Memory and Storage

- Sheetal Gosrani

Overview

Memory Hierarchy

RAM

Memory Chip Organization

ROM

Flash Memory

Memory Hierarchy

Slow and inexpensive

Increasing performance and

increasing cost

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

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

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

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

Memory Chip organization

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)

RAM Grid

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

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

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

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

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

Thank you