Understanding and Managing Memory

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Chapter 4

• Understanding and Managing Memory

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You Will Learn

• The types of physical memory housed on the system
  board and expansion boards
• How memory is used by DOS and Windows 9x
• How to manage memory using DOS and Windows 9x
• How to upgrade the memory in your computer

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Physical Memory

• ROM
• Stores system BIOS and startup BIOS programs in a
  microchip that does not lose its data when the
  power is turned off
• RAM
• Primary memory
• Temporarily holds data and instructions as the
  CPU processes them
• Loses stored data when PC is turned off
• Two categories
• SRAM (static RAM)
• DRAM (dynamic RAM)

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ROM on the System Board

• Consists of memory on chips that contain
  permanent programs
• Flash ROM (also called EEPROM or electrically
  erasable programmable ROM) chips do allow their
  programs to be changed
• Contains the programming the computer uses to
  boot itself
• Contains much of the BIOS
• Is usually socketed onto the system board

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ROM BIOS
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Flash Memory

• Uses EEPROM chips
• Acts like secondary storage (does not lose its
  data when power is turned off)
• Holds data electronically

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Dynamic RAM
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Types of Memory
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Static RAM (SRAM)

• Provides faster access than DRAM because data
  does not need to be constantly rewritten to SRAM
• SRAM chips are made up of transistors that can
  hold a charge
• DRAM chips are made of capacitors that must be
  recharged
• Due to expense of SRAM chips, most computers have
  a little SRAM and a lot of DRAM

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Memory Caching

• Method used to store data or programs in SRAM for
  quick retrieval

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Memory Caching
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Kinds of Static Cache Memory

• L1
• Contained on the CPU microchip
• L2
• External to the chip
• Housed either on the system board or inside the
  CPU case

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SRAM on the System Board
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Varieties of SRAM Memory

• Synchronous SRAM
• Faster and more expensive than asynchronous SRAM
• Requires a clock signal to validate control
  signals, enabling the cache to run in step with
  the CPU
• Asynchronous SRAM
• Does not work in step with the CPU clock, and is,
  therefore, slower than synchronous SRAM

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Main Memory SIMMs and DIMMS

• SIMM technologies
• Width of data path 32 bits
• FPM (fast page mode) memory and EDO (extended
  data output) memory
• DIMM technologies
• Width of data path 64 bits
• Use burst EDO or synchronous SRAM

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The DIMM Module
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Other RAM Technologies

• Synchronous DRAM
• Currently most popular memory type
• Rated by system bus speed and operates in sync
  with the system clock older types of memory
  (FPM, EDO, and BEDO) run at a constant speed
• Comes in three variations
• Regular SDRAM
• SDRAM II (DDR)
• SyncLink (SLDRAM)

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SDRAM Variations

• Regular SDRAM
• Runs at same speed as system bus
• Width of data path 64 bits (50 faster than its
  predecessor, EDO memory)
• SDRAM II (DDR)
• Runs twice as fast as regular SDRAM and doubles
  the rate of memory
• Width of data path 64 bits
• SyncLink (SLDRAM)
• Increases number of memory banks that can be
  accessed simultaneously from four to sixteen

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Direct Rambus DRAM

• Uses a narrow 16-bit data path which allows high
  speeds of 400 MHz to 800 MHz
• Uses a proprietary memory module called a RIMM
  and is not stored on a DIMM

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Direct Rambus DRAM
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ECC, Parity, and Nonparity DRAM

• Error checking and correction (ECC)
• A chip set feature on a system board that checks
  the integrity of data stored on DIMMs and can
  correct single-bit errors in a byte
• Parity memory
• Supported by some older system boards
• Nine-bit memory in which the 9th bit is used for
  error checking
• Nonparity memory
• Slightly less expensive, 8-bit memory without
  error checking

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Installing Memory in a PC
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Memory Banks

• Memory modules (SIMMs and DIMMs) must be
  installed in banks
• The makeup of a bank of memory depends on the
  type of memory module being used and the
  microprocessor theyre being used with

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Memory Banks

• Each type of memory module provides a certain
  number of bits of stored data at a time
• Each microprocessor has a certain DATA BUS WIDTH
  (32 bits, 64 bits)
• The number of bits provided by the memory modules
  must match the data bus width of the processor

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Data Bus Widths
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Memory Module Bit Sizes
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Memory Banks for Specific Processors
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How Much Memory Can Fit on the System Board?
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How Much Memory Can Fit on the System Board?
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How Much Memory Can Fit on the System Board?
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General Guidelines for Installing Memory Modules

• A bank must be full to be recognized by the
  BIOS
• All of the memory modules in a bank must be the
  same capacity (4, 6, 16, 32, 64, 128 MB, etc.)
• In some cases, Bank 0 must be filled first, then
  Bank 1, etc.
• Try to match memory speeds within a bank

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Upgrading Memory

• How much memory do I need?
• How much memory can my computer physically
  accommodate?
• What increments of memory does the system board
  support?
• How much additional memory is cost effective?
• What kind of memory can fit on the system board?
• What memory is compatible already with memory
  installed?

