Any electronics are complex devices, the principle of operation of which is not clear to every average person. What is ROM and why is this device needed? 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 a few kilobytes on the simplest devices, which have only a single silicon chip, 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.

Using 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 the hard drive. Today they are in every computer. This element is 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 through the use of 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. For this purpose, magnetic tapes, magnetic disks, punched cards, drums, ferrite tips, dielectrics with their property of accumulating electrostatic charges can be used.

ROM: schematic structure

This electronics object is usually depicted as a device that resembles the connection of a number of single-digit 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 ROM 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, 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, technology is not required to perform complex calculations. Modern TVs may require something more complex. The pinnacle of ROM circuit complexity is found in computer hardware such as 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 ensured the popularity of this type of device. 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 has occupied the minds of developers for quite some 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 version. The peculiarity of these ROMs is that the silicon in this case 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 use, an electrically erasable ROM design would 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 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, but ordinary users have absolutely no need for such supernatural capabilities.

| Read Only Memory (ROM)

Intel 1702 EPROM chip with UV erase
Read-only memory (ROM)- non-volatile memory, used to store an array of immutable data.

Historical types of ROM

Read-only storage devices began to find application in technology long before the advent of computers and electronic devices. In particular, one of the first types of ROM was a cam roller, used in barrel organs, music boxes, and striking clocks.

With the development of electronic technology and computers, the need for high-speed ROMs arose. In the era of vacuum electronics, ROMs were used based on potentialoscopes, monoscopes, and beam lamps. In computers based on transistors, plug matrices were widely used as small-capacity ROMs. If it was necessary to store large amounts of data (for first-generation computers - several tens of kilobytes), ROMs based on ferrite rings were used (they should not be confused with similar types of RAM). It is from these types of ROM that the term “firmware” originates - the logical state of the cell was set by the direction of winding the wire surrounding the ring. Since a thin wire had to be pulled through a chain of ferrite rings, metal needles similar to sewing needles were used to perform this operation. And the operation of filling the ROM with information itself was reminiscent of the sewing process.

How does ROM work? Modern types of ROM

Very often, in various applications, it is necessary to store information that does not change during the operation of the device. This is information such as programs in microcontrollers, boot loaders and BIOS in computers, tables of digital filter coefficients in signal processors. Almost always this information is not required at the same time, so the simplest devices for storing permanent information can be built on multiplexers. The diagram of such a permanent storage device is shown in the following figure

Read-only memory circuit based on a multiplexer
In this circuit, a read-only memory device with eight single-bit cells is built. Storing a specific bit into a single-digit cell is done by soldering the wire to the power source (writing a one) or sealing the wire to the case (writing a zero). On circuit diagrams such a device is designated as shown in the figure

Designation of a permanent storage device on circuit diagrams
In order to increase the capacity of the ROM memory cell, these microcircuits can be connected in parallel (the outputs and recorded information naturally remain independent). The parallel connection diagram of single-bit ROMs is shown in the following figure

Multi-bit ROM circuit
In real ROMs, information is recorded using the last operation of chip 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 of a demultiplexer in addition to a multiplexer. This solution makes it possible to turn a one-dimensional storage structure into a multidimensional 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:

Mask read-only memory circuit
Mask ROMs are depicted in circuit diagrams as shown in the figure. The addresses of memory cells in this chip are supplied to pins A0 ... A9. The chip is selected by the CS signal. Using this signal, you can increase the volume of ROM (an example of using the CS signal is given in the discussion of RAM). The microcircuit is read using the RD signal.

Programming of the mask ROM is carried out at the manufacturer's factory, which is very inconvenient for small and medium-sized production batches, 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 present. For small and medium-sized production series of radio equipment, microcircuits have been developed that can be programmed in special devices - programmers. In these chips, the permanent connection of conductors in the memory matrix is ​​replaced by fusible links made of polycrystalline silicon. During the production of a microcircuit, all jumpers are made, which is equivalent to writing logical units to all memory cells. During the programming process, increased power is supplied to the power pins and outputs of the microcircuit. In this case, if the supply voltage (logical unit) is supplied to the output of the microcircuit, then no current will flow through the jumper and the jumper will remain intact. If a low voltage level is applied to the output of the microcircuit (connected to the case), then a current will flow through the jumper, which will evaporate this jumper and when the information is subsequently read from this cell, a logical zero will be read.

Such microcircuits are called programmable ROM (PROM) and are depicted on circuit diagrams as shown in the figure. As an example, we can name microcircuits 155PE3, 556PT4, 556PT8 and others.

Designation of programmable read-only memory on circuit diagrams
Programmable ROMs have proven to be very convenient for small- and medium-scale production. However, when developing radio-electronic devices, it is often necessary to change the program recorded in ROM. In this case, the EPROM cannot be reused, so once the ROM is written down, if there is an error or an intermediate program, it has to be thrown away, which naturally increases the cost of hardware development. To eliminate this drawback, another type of ROM was developed that could be erased and reprogrammed.

