Read only memory (ROM) - memory device intended for storing unchangeable 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, one can point to LSI ROMs used in the PC for storing BIOS (Basic Input Output System).

In general, a ROM drive (an array of its memory cells) with a capacity of EPROM words, length r+ 1 bits each, usually represents a system of horizontal (address) and r + 1 vertical (bit) conductors, which at the intersection points can be connected by communication elements (Fig. 1.46). Communication elements (ES) are fuse-links or p-n-transitions. The presence of a link between j-m horizontal and i-m vertical conductors means that in i-th place of memory cell number j 1 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 ROM goes like this.

Figure: 1.46. ROM drive with a capacity of EPROM words, length r + 1 digits each

Address code A = j is decoded, and on the horizontal conductor the number j the drive is supplied with voltage from the power supply. Those of the discharge conductors that are connected to the selected address conductor by communication elements are energized U1 unit level, the remaining discharge conductors remain energized U0 level zero. A set of signals U0 and U1 on the discharge conductors and forms the content of the PL number j, namely the word at the address AND.

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

- mask (MPZU);

- programmable (EPROM);

- reprogrammable (EPROM).

Mask ROMs (ROM - from Read Only Memory) - ROMs, information into which is written from a photomask in the process of growing a crystal. For example, LSI ROM 555RE4 with a capacity of 2 kbytes is a symbol generator for the KOI-8 code. The advantage of mask ROMs is their high reliability, and the disadvantage is their low manufacturability.

Programmable ROM (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 discharge conductors. This increases the manufacturability of such LSIs, and hence the mass production and application. The recording (programming) of information in the EPROM is performed by the user at the place of their application. This is done by burning out the connection elements at the points at which the zeros should be written. Let us point out, for example, TTLSh-BIS PROM 556RT5 with a capacity of 0.5 kbytes. The reliability of the LSI PROM is lower than that of the mask LSI. Before programming, they must be tested for the presence of ES.

It is impossible to change the contents of their PL in the MPROM and EPROM. Reprogrammable ROM(EPROM) allow multiple changes of the information stored in them. In fact, an EPROM is a RAM that has tRFP \u003e\u003e tTHU. Replacing the contents of the EPROM begins with the erasure of the information stored in it. EPROMs are available with electrical (EEPROM) and ultraviolet (UVEPROM) erasure. For example, a KM1609RP2A LSI with electrical erasure KM1609RP2A 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 for at least 10 years in the off state. LSI EPROM with ultraviolet erasure K573RF4A with a capacity of 8 kbytes allows at least 25 rewriting 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 EPROMs 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.

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

In fig. 1.47 shows a conventional image of the ROM on the diagrams.

Figure: 1.47. Conditional image of ROM

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

Figure: 1.48. Functional diagram of ROM

According to the terminology accepted by specialists in storage devices, the input code is called the address, 2 n vertical tires - with numeric rulers, m outputs - by the digits 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, the connection of which with this number ruler is not destroyed, 1 appears. This means that 1 is written in the given bit of the selected word (or number ruler). At the outputs of those bits whose connection with the selected number ruler is burned out, zeros will remain. The programming law can also be inverse.

Thus, ROM is a functional unit with n inputs and m outputs storing 2 n m-bit words that do not change during the operation of a digital device. When the address is fed to the ROM input, the corresponding word appears at the output. In logical design, a permanent memory is considered either as a memory with a fixed set of words, or as a code converter.

In the diagrams (see Fig. 1.47), ROM is designated as ROM. Read-only memory devices usually have a permission input E. When the level at input E is active, the ROM performs its functions. In the absence of permission, the outputs of the microcircuit are inactive. There can be several permissive inputs, then the microcircuit is unlocked by coincidence of signals at these inputs. In ROM, the E signal is often called reading RT (read), selecting a VM chip, selecting a VC crystal (chip select - CS).

ROM chips are adapted for expansion. To increase the number of bits of stored words, all the inputs of the microcircuits are connected in parallel (Fig. 1.49, and), and from the increased total number of outputs, the output word is removed with a correspondingly increased bit width.

To increase the number of stored words themselves (Fig. 1.49, b) the address inputs of microcircuits are connected in parallel and considered as the least significant bits of the new, extended address. The added high-order bits of the new address go to the decoder, which selects one of the microcircuits at the E inputs. With a small number of microcircuits, the decoding of the most significant bits can be done on the conjunction of the enable inputs of the ROMs themselves. The outputs of the bits of the same name with an increase in the number of stored words should be combined using the OR functions. Special OR elements are not required if the outputs of the ROM microcircuits are made either according to the open collector circuit for combining by the wired OR method, or according to the buffer circuit with three states, allowing direct physical combination of the outputs.

