RAM and ROM

12 Key excerpts on "RAM and ROM"

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  Digital Electronic Circuits

  Principles and Practices

  • Shuqin Lou, Chunling Yang(Authors)
  • 2019(Publication Date)
  • De Gruyter

    (Publisher)

  One is read only memory (ROM) and another is random access memory (RAM). ROMs are used to store the binary data or information permanently or semiper-manently. Generally, data can only be read from the ROM. Although some types of data can be written into, the writing process is slow and usually all the data in the chips must be rewritten at the same time. A ROM stores data that are used repeatedly in system, such as tables, conversions, or programed instructions for system initi-alization and operation. Especially, it can be used to store system software, which must be immediately accessible to the computer, such as the BIOS program using for the start up of computer, and the software (microcode) for portable devices and embedded computers such as microcontrollers. Because ROMs maintain the stored data even if power is turned off, they belong to the nonvolatile memories. In contrast to ROMs, RAMs have both read and write capacities. Because RAMs lose stored data when the power is turned off, they belong to volatile memories. Although the volatile memory loses its stored data when the power to the memory chip is turned off, it can be faster and less expensive than nonvolatile memory. It is widely used for the main memory in most computers and other digital systems. 9.3 Read only memory A ROM stores permanent or semipermanent data, which can be read from the memory but either cannot be changed at all or cannot be changed without specialized equipment. In terms of electronic components used as memory cells, semiconductor memory can be divided into two categories: bipolar memory and metal-oxide-semiconductor (MOS) memory (also called unipolar memory ). Bipolar memory is Old address New data Data bus D 0 ~ D 7 t p t s WRITE Old data Old data New address New data Address bus A 0 ~ A 2 Stored data Q 0 ~ Q 7 Figure 9.2.6: Timing Diagram of a write operation. 9.3 Read only memory 311 generally composed of transistors with the advantage of high speed.

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  Digital Electronic Circuits

  The Comprehensive View

  • Alexander Axelevitch(Author)
  • 2018(Publication Date)
  • WSPC

    (Publisher)

  Semiconductor memory may be divided into three big groups, each one of them used to solve different problems. The first group is the random-access memory or RAM which enables us to store instructions, data, and various constants. The main property of RAM is our ability to access any piece of information stored in memory in the same amount of time independent of its location in memory. Due to the general principle of information storage, each information bit may be stored in two ways. The first method of storage is the application of latch schemes; the second one is the application of a capacitor for this goal. Consequently, all RAM systems are divided into systems that use static random-access memory (SRAM) and systems that use dynamic random-access memory or DRAM. SRAM is built on the base of latch-schemes and capable of storing information for a long time, up to the supply cutoff. DRAM requires a periodic refresh of the stored information due to leakages in the semiconductor capacitors.

  Fig. 5.8. Two integral circuits produced with an interval of about 30 years between them: the “Fairchild” logic gate consisting of four transistors and two resistors and the Intel-486 microprocessor.

  Fig. 5.9. A structure organization of the semiconductor memory.

  The second big group of memories is read-only memory (ROM). ROM is used to store significant information which should be stored without any changes throughout the lifetime of the device. The main operating system of a computer and very important information are usually written in ROM, for example a game which may only be changed with the ROM exchange. The construction of ROM has changed with the development of novel transistor technologies. The first ROMs were programmed with fuses firing in memory; these devices were called PROM. After that, the electrically programmed read-only memory devices (EPROM) were designed. We will discuss the construction of EPROM later. With the goal of changing information in the ROM, erasable devices called EEPROM were designed and applied in industry.

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  Foundation of Digital Electronics and Logic Design

  • Subir Kumar Sarkar, Asish Kumar De, Souvik Sarkar(Authors)
  • 2014(Publication Date)
  • Jenny Stanford Publishing

    (Publisher)