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Reading Ads About Memory Modules

• Number of pins
• Speed
• Size
• Type of module

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Typical Ad for Memory Modules
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When Installing Memory

• Protect chips against static electricity
• Use a ground bracelet
• Turn power off and remove the cover to the case
• Handle memory modules with care
• Ground yourself before unpacking or picking up a
  card
• Dont stack cards

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Installing Memory Modules

• SIMMs and DIMMs are physically keyed for proper
  installation
• 30-Pin SIMMs
• Notch on one end of module

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Installing Memory Modules

• 72-Pin SIMMs
• Notch on one end and offset notch on connector
• Place in at approx. 45-degree angle and lift to
  snap in place

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Installing Memory
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Installing Memory Modules

• DIMMs
• Two specially placed notches on connector allow
  the module to only be installed the correct way
• Module installs straight into slot and is held in
  place with tabs

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Installing Memory
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How Physical Memory and the Operating System Work
Together
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Processor Modes

• Real Mode
• Single-tasking operating mode where a program
  only has 1024 KB of memory addresses, has direct
  access to RAM and uses 160bit data path

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Processor Modes

• Protected Mode
• Operating mode that supports multitasking where
  the OS manages memory, programs have more than
  1024 KB of memory addresses and programs can use
  a 32-bit data path

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How DOS and Windows 9x View Memory
continued
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How DOS and Windows 9x View Memory
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Physical Memory and Memory Addresses

• Memory
• Physical microchips that can hold data and
  programming
• Located on the system board or on expansion
  boards as single chips or modules
• Can be either ROM or RAM
• Memory address
• A number the CPU assigns to physical memory
• Both ROM and RAM must be assigned memory
  addresses in order for the CPU to access it

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Memory Addresses
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Memory Management Background

• Original IBM PC used Intel 8088 microprocessor
• 8088 had a 20-bit address bus
• Max. memory 1,048,576 bytes
• All memory functions had to fit in 1 MB

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Memory Organization

• Types of memory
• Conventional first 640K of memory addresses
• Upper memory addresses from 640K to 1024K
• Extended memory above 1024K
• Expanded
• Accessed in 16K segments by a window in upper
  memory
• Falls outside of linear addressing of memory

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Areas of the Memory Map
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Conventional Memory

• Designed in the 1980s
• Inadequate today due to memory requirements of
• Large size of most applications
• Running more than one application at a time
• Graphic user interfaces

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Upper Memory
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Extended Memory and the High Memory Area

• Extended memory
• Any memory installed beyond 1 MB
• Managed by the OS as a device controlled by a
  device driver called a memory manager
• Amount is limited by amount of RAM and number of
  supportable memory addresses
• High memory area
• First 64K of extended memory
• Result of a bug in programming for 286 CPU

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Expanded Memory
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Memory Management

• Process of increasing available conventional
  memory by loading device drivers and TSRs into
  upper memory

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Managing Memory with DOS

• HIMEM.SYS
• Device driver for all memory above 640K
• Often executed by the line DEVICE
  C\DOS\HIMEM.SYS in a CONFIG.SYS file
• EMM386.EXE
• Manages memory addresses in upper memory
• Emulates expanded memory

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Using HIMEM.SYS
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Using EMM386.EXE
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Creating and Using Upper Memory Blocks

• Upper memory block (UMB)
• A group of consecutive memory addresses in RAM
  from 640K to 1 MB that can be used by device
  drivers and TSRs
• Loading high
• Process of loading a driver or TSR into upper
  memory

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Creating and Using Upper Memory Blocks
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Loading Device Drivers High

• Use the DEVICEHIGH command in CONFIG.SYS
• Device driver needs space immediately above it to
  hold its data and extra room to initialize itself

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Loading TSRs High

• Load TSRs that are not device drivers into upper
  memory from AUTOEXEC.BAT or from DOS prompt

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Loading TSRs High
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Simulating Expanded Memory
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Memory Reports Using the MEM Command

• MEM command
• A DOS utility used to display how programs and
  drivers are using conventional, upper, and
  extended memory
• Example MEM/C/P

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Memory Reports Using the MEM Command
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Memory Reports Using the MEM Command
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Memory Reports Using the MEM Command
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Memory Reports Using the MEM Command
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Using MemMaker with DOS 6

• A DOS utility that can increase the amount of
  conventional memory available to DOS-based
  software applications by loading drivers and TSRs
  into upper memory

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Summary of How Memory Is Managed

• Memory management makes greatest amount of
  conventional memory available to an application
• During boot process
• ROM and RAM from expansion boards acquire upper
  memory addresses
• Unused addresses in upper memory are used to hold
  TSRs and device drivers
• Applications must be able to access extended and
  expanded memory

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Virtual Memory

• Hard disk space used when a system starts to run
  low on RAM
• Used to increase the amount of memory available
• Works at slower speed than real memory
• Available only if OS operates in protected mode

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RAM Drives

• A RAM area configured as a virtual hard drive so
  that frequently used programs can be accessed
  faster
• Opposite of virtual memory

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Managing Memory with Windows 9x

• Improvements in Windows 9x
• New 32-bit protected mode drivers (virtual device
  drivers or VxD drivers) are automatically loaded
  into extended memory when Windows 9x loads,
  eliminating the need for DEVICE entries in
  CONFIG.SYS

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Windows 9x Swap File

• Automates virtual memory management

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Windows 9x Swap File
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The Ultimate Solution Windows NT

• Memory mapping for Windows NT
• Is one continuous, linear, 32-bit address space
• Allows each program and driver using Windows NT
  access to any part of this memory

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Chapter Summary

• Location of physical memory
• The kinds of memory chips and modules found in a
  computer
• How to upgrade the RAM on your computer
• How the operating system uses the memory located
  on system boards
• How to manage memory to meet the needs of the
  software you are using

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