Memory and Storage

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Memory and Storage

• - Sheetal
  Gosrani

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Overview

• Memory Hierarchy
• RAM
• Memory Chip Organization
• ROM
• Flash Memory

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Memory Hierarchy
Increasing performance and increasing cost

• Slow and inexpensive

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Memory Hierarchy (contd)?

• Registers
• top in the hierarchy
• matched in speed to the CPU
• but are large and consume significant power
• number of registers in a processor are small
• Secondary and off-line storage devices
• at the bottom of the hierarchy
• cost per stored bit small in terms of money and
  power
• but access time is very long compared to
  registers

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Random Access Memory(RAM)?

• The term random means any memory location can
  be accessed in the same amount of time regardless
  of its position in the memory.
• Volatile memory
• Types of RAM
• SRAM
• DRAM

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Static RAM

• Static RAM (SRAM) based on flip flops
• Contents of each location persist as long as
  power is applied.
• Fast but relatively large
• Consume a lot of power
• Used for memory applications that are small but
  fast

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Dynamic RAM

• Dynamic RAM (DRAM) employ capacitors
• Capacitor stores electric charge whose level
  represents a 1 or 0
• Capacitors dissipate with time and hence the
  charge must be restored frequently
• DRAMs
• smaller, slower than SRAMs
• support low cost, low power and high density and
  hence used in main memory

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Memory Chip organization
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RAM chip

• A0 Am-1 Address lines from 0 to m-1
• CS Chip Select (CS 0, chip selected)?
• WR ReadWrite (WR 0, write operation)?

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RAM Grid
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RAM Grid

• During read operation
• Entire row is selected
• It is fed into the column MUX
• MUX selects a single bit for output
• During write operation
• Single bit to be written is distributed by the
  DEMUX to the target column
• Row decoder selects the proper column to be
  written

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Read Only Memory(ROM)?

• Data stored in these chips is nonvolatile -- it
  is not lost when power is turned off.
• On turn on, the computer loads BIOS from ROM
• Data stored in these chips is either unchangeable
  or requires a special operation to change
• Applications Videogames, calculators, microwave
  oven, etc

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Read Only Memory(ROM) (contd)?

• Lets see the circuit for a ROM that stores four
  four-bit words(0101, 1011, 1110, 0000)?
• PROM burner allows contents of the ROM to be
  overwritten only once
• EPROMs could be written several times
• EEPROMs allow the content to be rewritten
  electrically

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Flash Memory

• A section of memory cells can be erased in a
  single step, or 'flash', thus the name
• Nonvolatile memory
• Flash memory is used for easy and fast
  information storage in such devices as digital
  cameras, mobile, video game consoles

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References

• Textbook Computer Organization and Architecture
• Wiki pages
• http//en.wikipedia.org/wiki/Computer_storage
• http//en.wikipedia.org/wiki/Dynamic_random_access
  _memory
• http//computer.howstuffworks.com/ram.htm
• http//computer.howstuffworks.com/computer-memory1
  .htm

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Thank you