What is ROM memory? Read-only storage devices. Main characteristics, scope. Schematic structure of ROM

PERMANENT MEMORY (ROM)

There is a type of memory that stores data without electrical current, namely ROM (Read Only Memory), or sometimes it is called non-volatile memory, used to store system and additional programs intended for constant use by the microprocessor, which does not allow changing or erasing information.

ROM (read-only memory) is a chip on the motherboard that contains programs and data recorded during the manufacture of the computer and used for internal testing of devices after the computer is turned on and the operating system is loaded into RAM. The set of these microprograms is called BIOS (Basic Input-Output System) - the basic input-output system. The BIOS contains the computer configuration setup program (SETUP). It allows you to set some characteristics of the computer devices (type of video controller, hard drives and floppy drives, often also modes of working with RAM, asking for a password at boot).

Data is written to ROM during production. To do this, a stencil with a certain set of bits is made, which is applied to the photosensitive material, and then parts of the surface are etched.

There are:

PROMs (Programmable ROMs) were developed in the late 70s by a company called Texas Instruments. In other words, under operating conditions it is possible to program. Such ROMs usually contain an array of tiny jumpers. In which it is possible to burn a specific jumper by selecting the desired row and column, and then apply high voltage to a specific pin of the microcircuit.

EPROM (erasable programmable ROM) allows, when using a special device, programming under operating conditions and erasing information. To do this, the chip is exposed to strong ultraviolet light with a certain wavelength for 15 minutes.

EEPROM (Electronically Ready Programmed ROM), also an erasable EPROM, but unlike EPROMs, they can be reprogrammed by applying pulses and do not require special additional devices. But they work 10 times slower with much less capacity and are more expensive.

Flash memory is erased and written in blocks. It is produced on printed circuit boards and has a capacity of up to several tens of megabytes.

Modules and ROM cartridges installed on the PC motherboard have a capacity, as a rule, not exceeding 128 KB. The performance of permanent memory is lower than that of random access memory, therefore, to increase performance, the contents of the ROM are copied into RAM, and only this copy, also called shadow ROM, is directly used during operation.

“Currently, PCs use “semi-permanent”, reprogrammable storage devices - flash memory. Flash memory modules, or cards, can be installed directly into the motherboard connectors and have the following parameters: capacity up to 512 MB (BIOS ROM uses up to 128 KB), read access time 0.035 -- 0.2 μs, write time per byte 2 -- 10 µs. Flash memory is a non-volatile storage device. An example of such memory is NVRAM -- Non Volatile RAM with a write speed of 500 KB/s. Typically, to rewrite information, it is necessary to apply a programming voltage (12 V) to a special flash memory input, which eliminates the possibility of accidentally erasing information. Flash memory reprogramming can be performed directly from a floppy disk or from a PC keyboard if a special controller is available, or from an external programmer connected to a PC. Flash memory can be very useful both for creating very high-speed, compact, alternative NMD storage devices - “solid-state drives”, and for replacing ROM that stores BIOS programs, allowing you to update and replace these programs with newer ones directly from a “floppy disk”. version when upgrading a PC" [Electronic resource] URL: http://library.tuit.uz/skanir_knigi/book/vich_sistemi/viches_sist_2.htm (Date of access: 05/15/2013)..

Comparative characteristics of RAM and ROM

Table 2 Comparative characteristics.

“Physically, to build a RAM-type memory device, dynamic and static memory chips are used, for which saving a bit of information means saving an electrical charge (this explains the volatility of all RAM, that is, the loss of all information stored in it when the computer is turned off).

RAM is physically executed on dynamic RAM elements, and to coordinate the operation of relatively slow devices (in our case, dynamic RAM) with a relatively fast microprocessor, a functionally designed cache memory built from static RAM cells is used. Thus, computers contain both types of RAM simultaneously. Physically, external cache memory is also implemented in the form of microcircuits on boards that are inserted into the corresponding slots on the motherboard” Nikolaeva V.A. Computer science and information technology. [Electronic resource] URL: http://www.junior.ru/wwwexam/pamiat/pamiat4.htm (access date: 05/15/2013).

