1. Computer Memory & Storage Devices

2. Memory
Computer memory is any physical device capable of storing information temporarily or permanently.
For example Random Access Memory(RAM), is a volatile memory that stores information on an integrated circuit used by the operating system, software, and hardware

3. CLASSIFICATION- BIG PICTURE
Registers
Cache (I, II, III)
Memory Types &
Storage Devices
Volatile
SRAM
DRAM
SDRAM
Primary
RAM
(Semiconductor – chip). Main Memory
EDRAM
EDO
FLASH RAM
PROM
Non-volatile
EPROM
ROM
EEPROM
Tape
Magnetic memory
HD, Zip Disk
Secondary
FDD
CD-ROM, CD-R, CD-RW
(Devices)
Optical Memory
DVD-ROM, DVD-R
DVD- RW

4. 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

5. Contents of each location persist as long as power is applied.
Static RAM
Static RAM is the most expensive. It requires more amount of space for a given amount of data than dynamic RAM.
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

6. 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

7. 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

8. Read Only Memory(ROM) (cont.)‏
Different types of ROM are
PROM (Programmable ROM)
EPROM (Erasable Program ROM)
EEPROM ( Electronically Erasable Program ROM.

9. 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

11. Storage Devices and Media
Objectives
Understand the difference between internal memory & backing storage
Be able to identify different types of storage devices & media
Identify the uses, advantages & disadvantages of different types of storage devices & media

12. Difference between internal memory & backing storage
Internal memory is the memory that is occupied by the current data and instruction being dealt with.
Its contents are lost when the computer is turned off.
Backing storage is the additional memory which is used to store data & instructions that we may need to use again.
Hard Disk & Drive
Floppy Disk & Drive
Optical Disk Drive
USB Storage Devices
Magnetic Tape
Its contents are NOT lost when the computer is turned off.

13. Difference between internal memory & backing storage
Internal memory, also known as Immediate Access Storage (IAS) is:
Small
Fast
Volatile
Backing storage is:
Large
Non –Volatile
Not very fast to read from.

14. How do we measure the size of memory?
Storing an individual character such as a letter, number or other character needs a set amount of space called a
Byte
To store 1024 characters we need 1024 Bytes or 1Kilobyte (1Kb)
1 million bytes (or 1000Kb) is called I Megabyte (Mb)
1000 Mb is called 1 Gigabyte (1Gb)

15. Hard disk drives
The Hard Drive is the DEVICE which reads & writes to the Hard Disk
The Hard Disk is the STORAGE MEDIA
It’s Magnetic media
Data is stored magnetically onto tracks on the disk
Disk rotates at high speed – passing under the read/write heads
Read/write heads READ the data into main memory and WRITE data back to the disk after it has been dealt with

16. Storing Information on a Hard Disk
Each platter is broken up into tracks and sectors
Tracks are concentric circles on the disk
Each track is broken up into a series of sectors
Track (yellow ring)
Sector (yellow ring
between the lines)

17. Sectors and Blocks
Sectors are further broken up into blocks
A block is a fixed size unit of storage
512 bytes/block is most commonly used
1024 bytes/block is common with SCSI disks
2048 bytes/block is used with CDs
If the user stores onto the hard disk a file which is larger than the block size, then multiple blocks are used.

18. Blocks and Files
If a file takes up multiple blocks, it is necessary to keep track of which blocks comprise that file
Each block is assigned an address
The location of a “file” is stored in what is called a “File Allocation Table” (or FAT)
When the hard disk is formatted, several blocks are reserved so that the Operating System can manage where files are stored on the disk
FATs are often used to keep track of the filename and directory as well.

19. Filesystems
Files are managed within a “filesystem”
The filesystem defines how and where files are stored within a hard disk (or partition)
Common filesystems include:
FAT16 (MSDOS)
VFAT (Windows 95)
FAT32 (Windows 98)
NTFS (Windows NT)
UFS (UNIX)
ext2/ext3 (Linux)
ISO9660 (CD Roms)

20. File systems
When a disk is formatted, a file system is placed on the disk
The file system reserves space for the FAT.
The remaining space is available for files
When a file is to be saved into the file system, the system looks for the best location to save the file
It is usually best to save the file in contiguous blocks.
If the disk is nearly full, it may not have enough contiguous blocks to save the file. In that case, the file system will place the file wherever it can find space.

21. Deleting Files
In order to make the delete operation faster, when a file is deleted ONLY the FAT is updated.
The actual data still remains on the disk
This is why it MAY be possible to recover a file which has been removed.
The FAT is told that the blocks where the file was stored are now available for writing
The data will remain until another file is stored in the same blocks.
As more files are written on the disk, it is less likely that a delete file can be recovered.

