Memory Organization

Memory-Internal storage areas in the computer. The term memory identifies datastorage that comes in the form of chips, and the word storage is used for memory that exists on tapes or disks. Moreover, the term memory is usually used as a shorthand for physical memory, which refers to the actual chips capable of holding data. Some computers also use virtual memory, which expands physical memory onto a hard disk.
Every computer comes with a certain amount of physical memory, usually referred to as main memory or RAM. You can think of main memory as anarray of boxes, each of which can hold a single byte of information. A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes (orcharacters) of information.


1.0 Types of Memory

•  RAM (random-access memory): This is the same as main memory. When used by itself, the term RAM refers to read and writememory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.

•  ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.

•  PROM (programmable read-only memory): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.

•  EPROM (erasable programmable read-only memory): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.

•  EEPROM (electrically erasable programmable read-only memory):An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.

2.0 Cache Memories
 Cache memories are small, fast SRAM-­‐based Memories managed automatically in hardware.
 Hold Frequently accessed blocks of main memory
 CPU Looks first for data in caches.
 Typical system structure:

General Cache Organizationon (S,E,B)

Cache Read

Example:Direct Mapped Cache (E=1)

Direct mapped: One Line per set

Assume: Cache block size 8 bytes

Example:Direct Mapped Cache (E=1)

Direct mapped: One Line per set

Assume: Cache block size 8 bytes

3.0 Types of External Memory

• Magnetic Disk

— Floppy

— Winchester

— RAID

— Removable

• Optical

— CD-ROM

— CD-Writable (WORM)

— CD-R/W

— DVD

• Magnetic Tape

Magnetic Disk

• Disk substrate coated with magnetizable material (iron oxide…rust)

• Substrate used to be aluminium

• Now glass

— Improved surface uniformity

– Increases reliability

— Reduction in surface defects

– Reduced read/write errors

— Lower flight heights (See later)

— Better stiffness

— Better shock/damage resistance

• Range of packaging

— Floppy (PC’s A: drive, B: drive)

— Winchester hard disk (PC’s C: drive)

— Removable hard disk

Floppy Disk

• 8”, 5.25”, 3.5”

• Small capacity

— Up to 1.44Mbyte (2.88M never popular)

• Slow

• Universal

• Cheap

Optical Storage CD-ROM

• Originally for audio

• 650Mbytes giving over 70 minutes audio

• Polycarbonate coated with highly reflective coat, usually aluminium

• Data stored as pits

• Read by reflecting laser

• Constant packing density

• Constant linear velocity

Magnetic Tape

• First kind of secondary memory

• Serial access

• Slow

• Very cheap

• Backup and archive

4.0 Data Organization and Formating

Disk Geometry

Disk Geometry (Muliple-Platter View)

Disk Operation (Single-Platter View)

Disk Operation (Multi-Platter View)