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Computer Organization

ROM and Secondary Storage

Powerpoint 2B – ROM and Secondary Storage

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RAM and ROM Compared

RAM

Random Access Memory

Fast

Expensive

 Volatile

ROM

Read Only Memory

Fast

Cheap

Non-Volatile

We studied RAM in the last Unit. It provided storage for instructions and data while the computer is running. RAM isvolatile, which means the contents are erased when you power down the computer.

Information in ROM is organized into bytes, just like RAM. However, ROM provides non-volatile storage, which meansthe contents are not erased when you shut down the computer. The computer can only read the instructions and datafrom ROM, while it can read from and write to RAM.

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ROM

Hardwired

Programmed with Diodes

Time consuming to produce

 Very cheap when mass produced

The picture at the right shows a ROM chip on a computerboard. Producing a single ROM chip is an expensive andtime consuming process. However, because each computerrequires exactly the same chip, they can be mass produced.We refer to the low cost of mass production as economiesof scale.

ROM is used in a computer to store the instructions that must run every time a computer starts up. It also containsinstructions that read and write data to secondary storage. Notice the label on the chip in the picture says AMIBIOS.

 AMI is the company that produced the ROM. BIOS stands for Basic Input Output System.

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ROM Variations

PROM

EPROM

EEPROM

Flash Memory 

Several variations of ROM are shown above. The top three areno longer part of the IB curriculum, so feel free to skip the nextthree slides and go directly to Flash Memory if you are notinterested.

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PROM

Programmable Read onlyMemory

Grid of Rows and columnsconnected by fuses

Fused connectionrepresents 1

Programmed byburning fuses

Programming ispermanent

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EPROM

Erasable Programmable Read Only Memory

Chips can be written many times

Programmed like PROM

Erased with ultraviolet light

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EEPROM

Electrically Erasable Programmable Read Only Memory

Programmed like PROM

Can be erased with electrical signals

Much slower than RAM

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Flash Memory

Special type of EEPROM

Provides non-volatile storage

Memory is erased in blocks instead of 1 byte at atime

Commonly used for storage in digital cameras andcell phones

Flash memory provides non-volatile storage that can be erased with a special signal.The picture at the right shows a flash memory chip that might look familiar to you. Itis Smart Media card that is commonly used in digital cameras. This particular cardprovides 256 MB of storage for pictures.

Other common applications for Flash Memory include USB Thumb drives, MP3players, some iPod’s, and cell phones. In addition to storing your data, flash memoryallow companies to provide updates for programs that run on many devices.

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Secondary Storage

Flash Memory

Magnetic Disks

Optical Disks

Magnetic Tape

In general, secondary storage refers to any storage that is not RAM. Secondarystorage is non-volatile. Common forms of secondary storage are shown above.

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Magnetic Disk 

Ferromagnetic iron oxide

Direct Access

High Capacity

Faster than flash or tape

Slower than RAM

Magnetic Disk storage is often called the hard drive. These areoften found mounted inside a computer. The picture at the rightshows an external hard drive, meaning it can be attached to acomputer with a cable.

Currently, magnetic disk can provide more storage than flashdrives. A good magnetic disk is also faster than a flash drive.

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Magnetic Disk Structure

Cylinders

Platters

Tracks

Sectors

The picture at the right shows the structure of a magnetic disk. Most hard drives actuallycontain several platters. Data is written onboth sides of the platter. Each platter isdivided into concentric circles called tracks.Each track is divided into equal sized

segments called sectors.

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Optical Disks

Large amount of storage on verysmall surface

Cheap to manufacture

Bits are represented by bumps on a

tightly wound spiral Tracks spacing

is .5 microns

I’m going to assume that everyone in class has seen an opticaldisk. You know them as CD’s and DVD’s. Most personalcomputers contain drives that read from and write to optical

disks.

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Magnetic Tape

Ferromagnetic iron oxide

Sequential Access

Cheap

High Capacity

Slow

Tape drives provide large amounts of secondarystorage, but are less common today than 20 yearsago. One practical application for tape drives isbacking up large amounts of information fororganizations or businesses. A common exampleof a tape drive that you may have seen in yourhome is a VCR or audio tape.

Tape drives provide sequential access. That means it is difficult to access any particular piece of information. (Haveyou ever tried to find a particular song on an audio tape, or a particular seen on your VCR?)

With sequential access, data is stored in order, and you need to search for data by looking through the previous items.In contrast, direct access allows you to go directly to the piece of information you want. Other forms of secondarystorage provide direct access.

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Typical Storage Capacities

Flash Memory 128 MB – 2 GB

Magnetic Tape 300+ GB

Magnetic Disk 10 – 300 GB

Optical Disk (CD) 750 MB

Optical Disk (DVD)

4.7 – 8.5 GB

The table to the right shows typicalcapacities for different storage devices(2006). Each year storage capacitiesincrease with technological improvements.

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Units of Storage

Bit Binary Digit

Byte 8 Bits

Kilobyte (KB) 1024 bytes

Megabyte (MB) 1024 KB

Gigabyte (GB) 1024 MB

Terabyte (TB) 1024 GB

This slide should be review, since wecovered these units in the lastlesson. In computer science, mostnumbers are base 2 rather than base10 like you learned in otherdisciplines.

Term SI Computer

Kilo 1000 103 1024 210

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 Virtual MemoryWe studied RAM in the last Unit. It provided storage for instructions and data while the computer is running. RAM isexpensive. On many computers, applications require more storage than is available on RAM.

 Virtual Memory allows primary storage (RAM) can be extended using secondary storage. The available storage in RAM isdivided into equal size blocks called pages. A much larger chunk of secondary storage contains a copy of all the pagesthat are in RAM, plus many more that have been swapped out of RAM. Wnenever necessary, pages are moved fromsecondary storage to RAM (swapped in) or moved from RAM to secondary storage.

 A computer with 512 MB of RAM might have as much 4 GB of Virtual Memory. In the simplified diagram below, the red

pages are swapped in - they are present in both primary and secondary memory. The green pages are swapped out.Notice also how the pages in primary storage are not in the same order as they are in secondary storage.

Page 0

Page 3

Page 10

Page 9

Page 8

Page 5

Page 0

Page 1

Page 2

Page 3

Page 4

Page 5

Page 6

Page 7

Page 8

Page 9

Page 10

Secondary Storage.Primary Storage.

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Online References

http://computer.howstuffworks.com/removable-storage.htm 

http://computer.howstuffworks.com/flash-memory.htm