The outputs of ROM microcircuits are usually inverse, and the E input is often also inverse. The expansion of 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), increasing the ROM usually does not give rise to fundamental problems.

Figure: 1.49. An increase in the number of bits of stored words when the inputs of microcircuits are connected in parallel and the number of stored words is increased when the address inputs of microcircuits are connected in parallel

Types of ROM

ROM - stands for read-only memory, which provides non-volatile storage of information on any physical medium. According to the method of storing information, ROMs can be divided into three types:

1. ROM based on the magnetic principle of information storage.

The principle of operation of these devices is based on changing the direction of the magnetization vector of the sections of the ferromagnet under the influence of an alternating magnetic field in accordance with the values \u200b\u200bof the bits of the recorded information.

Ferromagnet - a substance capable of being magnetized at temperatures below a certain threshold (Curie point) in the absence of an external magnetic field.

The readout of recorded data in such devices is based on the effect of electromagnetic induction or magnetoresistive effect. This principle is implemented in devices with a moving medium in the form of a disk or tape.

Electromagnetic induction is the effect of an electric current in a closed loop when the magnetic flux passing through it changes.

The magnetoresistive effect is based on a change in the electrical resistance of a solid-state conductor under the influence of an external magnetic field.

The main advantage of this type is a large amount of stored information and a low cost per unit of stored information. The main disadvantage is the presence of moving parts, large dimensions, low reliability and sensitivity to external influences (vibration, shock, movement, etc.)

2. ROM based on the optical principle of information storage.

The principle of operation of these devices is based on changing the optical properties of a section of the carrier, for example, by changing the degree of transparency or reflection coefficient. An example of a ROM based on the optical principle of information storage can be CD, DVD, BluRay discs.

The main advantage of this type of ROM is the low cost of the carrier, ease of transportation and the possibility of replication. Disadvantages - low read / write speed, limited number of rewrites, the need for a reader.

3. ROM based on the electrical principle of information storage.

The principle of operation of these devices is based on threshold effects in semiconductor structures - the ability to store and register the presence of a charge in an isolated area.

This principle is used in solid-state memory - memory that does not require moving parts to read / write data. An example of a ROM based on the electrical principle of information storage is flash memory.

The main advantage of this type of ROM is its high read / write speed, compactness, reliability, and economy. Disadvantages - limited number of rewrites.

At the moment, there are other, "exotic" types of persistent memory or are under development, such as:

Magnetic-optical memory - memory that combines the properties of optical and magnetic storage. Recording on such a disk is carried out by heating the cell with a laser to a temperature of about 200 o C. The heated cell loses its magnetic charge. Then the cell can be cooled, which will mean that a logical zero has been written to the cell, or it can be recharged with the magnetic head, which will mean that a logical one has been written to the cell.

After cooling, the magnetic charge of the cell cannot be changed. The reading is performed with a laser beam of lower intensity. If the cells contain a magnetic charge, the laser beam is polarized and the reader determines whether the laser beam is polarized. Due to the "fixing" of the magnetic charge during cooling, the magneto-optical have high reliability of information storage and theoretically can have a recording density higher than the ROM based only on the magnetic principle of information storage. However, they cannot replace "hard" disks due to the very low write speed caused by the need for high heating of the cells.

Magnetic-optical memory is not widely used and is used very rarely.

Molecular memory - memory based on the technology of atomic tunneling microscopy, which allows the removal or addition of individual atoms to molecules, the presence of which can then be read by special sensitive heads. This technology was presented in mid-1999 by Nanochip, and theoretically made it possible to achieve a packing density of about 40 Gbit / cm 2, which is ten times higher than the existing serial samples of "Hard" disks, but too low write speed and technology reliability does not allow us to talk about practical use of molecular memory for the foreseeable future.

Holographic memory - differs from the existing most common types of permanent memory, which use one or two surface layers for recording, by the ability to record data over the "entire" memory volume using various laser tilt angles. This type of memory is most likely to be used in ROM based on optical storage of information, where optical disks with multiple information layers are no longer new.

There are other, completely exotic types of permanent memory, but even in laboratory conditions they balance on the brink of science fiction, so I will not mention them, wait and see.

