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

RAM

• 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

ROM

• 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

• 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

• 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

Addresses

• 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