22. Hard Disk Fragmentation
When files are deleted, their blocks are open for writing.
What if the next file which is written in the same blocks is larger than the file which was originally written there?
The system has to break the file up into pieces throughout the filesystem
This is called fragmentation. It considerably slows the access of files.
Most modern Filesystems contain automatic defragmentation utilities.

23. Hey! Wait a minute!
When you purchase a hard disk, the manufacturer indicates its capacity.
When you format that hard disk, you’ll notice that its capacity is less than what the manufacturer told you.
Formatting the disk requires space for the FAT
Manufacturers use decimal numbers to represent number of bytes
1GB = 1,000,000,000 bytes
There is a class action suit currently filed against the major disk manufacturers because of this

24. Hard disk drives Uses Advantages Disadvantages Stores software
Stores your data files
Advantages
Large storage capacity
Stored items are not lost when the computer is switched off.
Usually fixed inside the computer, so can’t get lost.
Disadvantages
Slower than IAS
If the hard disk crashes the computer will not work & you have lost your work!!!

25. What is a floppy disk?
Portable, inexpensive storage medium (also called diskette)
Thin, circular, flexible film enclosed in 3.5” wide plastic shell

26. Floppy disks & drives Uses Advantages Disadvantages
To keep personal data
Keep extra copies of data
Advantages
Can be carried with you
Disadvantages
Limited capacity (typically 1.44Mb)
Unlikely to store your ICT coursework on one disk

27. Zip Drives
Zip drives are similar to floppy drives because the individual disks are removable and portable but they hold much larger amounts of data
Typically between 100 MB and 2 GB.

28. Optical disk drives CD Rom Uses Advantages Disadvantages
Compact Disk Read Only Memory
Known as WORM devices Write Once Read Many times.
Uses
Storing software
Reference material (multimedia)
Advantages
Data cannot be erased
Portable
Much larger capacity than floppy disks (about 650Mb)
Disadvantages
Can get lost
Can’t write data to a CD Rom
Data access can be slower than a hard drive.

29. DVD – Digital Versatile Disk
DVDs hold approx 7 times the information that CDs do in the same amount of storage space
DVDs come in 3 types
Single Sided/Single Layer (4.7 GB)
Single Sided/Double Layer (8.5 GB)
Double Sided/Double Layer (17 GB)
DVD uses a laser with a shorter wavelength so the pits are smaller
More pits can be stored on a DVD
Narrower track

30. DVD – Multiple Layers
Each side of a DVD can contain 2 layers, doubling the amount of data that can be stored
One layer is semi-transparent. The laser can be focused “through” that layer onto the second layer.
If you took the track off of a single layer of a DVD and stretched it out into a straight line, it would be 7.5 miles long!
If you did the same with a double layer, double sided disc, the track would stretch to over 30 miles!

31. DVD-R and DVD-RW
These two technologies are still relatively new and still fairly expensive.
DVD-R and DVD-RW use similar techniques as CD-R and CD-RW
A dye layer is used for data writing
The dye layer in DVD-RW is erasable

32. Optical disk drives DVD Uses Advantages Disadvantages
Digital Versatile Disk
Uses
Storing high quality audio & video
Advantages
Data cannot be erased
Portable
Much larger capacity than floppy disks and CD Rom (about 4.7 Gb)
Disadvantages
Can get lost
Can’t write data to a standard DVD

33. Optical disk drives CD Rewriter Uses Advantages Disadvantages
Back up software & data files
Copy music files
Advantages
Portable
Can store much larger files than floppy disks
Disadvantages
Can get lost
Can’t always be read in some Optical drives

34. Optical disk drives DVD Rewriter Uses Advantages Disadvantages
Back up software & data files
Copy video files
Advantages
Portable
Can store much larger files than CD R’s
Disadvantages
Can get lost
Can’t always be read in some Optical drives or DVD players

35. USB storage devices Recent development Fits on a key ring
Smallest storage capacity is about 32Mb = about 22 floppy disks
1Gb versions now available
Is this the end of the floppy disk?

36. Comparing storage device capacity:
1 CD-ROM (650 MB) = 451 Floppy disks (1.44 MB)
1 DVD (4.7 GB) = 7 CD-ROM's (650 MB)
Access speed
Hard drive KB/s
CD-ROM KB/s
Floppy disk - 36 KB/s

37. Direct and serial access
Floppy disks, hard disks CDs and USB storage devices all allow direct access to data.
Direct access means that the required data can be found straight away without having to read through all the data on the disk.
Magnetic tape allows only serial access to data. To locate data on a magnetic tape it has to be searched from the beginning until the required data is found.

38. File compression
File compression software can be used to make files smaller so that more data can be stored in the same amount of space on backing store.
When a compressed file on backing store needs to be used it must be decompressed.
This can be done using decompression software or by setting files up to be self-extracting which means that they can automatically decompress themselves.
Winzip is an example of software that can be used to compress and decompress files.