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Novgorod State University I. Wise

abstract

On the topic “Permanent storage devices. Main characteristics, scope "

Completed: 1st year student gr. 5261

Bronina Ksenia

Checked by: Arkhipova Gelirya Askhatovna

Veliky Novgorod, 2016

1. The concept of permanent storage

1.1 Main characteristics of ROM

1.2 Classification of ROM

1.2.1 By type of execution

1.2.2 By types of ROM chips

1.2.3 By the method of programming microcircuits (writing firmware to them)

2. Application

3. Historical types of ROM

Literature

1. The concept of permanent storage

Read-only memory (ROM, or ROM - Read Only Memory, read-only memory) is also built on the basis of modules (cassettes) installed on the motherboard and is used to store unchanged information: operating system boot programs, computer device testing programs and some drivers basic input / output system (BIOS), etc.

Permanent memory includes read-only memory, ROM (in English literature - Read Only Memory, ROM, which literally translates as "read-only memory"), programmable ROM, PROM (in English literature - Programmable Read Only Memory, PROM), and flash memory. The name of the ROM speaks for itself. The information in the ROM is recorded at the factory-manufacturer of memory chips, and its value cannot be changed later. The ROM stores information that is critical for the computer, which does not depend on the choice of the operating system. The programmable ROM differs from the usual one in that the information on this microcircuit can be erased by special methods (for example, ultraviolet rays), after which the user can re-write information on it. This information cannot be erased until the next erasing operation.

It is customary to refer to ROM as non-volatile permanent and "semi-permanent" storage devices, from which you can only read information on-the-fly, recording information in ROM is performed outside the PC in laboratory conditions or in the presence of a special programmer and in the computer. According to the information recording technology, ROMs of the following types can be distinguished:

§ microcircuits, programmable only during manufacture - classic or masked ROM or ROM;

§ microcircuits, programmable once in a laboratory, - programmable ROM (PROM), or programmable ROM (PROM);

§ microcircuits programmable repeatedly - reprogrammable ROM or erasable PROM (EPROM). Among them should be noted electrically reprogrammable EEPROM (Electrical Erasable PROM) microcircuits, including flash memory.

1.1 Main characteristics of ROM

Data in read only memory (ROM) is stored permanently. Data stored permanently is referred to as non-volatile, which means that it remains in ROM even when the power is turned off. Once data is written to ROM, it can be read by other devices, but new data cannot be written to ROM.

ROM is most commonly used to store what is called a “monitor program”. A monitor program is a machine program that allows the user of a microcomputer system to view and modify all system functions, including memory. Another widespread use of ROMs is storing fixed tables of data, such as mathematical functions, that never change.

Four types of ROM are widely used by digital computer systems: mask-programmed ROM, programmable ROM (EPROM), erasable programmable ROM (EPROM), and electrically programmable ROM (EEPROM).

1.2 Classification of ROM

1.2.1 By type of execution

The data array is combined with the sampling device (a reader), in this case the data array is often called "firmware" in conversation:

§ rOM chip;

§ One of the internal resources of a single-chip microcomputer (microcontroller), usually FlashROM.

The dataset exists on its own:

§ CD;

§ punch card;

§ punched tape;

§ bar codes;

§ mounting "1" and mounting "0".

1.2.2 By types of ROM chips

By crystal manufacturing technology:

§ ROM eng. read-only memory - read-only memory, masked ROM, manufactured by the factory method. There is no way to change the recorded data later.

Figure 1. Mask ROM

§ PROM eng. programmable read-only memory - programmable ROM that can be flashed once by the user.

Figure 2. Programmable ROM

§ EPROM eng. erasable programmable read-only memory - reprogrammable / reprogrammable ROM (EPROM / EPROM)). For example, the contents of the K573RF1 microcircuit were erased using an ultraviolet lamp. For the passage of ultraviolet rays to the crystal, a window with quartz glass was provided in the microcircuit case.

Figure 3. Flash ROM

§ EEPROM eng. electrically erasable programmable read-only memory - electrically erasable programmable ROM). This type of memory can be erased and filled with data tens of thousands of times. Used in solid state drives. One of the varieties of EEPROM is flash memory.

Figure 4. Erasable ROM

§ ROM on magnetic domains, for example K1602RTs5, had a complex sampling device and stored a rather large amount of data in the form of magnetized regions of the crystal, while not having moving parts (see Computer memory). An unlimited number of rewriting cycles was provided.

§ NVRAM, non-volatile memory - "non-volatile" memory, strictly speaking, is not a ROM. This is a small amount of RAM, structurally combined with a battery. In the USSR, such devices were often called "Dallas" after the company that launched them on the market. In NVRAM of modern computers, the battery is no longer structurally connected to the RAM and can be replaced.