  It contains only the programs and data currently used by the processor reside inside in main memory. It determines the size and the number of programs that can be stored within the computer as well as the amount of data that can be processed. This is volatile, expensive, and slow read/write (R/W) memory. (Volatile means, as long as the 178 Memory power is there it retains the contents, when power is switched off the content is lost.) It is also random access memory (RAM). The primary memory (PM) is a must for every microprocessor-based system. It is this memory with which the microprocessor directly communicates through its system buses. The size of the PM is limited due to large cost as well as architectural limitation of microprocessor. Some part of primary memory is dedicated for OS process. It is the PM where the operating system (OS) resides, but in some PM area. 3.2.4.1 Classification of primary memory Primary memory (PM) can be two types. RAM and ROM. The general structure of PM is given in the Fig. 3.2 ↓ ↓ ↓ Read/write RAM Read only Memory Memory (both) (ROM) (R/W Memory) Figure 3.2 The RAM and ROM are the primary memories. 3.2.4.2 Random access memory The random access memory (RAM) is a volatile storage device in which any memory location can be accessed at random for reading or writing. It retains the stored information as long as the power is not switched off. When power supply is switched off or interrupted the stored information in the RAM is lost. The RAM may be (i) static or (ii) dynamic Static RAM retains stored information only as long as the power supply is on. However, dynamic RAM loses its stored information in Classifications of Memory 179 a very short span of time even though power supply is on. Therefore, dynamic RAM has to be refreshed periodically. 3.2.4.3 Read only memory Read only memory (ROM) is a onetime programmed, nonvolatile memory device. The user can only read it but cannot write onto ROM.

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  Microelectronics - Systems and Devices

  • Owen Bishop(Author)
  • 2013(Publication Date)
  • Routledge

    (Publisher)

  The two main types of memory are random access memory (RAM) and read only memory (ROM). Two types of RAM are static RAM (SRAM) and dynamic RAM (DRAM). There are several types of ROM, including mask-programmed ROM, PROM, EPROM and EEROM. Decoders are used to route signals to or from a specified location in memory. A real time clock relieves the CPU of timekeeping duties.

  There are two main types of memory:

  • Random access memory – used for temporary storage. Data may be written into it at any time, and later read from it The data is lost when the power supply is switched off. Used for the storage of data and for programs copied from more permanent data stores such as magnetic disks.
  • Read only memory – used for permanent storage. In most types, the data once written into it can not be changed. Used for storing programs and data tables.

  Random access memory

  This occupies most of the available address space in a typical microcomputer. A PC when purchased may be equipped with 32 Mb of RAM, with room for expansion to 64 Mb or more. The RAM is used for the temporary storage of programs and data. There is usually less need for RAM in a microcontroller system, as their programs are permanent and are stored in ROM instead (see below). The RAM is usually included on the microcontroller chip, and may consist of as few as 64 bytes.

  Mb: the symbol for megabyte, which is approximately one million bytes, but more exactly is 220 , or 1048576 bytes.

  Whenever data is read from RAM, it is copied in a register of the CPU. Conversely, it is copied from the CPU when it is written into RAM. In either case, the original data remains unaltered after copying. This is known as non-destructive readout.

  Random access: data may be read from or written to any location at any time. This is different from, say, a shift register, from which data must be read in the order in which it was written.

  Once data is stored in RAM, it remains there until it is either:

  • Overwritten and replaced by new data, or
  • The power supply is switched off.

  There are two types of RAM:

  Static RAM (SRAM). Each SRAM IC contains an array of flip-flops, each one of which can be set or reset to one of its two states and so represent a logic 0 or a logic 1. The flip-flops may be individually addressable, or they may be addressed in groups of 4 (a nybble) or 8 (a byte

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  Wiley Pathways Personal Computer Hardware Essentials

  • David Groth, Ron Gilster, Megan Miller(Authors)
  • 2015(Publication Date)
  • Wiley

    (Publisher)

  5.1 Examining Memory For the CPU to perform its functions, it must receive data and instructions. Data and software instructions can be stored permanently in PCs on hard drives, but for the CPU to work quickly, they are loaded into storage areas called memory. Memory is composed of electronic components, such as transistors and semi- conductors, which hold signals as binary 1s and 0s. 5.1.1 Memory and the PC There are two categories of memory: volatile and nonvolatile. Volatile memory only holds data or signals while it receives power. When power is switched off, the data it holds is lost. Nonvolatile memory retains its signals, or data, even when the power is switched off. You will find two basic types of memory in a PC: ▲ Read-only memory (ROM): ROM is nonvolatile memory, typically used to store essential, small programs such as the BIOS. ▲ Random-access memory (RAM): RAM is memory used to hold the instructions and data in use by the operating system and software applica- tions before and after the data is passed to the CPU. It is called “random” because the data stored in it is accessed nonsequentially: The CPU doesn’t need to read each successive bit to find a particular piece of data. (A tape drive, in contrast, is sequential; as the data on it is recorded in a sequential 5.1 EXAMINING MEMORY 165 166 MEMORY stream.) The RAM used for the main memory on a PC is volatile. RAM is by far the faster of the two types of memory. In fact, RAM is often used to shadow the BIOS ROM to improve its performance during the boot process. Memory locations in the PC include: ▲ System memory, installed in banks of RAM memory modules on the motherboard (Figure 5-1) ▲ CPU cache memory, installed as part of the CPU (internal cache) or nearby on the motherboard (external cache) ▲ CPU registers ▲ BIOS chips on the motherboard or other expansion boards In general, the transfer of data into memory in the PC follows this order: 1.

