Read Only Memory (ROM)

• Permanent storage

—   Nonvolatile

• Microprogramming (see later)


• Library subroutines


• Systems programs (BIOS)


• Function tables

Types of ROM

• Written during manufacture

—   Very expensive for small runs

• Programmable (once)

—   PROM

—   Needs special equipment to program

• Read “mostly”

—   Erasable Programmable (EPROM)

–     Erased by UV

—   Electrically Erasable (EEPROM)

–     Takes much longer to write than read

—   Flash memory

–     Erase whole memory electrically

Characteristics

• Location


• Capacity


• Unit of transfer


• Access method


• Performance


• Physical type


• Physical characteristics


• Organisation

Location

• CPU


• Internal


• External

Capacity

• Word size

—   The natural unit of organisation

• Number of words

—   or Bytes

Unit of Transfer

• Internal

—   Usually governed by data bus width

• External

—   Usually a block which is much larger than a word

• Addressable unit

—   Smallest location which can be uniquely addressed

—   Word internally

—   Cluster on  disks

Access Methods

• Sequential

—   Start at the beginning and read through in order

—   Access time depends on location of data and previous location

—   e.g. tape

• Direct

—   Individual blocks have unique address

—   Access is by jumping to vicinity plus sequential search

—   Access time depends on location and previous location

—   e.g. disk

• Random

—   Individual addresses identify locations exactly

—   Access time is independent of location or previous access

—   e.g. RAM

• Associative

—   Data is located by a comparison with contents of a portion of the store

—   Access time is independent of location or previous access

—   e.g. cache