By type of access:

§ With parallel access (parallel mode or random access): This ROM can be accessed in the system in the RAM address space. For example, K573RF5;

§ With sequential access: such ROMs are often used for one-time loading of constants or firmware into a processor or FPGA, used to store TV channel settings, etc. For example, 93C46, AT17LV512A.

1.2.3 By the method of programming microcircuits (writing firmware to them)

§ Non-programmable ROM;

§ ROM, programmable only with a special device - ROM programmer (both one-time and multiple-flash). The use of a programmer is necessary, in particular, to supply non-standard and relatively high voltages (up to +/- 27 V) to special outputs.

§ In-circuit (re) programmable ROM (ISP, in-system programming) - such microcircuits have a generator of all the necessary high voltages inside, and can be flashed without a programmer and even without soldering from the printed circuit board, by software.

memory chip programming monoscope

2. Application

The read-only memory is often recorded with the firmware for controlling a technical device: a TV, a cell phone, various controllers, or a computer (BIOS or OpenBoot on SPARC machines).

BootROM is a firmware, such that if it is written into a suitable ROM chip installed in a network card, it becomes possible to load the operating system onto a computer from a remote node of the local network. For embedded network cards, BootROM can be activated through the BIOS.

ROM in IBM PC-compatible computers is located in the address space from F600: 0000 to FD00: 0FFF

3. Historical types of ROM

Permanent 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, which was used in barrel organs, music boxes, and striking clocks.

With the development of electronic technology and computers, the need arose for high-speed ROMs. In the era of vacuum electronics, ROMs based on potentioscopes, monoscopes, and beam lamps were used. In computers based on transistors, plug matrices were widely used as ROMs of small capacity. If it was necessary to store large amounts of data (for computers of the first generations - 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, covering the ring. Since a thin wire had to be pulled through a chain of ferrite rings, metal needles similar to sewing ones were used to perform this operation. And the very operation of filling the ROM with information resembled the process of sewing.

Literature

Ugryumov E.P. Digital circuitry BHV-Petersburg (2005) Chapter 5.

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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 with the help of a photomask of the switching layer, connections are realized between the lines of address, data and the actual storage element. 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 shown in Fig. 12.1. Here, horizontal lines are address lines, and vertical lines are data lines; in this case, 8-bit binary numbers are removed from them. In this diagram, the GE 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 if there is a diode at the line crossing 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.

Masked ROMs based on MOSFET matrix

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 you select 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 in the one 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 will be code 0010 2.

Masked ROMs based on a matrix of bipolar transistors

An example of the scheme of this ROM 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 the ZE line per address line 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.

Programmable ROM

Programmable ROM (EPROM) are the same diode or transistor matrices, as well as masked ROMs, but with a different design 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, which 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 and the logical 1 are written in the ZE.

It is important to know difference between RAM and ROM. If you understand this difference you will be able to better understand how a computer works. RAM and ROM are like different types of storage devices, and they both store data in a computer. In this article, we are going to tell you about the main differences between these two memories, namely RAM and ROM.

Random Access Memory (RAM)

Random access memory is a type of memory that allows you to access stored data in any order and from any physical location in memory. RAM can be read and written with new data. The main advantage of RAM is that it takes almost the same time to access any data, regardless of where the data is located. This makes RAM a very fast memory. Computers can read from memory very quickly, and they can also write new data into RAM very quickly.

What does RAM look like?

Commercially available conventional memory chips can be easily plugged into and plugged into the output of a computer's motherboard. The following figure shows the memory chips.

Read only memory (ROM)

As the name suggests, data is written to ROM only once and forever. After that, the data can only be read by computers. Read-only memory is often used to set persistent instructions into a computer. These instructions will never change. ROM chips store basic input / output system (BIOS) of the computer. The following figure shows a commercially available ROM BIOS chip.

Difference between RAM and ROM

The following table lists the main differences between random access and only for reading memory.

RAM and ROM are integral parts of modern computer systems. Do you want to know when the disk is working and when the RAM is in the game? Well, when you switch on your computer, you might see a black screen with some white text. This text is from ROM. The ROM instructions control your computer for the first few seconds when you turn it on. During this period, as instructions " , how to read from hard disk ", "how to print on the screen" loaded from ROM. After the computer is able to do these basic operations, the operating system (Windows / Linux / OSX, etc.) is read from the hard disk and loaded into RAM. The following video explains RAM vs ROM concept additionally.

When you open a program like Microsoft Word, the program is loaded from your computer's hard drive into RAM.

We hope this article helped you understand the main differences between RAM and ROM. If you have any questions related to this topic please feel free to ask in the comments section. We will try to help you. Thank you for using TechWelkin!