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  Digital Electronic Circuits

  Principles and Practices

  • Shuqin Lou, Chunling Yang(Authors)
  • 2019(Publication Date)
  • De Gruyter

    (Publisher)

  ROM stores data that are used repeatedly in system applications, such as tables, conversions, or programmed instructions, for system initialization and operation. ROM keeps the stored data when the power is turned off, therefore ROM is nonvolatile memory.

  9.4 Random access memory

  Unlike ROM, RAM is also called read/write memory , in which data can be written into or read from any selected address in any sequence. When the power is turned off, the stored data in RAM will be lost, so RAM is a volatile memory and is mainly used to store short-term data and temporary programs.

  The objectives of this section are to

  • – Explain the basic structure of RAM
  • – Explain the difference between static RAM and dynamic RAM
  • – Describe the SRAM stage cell
  • – Describe the dynamic RAM stage cell
  • – Explain the operation modes of the IC RAMs
  • – Discuss some special RAMs

  9.4.1 Basic structure of RAM

  The basic structure of RAM contains three parts: address decoder, memory array, and read/write control circuit, as shown in Figure 9.4.1 . The address lines A 0 through

  An

  -1 are applied on the address decoder and 2n addresses are produced. The read and write control circuit is used to select the read/write operation. In READ mode, the data stored in a selected address appears on the data output lines. In WRITE mode, the data applied to the data input lines is stored at a selected address. The data input and output lines share the same lines. During READ, they act as output lines. During WRITE, they act as input lines. Therefore, the data lines are bidirectional. If the RAM has a larger capacity, dual address decoders containing row decoder and column decoder are adopted in RAM organization to simplify the RAM structure.

  Figure 9.4.1: Basic structure of RAM.

  9.4.2 Types of RAMs

  The two widely used types of RAM are static RAM (SRAM ) and dynamic RAM (DRAM ) [44 ].

  SRAM generally uses latches as storage elements and can store data infinitely as long as power is turned on. SRAM is more expensive to manufacture, but is generally faster and requires less dynamic power than DRAM. In modern computers, SRAM is often used as cache memory for the CPU.

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  Embedded Systems

  A Contemporary Design Tool

  • James K. Peckol(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  dynamic allocation in an embedded context. The topic of memory test closes the chapter. Several approaches for testing RAM and ROM memories are introduced and studied.

  4.2 Classifying Memory

  read only memory, ROM

  random access memory, RAM

  static, dynamic

  As a first step in studying memory systems, it is important to recognize that the term memory is generic. There are many different kinds of memory, each with its strengths and weaknesses. Understanding the characteristics of each and how to design with them can lead to more robust and effective design solutions. We begin by classifying memory into two general categories: ROM and RAM. We further subdivide RAM into static RAM (SRAM) and dynamic RAM (DRAM). These classifications lead to a number of different subcategories of both ROM and RAM. Some of the more common subclassifications are described here.

  • RAM – Random Access Memory. As the name suggests, any location in memory is visible for immediate access rather than having to sequence through predecessor locations. The times for a read operation and a write operation are comparable. A RAM may be organized as bits, bytes, or words.
  • DRAM – Dynamic RAM. A simple memory cell design with bit storage implemented using a stored charge mechanism. The stored charge can leak away if it is not repeatedly restored. These devices are used for larger memory systems. I/O is asynchronous with respect to any external system clocks.
  • SRAM – Static RAM. A more complex memory cell design with bit storage implemented using a latch‐type mechanism. The stored data does not have to be refreshed. These devices are used for higher speed memory systems because they are faster than DRAM designs. I/O is asynchronous with respect to any external system clocks.
  • Semistatic RAM. The periphery is clock activated (dynamic). Only one memory cycle is permitted per clock. The periphery circuitry must be allowed to reset after each active memory cycle for minimum pre‐charge time. No refresh is required.
  • SDRAM – Synchronous DRAM