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

UV- and electrically erasable ROM memory cell
The cell is a MOS transistor in which the gate is made of polycrystalline silicon. Then, during the manufacturing process 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 microcircuit, 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 tunnel effect. After the programming voltage on the floating gate is removed, the induced charge remains and, therefore, the transistor remains in a conducting state. The charge on a floating gate can be stored for decades.

The block diagram of the read-only memory does not differ from the previously described mask ROM. The only thing that is used instead of a jumper is the cell described above. In reprogrammable ROMs, previously recorded information is erased using ultraviolet radiation. In order for this light to pass freely to the semiconductor crystal, a quartz glass window is built into the chip body.

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

The number of write-erase cycles of microcircuits ranges from 10 to 100 times, after which the microcircuit fails. This is due to the damaging effects of ultraviolet radiation. As an example of such microcircuits, we can name microcircuits of the 573 series of Russian production, microcircuits of the 27cXXX series of foreign production. These chips most often store BIOS programs for general purpose computers. Reprogrammable ROMs are depicted in circuit diagrams as shown in the figure

Designation of a reprogrammable read-only memory device on circuit diagrams
So, cases with a quartz window are very expensive, as well as the small number of write-erase cycles, which led to the search for ways to erase information from the EPROM electrically. There were many difficulties encountered along this path, which have now been practically resolved. Nowadays, chips with electrical erasure of information are quite widespread. As a storage cell, they use the same cells as in the ROM, but they are erased by electrical potential, so the number of write-erase cycles for these microcircuits reaches 1,000,000 times. The time to erase a memory cell in such microcircuits is reduced to 10 ms. The control circuit for such microcircuits turned out to be complex, so two directions for the development of these microcircuits have emerged:

1. -> EEPROM
2. -> FLASH – ROM

Electrically erasable PROMs are more expensive and smaller in volume, but they allow you to rewrite each memory cell separately. As a result, these microcircuits have a maximum number of write-erase cycles. The area of ​​application of electrically erasable ROM is the storage of data that should not be erased when the power is turned off. Such microcircuits include domestic microcircuits 573РР3, 558РР and foreign microcircuits of the 28cXX series. Electrically erasable ROMs are designated on the diagrams as shown in the figure.

Designation of electrically erasable read-only memory on circuit diagrams
Recently, there has been a tendency to reduce the size of EEPROM by reducing the number of external legs of the microcircuits. To do this, the address and data are transferred to and from the chip via a 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 microcircuits 558PPX.

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

When accessing a permanent storage device, you first need to set the address of the memory cell on the address bus, and then perform a read operation from the chip. This timing diagram is shown in the figure

Designation of FLASH memory on circuit diagrams
The arrows in the figure show the sequence in which control signals should be generated. In this figure, RD is the read signal, A is the cell address selection signals (since individual bits in the address bus can take on different values, transition paths to both the one and zero states are shown), D is the output information read from selected ROM cell.

Last file update date: 10/23/2009

Read Only Memory (ROM)

Very often, in various applications, it is necessary to store information that does not change during the operation of the device. This is information such as programs in microcontrollers, boot loaders (BIOS) in computers, tables of digital filter coefficients in , and , tables of sine and cosine 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 translated literature, permanent storage devices are called ROM (read only memory - read-only memory). The diagram of such a read-only memory (ROM) is shown in Figure 1.

Figure 1. Read-only memory (ROM) circuit built on a multiplexer

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

Figure 2. Designation of a permanent storage device on circuit diagrams

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

Figure 3. Multi-bit ROM circuit diagram

In real ROMs, information is recorded using the last operation of chip 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 of, in addition to a multiplexer, a . This solution makes it possible to turn a one-dimensional storage structure into a two-dimensional one and, thereby, significantly reduce the circuit volume required for the operation of the ROM circuit. This situation is illustrated by the following figure:

Figure 4. Masked read-only memory (ROM) circuit diagram

Mask ROMs are depicted in circuit diagrams as shown in Figure 5. The addresses of memory cells in this chip are supplied to pins A0 ... A9. The chip is selected by the CS signal. Using this signal, you can increase the volume of ROM (an example of using the CS signal is given in the discussion). The microcircuit is read using the RD signal.

Figure 5. Mask ROM (ROM) on circuit diagrams

Programming of the mask ROM is carried out at the manufacturer's factory, which is very inconvenient for small and medium-sized production batches, 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 present. For small and medium-sized series of radio equipment production, microcircuits were developed that can be programmed in special devices - programmers. In these ROMs, the permanent connection of conductors in the memory matrix is ​​replaced by fusible links made of polycrystalline silicon. During ROM production, all jumpers are made, which is equivalent to writing logical units to all ROM memory cells. During the ROM programming process, increased power is supplied to the power pins and outputs of the microcircuit. In this case, if the supply voltage (logical one) is supplied to the output of the ROM, then no current will flow through the jumper and the jumper will remain intact. If a low voltage level is applied to the output of the ROM (connected to the case), then a current will flow through the jumper of the memory matrix, which will evaporate it and when the information is subsequently read from this ROM cell, a logical zero will be read.

Such microcircuits are called programmable ROM (PROM) or PROM and are depicted on circuit diagrams as shown in Figure 6. As an example of PROM, we can name microcircuits 155PE3, 556RT4, 556RT8 and others.

