Main Memory & Backing Storage
MEMORY and STORAGE are two DIFFERENT things.
MAIN MEMORY is made up of RAM and ROM. But these are NOT used to store all the programs and data you need on a computer for three reasons:
- Meomory is too SMALL to store all these programs and data
- The contents of RAM are LOST when the computer is turned off
- The contents of ROM CAN'T be RE-WRITTEN.
Data that is stoed on BACKING STORAGE is usually CHANGEABLE and can be stored LONG TERM.
As most computers need to store data for long periods of time and need to be able to use it, both MEMORY and BACKING STORAGE are required.
Storage Media and Devives
The STORAGE MEDIUM is the part which HOLDS the data.
The STORAGE DEVICE is the machine that READS data from the medium.
There are three types of storage device
MAGNETIC STORAGE MEDIA hold data magnetically. Tiny areas on the surface of the medium are magnetised to store bits (unit of computer storage, 8 bits in a byte) in codes that represent characters. If the device reads the medium then the magnetised areas can be turned back into codes which the computer can use. Eg. Hardisks & Magnetic tapes
OPTICAL STORGE MEDIA store data on their surface by tiny pits burnt by a laser into the surface. The laser is shone on tpo the surface and where it hits the pits, the reflection differs from the normal surface. These differences can be used to store codes for characters. Eg. CDs
SOLID STATE STORAGE is different as it has NO MOVING PARTS and fits DIRECTLY into the computer.The device and the medium are the same thing. Eg. Memory Sticks
Magnetic Storage Media: Fixed Hard Disks
FIXED HARD DISKS are used for MAIN STORAGE. They look a bit like an old record player and are stored in the computer's case.
Each disk has a READ/WRITE HEAD which can WRITE data onto the disk so it can be stored and READ data from the disk when it needs to be used.
They are used as the MAIN STORAGE on most computers because:
- They can store very large amounts of data
- Access to the data is fast
- There is no limit to the amount of times it can delete data and reuse the medium
Disadv: They are fragile
Hard disks store three types of data: The operating system, the user's applications and the user's data. These types of data are stored on the hard disk and sent to the computer's memory (RAM) when needed.
Hard disks use DIRECT ACCESS. This means they don't need to read through the whole disk to find data. The read/write head just looks in an INDEX to find where it is.
Magnetic Storage Media: Portable Hard Disks
PORTABLE HARD DISKS are very similar to fixed hard disks and use the same technology. However they are not connected inside the computer case.
This means they can be easily connected to any computer then removed and taken elsewhere.
They are very fragile and need more protection than fixed hard drives as they are more likely to get knocked when moved about. They are useful for making backup copies.
- Store lots of data
- Very fragile
Magnetic Storage Media: Magnetic Tapes
MAGNETIC TAPES are similar to magnetic disks, however the data is stored in a long line on a tape, not scattered over the surface of a disk. A read/write head is used, however unlike hard disks, it stays still and the tape moves past it.
This means that data stored at the end of the tape takes a very LONG TIME to get to because all the other data has to be read first. This is called SERIAL STORAGE - all the data is stored one after the other.
SEQUENTIAL STORGAE is where the data is stored in some sort of ORDER, like alphabetical order. This is still very SLOW compared to using hard disks, but faster than using serial storage as the device can fast forward through the unwanted bits.
- Can store lots of data
- Cheaper than disk storage
- Less likely to be currupted
- Very slow
Optical Storage Media: CDs and DVDs
CD stands for COMPACT DISK. A CD can hold quite large files but is limited to about 1GB. This is great for things like MUSIC FILES, however for films, they are too small.
DVD stands for DIGITAL VERSATILE DISK. A DVD can hold about 5-10GB of data: Perfect for storing FILMS.
The disks are the STORAGE MEDIA. Its the disk drive that is the STORAGE DEVICE. Both CD and DVD drives use a LASER to read and write the data, but a DVD drive uses a more precise laser allowing the data to be closer together so it can store more data.
There are different types of CD and DVD.
- CD-ROM / DVD-ROM (Read-Only-Memory)
- CD-R / DVD-R (Recordable)
- CD-RW / DVD-RW (Re-Writable)
Optical Storage Media: Types of CD & DVD
CD-ROMs and DVD-ROMs CANNOT be written to, but CAN be READ from. This means their contents can NEVER be changed. They are used for the sale and distribution of software and encyclopedias.
Adv: Robust, Cheap, Can't be corrupted. Disadv: Can't change the contents if you want to.
CD-Rs and DVD-Rs can be WRITTEN to ONCE. After that the contents can't be changed. Used to store films, music or to make copies of files at the time, eg. for an archive.
Adv: Can be written to, Once written data is protected. Disadv: Not all CD/DVD devices can play them, Can only be written once.
CD-RWs and DVD-RWs can be written to multiple times. They are ideal for moving files from one computer to another and for making back-ups.They can store LOTS of data and are DIFFICULT to damage.
Blu-rau disks have much larger storage capacities (25-50GB) than regular DVDs. They can also cope with high speed transfers but are much more expensive. They are used for HD-films and in video recorders and games consoles.
SOLID STATE STORAGE devices have NO MOVING PARTS. This means that they are very ROBUST, much more so than other storage media. Used for transporting files.
- Very ROBUST
- SMALL and extremely PORTABLE.
- EASY to use as many are designed to fit straight into a computer port.
- More expensive than other forms of secondary storage.
- Lifespan is limited, lightweight protective cover
- Easy to lose
There are two types:
- MEMORY STICKS / USB PENDRIVES which can be used for backups for the main files on a computer system and transporting files.
- FLASH MEMORY CARDS which are often used in cameras and mobile phones.
Data can easily be lost from a computer sytem. If a computer gets a virus, it gets stolen or it gets damaged you could lose very important data. Even if it is just accidentally deleted, you could lose a lot of important info.
For these reasons it is sensible to make another copy of the data, just in case something does happen to the computer and you lose the inportant information stored on it. This is called a BACKUP COPY.
It is best to make backup copies on other storage devices not just the computers hard drive as imagine if there was a fire or the computer was stolen. If the backup was on the same computer it would still be lost.
Backups of important data need to be stored away from the computer to keep it safe.
A backup is a copy of material on the main computer storage, taken incase the main system is corrupted.