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  Microprocessor Engineering

  • B. Holdsworth(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  In effect, cycle time is a measure of memory speed. 3.4 Memory classification Memory can be broadly divided into two classes: (a) read only memory (ROM) and (b) read/write memory (RWM). Read/write memories are normally referred to as RAM, but is should be recognized that a ROM is also a random access memory and the exclusive usage of this term for read/write memories is not strictly correct. ROM is used for program storage and invariable data patterns. It is used in situations where memory contents are not required to change and a main area of application for ROM is in dedicated microprocessor systems. 68 Memory A ROM functions as a memory array whose contents once programmed are permanently fixed and cannot be altered. It is a non-volatile memory, which means that if there is a loss of power or some other circuit malfunction the contents of the ROM are not destroyed. Because the state of a memory cell in a ROM is permanently defined its structure is considerably simpler than that of a memory cell in RWM. There are four categories of ROMs presently available. (1) Mask programmed when manufactured. The contents are programmed by the manufacturer during fabrication accord- ing to customer specification. It is not possible to change the program after packaging. (2) PROMs, or programmable ROMs. The contents are written into a PROM by the user with the aid of a PROM programmer. Once programmed, the contents of the memory are permanently fixed. (3) EPROMs y or erasable PROMs. The contents are programmed by the user but can be subsequently erased by an application of ultraviolet light through a window on the upper surface of the memory chip. (4) EAROMSy or electrically alterable ROMs. This type of ROM can have its program erased by electrical means while still in circuit. Both EPROMs and EAROMs can be reprogrammed and used again after erasure. RWMs are used in microcomputer systems for the temporary storage of data and intermediate results of calculations.

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  Computer Organisation and Architecture

  An Introduction

  • B.S. Chalk, Antony Carter, Robert Hind(Authors)
  • 2017(Publication Date)
  • Red Globe Press

    (Publisher)

  This type of memory requires the continued presence of an electricity supply to maintain the data. Once the electricity is removed the data is lost. The former type of memory is known as non-volatile memory while the latter is volatile . ROM memory is an example of non-volatile microelectronic memory but more on that later. 6.3.1 Random Access Memory (RAM) There are two basic types of semiconductor RAM: Static RAM (SRAM) and Dynamic RAM (DRAM). The internal organisation of a typical SRAM memory chip is shown in Figure 6.2. The cells are arranged in a matrix with rows and columns. An address supplied on the address line A 0 , …, A 9 is used to select a particular row, each cell in the row being connected to a separate column line. The cells are arranged in 8s which we call byte organised . The column lines are used to transfer data bytes between the addressed rows and the data lines D 0 , …, D 7 , which connect the chip to the data bus. Primary memory 91 Figure 6.2 1024 8-bit static RAM chip organisation Row select A 0 A 1 A 2 : : : A 9 R/W Column I/O circuits D 0 D 1 D 2 D 3 D 4 D 5 D 6 D 7 Selected row Memory cell Column line TQ 6.2 How many rows can the 10 address lines A 0 , …, A 9 select? Reading and writing data is controlled by the read–write (R/W –– ) control line, which is connected to the column I/O circuit. A read operation is selected by taking this line high, and a write operation is selected by taking the line low. This latter state is indicated by the bar over the W. SRAM cells store data by using cross-coupled transistor circuits acting as R-S flip-flops (see Section 3.3). (A transistor is a sort of microelectronic switch. It is the basic building block of logic gates.) Because these cells require four or six transistors per cell, SRAM chips can store fewer bits per mm 2 of silicon than DRAM. They are also more expensive in terms of the costs per bit of storage space. The main advantage of SRAM over DRAM is its shorter access time.