Figure 6. Graphic designation of a programmable read-only memory (PROM) on circuit diagrams

Programmable ROMs have proven to be very convenient for small- and medium-scale production. However, when developing radio-electronic devices, it is often necessary to change the program recorded in ROM. In this case, the EPROM cannot be reused, so once the ROM is written down, if there is an error or an intermediate program, it has to be thrown away, which naturally increases the cost of hardware development. To eliminate this drawback, another type of ROM was developed that could be erased and reprogrammed.

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

Figure 7. UV- and electrically erasable ROM memory cell

The cell is a MOS transistor in which the gate is made of polycrystalline silicon. Then, during the manufacturing process of the chip, 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 the programming voltage is removed, the induced charge remains on the floating gate and hence the transistor remains in a conducting state. The charge on the floating gate of such a cell can be stored for decades.

The described read only memory does not differ from the previously described mask ROM. The only difference is that instead of a fusible jumper, the cell described above is used. This type of ROM is called reprogrammable read only memory (EPROM) or EPROM. In RPOM, previously recorded information is erased using ultraviolet radiation. In order for this light to pass freely to the semiconductor crystal, a quartz glass window is built into the housing of the ROM chip.

Figure 8. Appearance of an erasable read only memory (EPROM)

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

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) EEPROM 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 555RE4 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. These LSIs are manufactured with a full set of ES at all points of intersection 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 LSI RPOM with electrical erasure KM1609RR2A 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 software development and debugging systems, 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 of ROM

According to the terminology accepted 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 binary code arrives at the ROM input, 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 in this bit of the selected word (or number line) 1 is written. 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 that do not change during operation of a digital device. When a ROM address is applied to the input, the word corresponding to it appears at the output. In logic design, read-only 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 memories 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 inputs E. 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-named 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 relatively small amounts of memory, which is typical for many control centers ( for example, automation devices), expanding 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

Personal computers have four hierarchical memory levels:

microprocessor memory;

main memory;

register cache memory;

external memory.

Microprocessor memory is discussed above. Main memory is designed for storing and quickly exchanging information with other computer devices. Memory functions:

receiving information from other devices;

remembering information;

issuing information on request to other devices of the machine.

Main memory contains two types of storage devices:

ROM - read-only memory;

RAM is a random access memory device.

ROM is designed to store permanent program and reference information. Data is entered into ROM during manufacture. Information stored in ROM can only be read, but 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 you turn on the computer;

programs for controlling the display, keyboard, printer, external memory;

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

ROM is non-volatile memory; information is retained in it when the power is turned off.

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

The main advantages of RAM are its high speed and the ability to access each memory cell separately (direct 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 memory capacity 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 motherboard also has non-volatile CMOS memory, which is constantly powered by its battery. It stores computer configuration settings that are checked every time the system is turned on. This is a semi-permanent memory. To change computer configuration settings, the BIOS contains a computer configuration program - SETUP.

To speed up access to RAM, a special ultra-fast cache memory is used, which is located “between” the microprocessor and RAM; it stores copies of the most frequently used sections of RAM. Cache registers are not accessible to the user.

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

Microprocessors, starting from MP 80486, have their own built-in cache memory. Pentium and Pentium Pro microprocessors have cache memory separately for data and separately for instructions. All microprocessors can use additional cache memory located on the motherboard outside the microprocessor, the capacity of which can reach several MB. External memory refers to the external devices of a computer 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 memory devices - external storage devices - are very diverse. They can be classified by type of media, by type of design, by the principle of recording and reading information, by access method, etc.

The most common external storage devices are:

hard magnetic disk drives (HDD);

floppy magnetic disk drives (FMD);

optical disk drives (CD-ROM).

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

Disk drives are devices for reading and writing from magnetic or optical media. The purpose of these drives is to store large amounts of information, record and release stored information upon request into a random access memory device.

Hard disk drives and flat disk drives differ only in design, the volume of stored information and the time it takes to search, record and read information.

As a storage medium for magnetic disks, magnetic materials with special properties are used that make it possible to record two magnetic states - two directions of magnetization. Each of these states is assigned binary digits 0 and 1. Information on magnetic disks is written and read by magnetic heads along concentric circles - tracks (tracks). The number of tracks on a disk and their information capacity depend on the type of disk, drive design, quality of magnetic heads and magnetic coating. Each track is divided into sectors. One sector typically holds 512 bytes of data. Data exchange between the magnetic disk drive and RAM is carried out sequentially by 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 storage media. This means that the computer can access the track on which the section with the required information begins or where new information needs to be written, directly, wherever the drive’s recording and reading head is located.

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

Hard magnetic disk drives are called “hard drives”. This term arose 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 a Winchester hunting rifle. Hard disk storage capacity is measured in MB and GB.

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

In recent years, optical disk drives (CD-ROM) have become the most widespread. Due to their small size, high capacity and reliability, these drives are becoming increasingly popular. The capacity of optical disk drives is from 640 MB and above.

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 manufacturers with information already recorded on them. Recording information on them is possible only in laboratory conditions, outside of a computer.

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

access time;

speed of reading consecutive bytes.