Computer Memory

  • Think of memory like a set of drawers
    • Each drawer has an address
  • Collection of cells with unique physical addresses
    • Number of bits in each location varies from one machine to the other
    • More common computers are byte addressable
      • Memory addresses are made up of a certain number of bytes
  • The contents of memory alone aren't too useful
    • Need to know how to interpret the contents
  • Main memory
    • Active programs and data stored
    • Volatile
  • Secondary memory
    • Nonvolatile
    • Data maintained even with power off


  • Random Access Memory
  • Each location can be accessed and changed
  • Volatile
    • Does not retain its bit configuration when the power is turned off
    • Loses data when power is off


  • Read Only Memory
  • Each location can be accessed but not changed
  • Contents are permanently burned into the chip
  • Used for storing instructions that the computer needs to start itself

Secondary Storage Devices

  • Used because main memory is volatile and limited
  • Places to store data outside of main memory

Magnetic Tape

Magnetic tape

  • First truly mass storage device
  • Cannot hold much data
  • Requires you to scrub to find data in the middle
    • Not efficient
  • Four measures of efficiency
    • Seek time
      • Time it takes for read/write head to get positioned over specified track
    • Latency
      • Time it takes for specified sector to spin to read/write head
    • Access time
      • Time it takes for block to start being read
      • Sum of seek time and latency
    • Transfer rate
      • Rate at which data moves from the disk to memory

Magnetic Disks

A disk and hard disk drive

  • Disk drive
  • Cross between CD player and tape recorder
  • Read/write head travels across a spinning magnetic disk
    • Retrieves or records data
  • Surface of each disk is organized into tracks and sectors
    • Tracks
      • Concentric circles around surface of disk
      • Divided into sectors
    • Sector
      • A division of a track
      • Holds a block of information as continuous sequence of bits
      • Blocks of data nearer center are more densely packed


  • Logical address
    • Reference to a stored value relative to the program making the reference
  • Physical address
    • Actual address in the main memory device
  • Address binding
    • Mapping from a logical address to a physical address
    • As long as we keep track of where a program is stored, we can always determine the physical address that corresponds to any logical address

Computer Science