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  Digital Computer Structure and Design

  • R. Townsend(Author)
  • 2014(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Non-destructive Read-Out An operation in which the contents of a memory location may be read without being destroyed. Latency This refers to the time necessary to wait with a drum, or disc memory, for the desired information to rotate to the reading head. This is a variable time depending on the position of the disc or drum at the time the data is requested. RAM Random access memory. This refers to the fact that any word can be accessed, (i.e. read or written) at random with the same access time. ROM Read only memory. A type of memory in which data is per-manent or semi-permanent, and can only be read by the processor and not written. PROM Programmable read only memory. A type of read only memory in which the contents can be entered on a separate instrument, usually at a much slower rate. This is sometimes referred to as 'blowing' a PROM. The PROM can also be erased for reuse. EAROM A type of PROM in which the contents are erased electrically. Hierarchy of Memories If an overall view of all types of memories is taken it is found that there is a trade-off between the capacity of the memory, its speed, and the 146 Computer Memories cost per bit of storage. In larger computers both fast and slow memories may be incorporated in a hierarchy of several speeds and capacities, and data is transferred to and from the slower high capacity memory in blocks to the fast memory for high speed processing and then returned to the slower memory for long term storage. This attempts to make the optimum use of the high speed of the smaller fast memory and the lower cost of the slower memory. Table 7.1 AREA OF MEMORY CAPACITY AND SPEED. Type of Memory Scratchpad Memory (high speed) (a) (b) Main Memory (a) (b) Special Purpose Memories Mass Memory (a) (b) (c) Archival Storage.

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  Computer Architecture

  Fundamentals and Principles of Computer Design, Second Edition

  • Joseph D. Dumas II(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  They are nonvolatile but have the major limitation of not being writable (although EPROMs can be reprogrammed in a separate circuit after erasure with ultraviolet light). Because they are not read/write memories, ROMs are only useful in limited applications, such as single-purpose embedded systems, video game cartridges, and the basic input/output system (BIOS) that contains the bootstrap code and low-level input/output (I/O) routines for most typical computer systems. Semiconductor “read-mostly” memories include electrically erasable programmable read-only memories (EEPROMs) and their technological descendants, fash memories . These memories are nonvolatile, but unlike ROMs, they are rewritable in-circuit. Writes, however, can take signif-cantly longer than reads to perform and in some cases must be done as “block” writes rather than individual memory locations. Also, these devices are more expensive than most other semiconductor memories and can only be rewritten a limited number (usually a few tens or hundreds of thousands) of times, so they are not suitable for populating the entire main memory space of a computer. Instead, read-mostly memories are typically used for special-purpose applications, such as digital cameras, portable thumb drives, hybrid drives, tablet computers, and smartphones. Magnetic memories have been in use much longer than semiconduc-tor memories—almost as long as there have been electronic computers. Mainframe computers of the 1950s often used rotating magnetic drums for storage; a few years later, magnetic core memory became the standard technology for main memory and remained so until it was replaced by integrated-circuit RAM and ROM in the 1970s. Magnetic core memory, like all magnetic memories, offered the advantage of nonvolatility (except in the presence of a strong external magnetic feld).

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  Embedded Systems Design

  • Steve Heath(Author)
  • 2002(Publication Date)
  • Newnes

    (Publisher)

  It is battery backed-up to ensure it is powered up while the computer is switched off. Some microcontrollers with on-chip SRAM support the connection of an external battery to backup the SRAM contents when the main power is removed. EPROM and OTP EPROM is used to store information such as programs and data that must be retained when the system is switched off. It is used within PCs to store the Toolbox and BIOS routines and power on software in the MAC and IBM PC that is executed when the computer is switched on. These devices are read only and cannot be written to, although they can be erased by ultraviolet (UV) light and have a transparent window in their case for this purpose. This Memory systems 79 window is usually covered with a label to prevent accidental erasure, although it takes 15–30 minutes of intense exposure to do so. There is a different packaged version of EPROM called OTP (one time programmable) which is an EPROM device packaged in a low cost plastic package. It can be programmed once only because there is no window to allow UV light to erase the EPROM inside. These are becoming very popular for small production runs. Flash Flash memory is a non-volatile memory which is electri-cally erasable and offers access times and densities similar to that of DRAM. It uses a single transistor as a storage cell and by placing a higher enough charge to punch through an oxide layer, the transistor cell can be programmed. This type of write operation can take several milliseconds compared to sub 100 ns for DRAM or faster for SRAM. Reads are of the order of 70–100 ns. FLASH has been positioned and is gaining ground as a replacement for EPROMs but it has not succeeded in replacing hard disk drives as a general-purpose form of mass storage. This is due to the strides in disk drive technology and the relatively slow write access time and the wearout mechanism which limits the number of writes that can be performed.

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