Computer Memory & Storage Devices

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

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

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

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

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

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

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

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

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.

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.

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)

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

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)

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.

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.

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)

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.

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.

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.

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

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!!!

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

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

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.

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.

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

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!

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

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

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

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

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?

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 - 1000 KB/s CD-ROM - 100 KB/s Floppy disk - 36 KB/s

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.

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.