ROM- fast, non-volatile memory, which is intended for read-only. 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) is a set of programs designed to automatically test devices after turning on the computer and loading the operating system into RAM.

The ROM contains:

Test programs that check the correct operation of its units every time you turn on the computer;

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, it is a memory into 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 mass production is involved, 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.

The central processor, when working with RAM, 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 (the processor's internal memory 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 accesses to RAM, 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 the necessary data is not 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 commands 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.

| 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

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

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 one) or sealing the wire to the case (writing 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 structural diagram of a read-only memory device does not differ from the mask ROM described earlier. 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, microcircuits 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 - an SPI port and an I2C port (chips of the 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.

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 the LSI ROM used in PCs to store the 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 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 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 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 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 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

In electronic devices, one of the most important elements that ensures the operation of the entire system is memory, which is divided into internal and external. Elements internal memory consider RAM, ROM and processor cache. External- these are all kinds of storage devices that are connected to the computer from outside - hard drives, flash drives, memory cards, etc.

Read-only memory (ROM) is used to store data that cannot be changed during operation, random access memory (RAM) is used to store information from processes currently occurring in the system in its cells, and cache memory is used for urgent signal processing by the microprocessor .

What is ROM

ROM or ROM (Read only memory) is a typical non-changeable information storage device included in almost every PC and phone component and required for startup and operation all elements of the system. The contents in ROM are written by the hardware manufacturer and contain directives for preliminary testing and startup of the device.

ROM properties are independence from power supply, the impossibility of rewriting and the ability to store information for long periods. The information contained in the ROM is entered by the developers once, and the hardware does not allow it to be erased and is stored until the end of the life of the computer or phone, or its breakdown. Structurally ROM protected from damage during voltage surges, therefore only mechanical damage can cause damage to the information contained.

By architecture they are divided into masked and programmable:

• Wearing masks devices, information is entered using a typical template at the final stage of manufacturing. The contained data cannot be overwritten by the user. The separating components are typical PNP elements of transistors or diodes.
• In programmable ROM, information is presented in the form of a two-dimensional matrix of conductive elements, between which there is a pn junction of a semiconductor element and a metal jumper. Programming such a memory involves eliminating or creating jumpers using a current of high amplitude and duration.

Basic functions

ROM memory blocks contain information on managing the hardware of a given device. The ROM includes the following subroutines:

• Directive start and control the operation of the microprocessor.
• Checking program performance and integrity all the hardware contained in a computer or phone.
• A program that starts the system and ends it.
• Subroutines that control peripheral equipment and input/output modules.
• Information about the address of the operating system on the physical drive.

Architecture

Read-only storage devices are designed as two-dimensional array. The elements of the array are sets of conductors, some of which are not affected, while other cells are destroyed. Conducting elements are the simplest switches and form a matrix by connecting them alternately to rows and rows.

If the conductor is closed, it contains a logical zero; if it is open, it contains a logical one. Thus, data in binary code is entered into a two-dimensional array of physical elements, which is read by a microprocessor.

Varieties

Depending on the manufacturing method of the device, ROM is divided into:

• Ordinary, created in a factory way. The data in such a device does not change.
• Programmable ROMs that allow the program to be changed once.
• Erasable firmware, which allows you to clear data from elements and rewrite them, for example, using ultraviolet light.
• Electrically cleanable rewritable elements that allow multiple change. This type is used in HDD, SSD, Flash and other drives. The BIOS on motherboards is written on the same chip.
• Magnetic, in which information was stored in magnetized areas alternating with non-magnetized ones. It was possible to rewrite them.

Difference between RAM and ROM

The differences between the two types of hardware are its safety when the power is turned off, speed and ability to access data.

In random access memory (RAM), information is contained in sequentially located cells, each of which can be accessed using software interfaces. RAM contains data about currently running processes in the system, such as programs, games, contains variable values ​​and lists of data in stacks and queues. When you turn off your computer or phone, RAM memory completely cleared. Compared to ROM memory, it has higher access speed and energy consumption.

ROM memory works slower and consumes less energy to operate. The main difference is the inability to change incoming data in ROM, while in RAM the information changes constantly.