rom and secondary storage
TRANSCRIPT
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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