting a MOS transistor at the appropriate points of the LSI. When selecting a specific address on the corresponding address line LA i, an active signal of logical 1 appears, i.e. potential close to the potential of the power supply + 5 V. This logic 1 is fed to the gates of all line transistors and opens them. If the drain of the transistor is metallized, on the corresponding data line D i, a potential of the order of 0.2-0.3 V appears, i.e. logic level 0. If the drain of the transistor is not metallized, the indicated circuit is not implemented, there will be no voltage drop across the resistance R i, i.e. at the point D i there will be a potential of +5 V, i.e. logical level 1. For example, if shown in fig. 12.2 ROM to the address apply the code 01 2, on the address line LA 1 will be active level 1, and on the data bus D 3 D 0 would be code 0010 2.

Masked ROMs based on a matrix of bipolar transistors

An example of a schema for this ROM is shown in Fig. 12.3. Information recording is also carried out by metallization or non-metallization of the area between the base and the address line. To select a line of ZE per line of address LA i is fed to logic 1. During metallization, it is fed to the base of the transistor, it opens due to the potential difference between the emitter (ground) and the base (approximately + 5 V). This closes the circuit: + 5 V; resistance R i; open transistor, ground at the emitter of the transistor. At the point D i in this case there will be a potential corresponding to the voltage drop across the open transistor - about 0.4 V, i.e. logical 0. Thus, zero is written in the ZE. If the section between the address line and the base of the transistor is not metallized, the indicated electrical circuit is not implemented, the voltage drop across the resistance R i is not, therefore, on the corresponding data line D i there will be a potential of +5 V, i.e. logical 1. When giving, for example, address 00 2 in the one shown in fig. 12.3 ROM code 10 appears on the stepper motor 2.

Examples of mask ROMs are shown in Fig. 12.4, and in table. 12.1 - their parameters.

Table 12.1. Mask ROM parameters

LSI designation	Manufacturing technology	Information capacity, bit	Sample time, ns
505RE3	pMOS	512x8	1500
K555RE4	TTLSh	2Kx8	800
K568RE1	nMOS	2Kx8	120
K596RE1	TTL	8Kx8	350

Programmable ROM

Programmable ROM (EPROM) are the same diode or transistor matrices as the masked ROMs, but with a different version of the ZE. Memory element EPROM is shown in Fig. 12.5. Access to it is provided by applying a logical 0 to the address line LA i. Writing to it is made as a result of deposition (melting) of PV fusible links connected in series with diodes, emitters of bipolar transistors, drains of MOS transistors. The PV fusible insert is a small metallization area that is destroyed (melted) when programmed by current pulses of 50-100 microamperes and a duration of about 2 milliseconds. If the insert is saved, then a logical 0 is written in the ZE, since a circuit is implemented between the power supply and ground on LA i through a diode (in transistor matrices - through an open transistor). If the insertion is destroyed, then the specified chain is written and logical 1 is written in the ZE.