ROM
A ROM chip is a memory chip that stores instructions and data permanently. The information on a ROM chip is stored on it by the manufacturers and cannot be modified by the user. A CPU can read and retrieve the instructions and data from the ROM chip, but it cannot change the contents in ROM. Information on the ROM chip is only for the computer to use. When you turn on a computer, messages are made possible by the information in ROM. The programs on a ROM chip are called firmware, this includes: (a) a wordprocessor ; (b) the operating system or the part of the operating system needed to start up the computer; utility programs, including anti-virus programs and disk management programs.
NOTE: The contents of ROM cannot be changed in any way (hence read only) not even when the computer is turned off.
Variations to ROM
i) Programmable ROM (PROM)
This can be thought of as blank or empty ROM, that is to say nothing is stored on the chip by the manufacturer; it is the user or the hired programmer who adds the contents of PROM. The user/customer can thus select the most appropriate program to suit his/her needs (customization). This customization however is only allowed once. Therefore the PROM acts just like ROM after the first instance of writing programs to it.
ii) Erasable Programmable ROM (EPROM)
This type of memory can be thought of as reusable PROM. After being customized, this device can be reset to its original state using a special process (using ultra violet light) and then it can be re-customized. The EPROM chip must be removed from the computer in order to be processed.
iii) Electrically Erasable PROM
This type of memory is similar to EPROM except that is uses electrical pulses to erase and to write to the chip. It is also possible to erase the chip while it is in the computer.
Cache Memory
Cache memory is a special high-speed memory designed to supply the processor with the most frequently requested instructions and data. Instructions and data located in cache memory can be accessed many times faster than instructions and data located in main memory. The more instructions and data the processor can access directly from cache memory, the faster the computer runs as a whole.
In the same way when the cache controller retrieves an instruction from main memory, it also takes the next instruction back to cache with it. This increases the chances that the next instruction requested by the CPU is already in cache. When a request from the CPU is found in cache, this is referred to as a cache hit. When it is not found it is referred to as a cache miss.
Buffers
A buffer is an internal memory area used for temporary storage of data records during input or output operations. For example most modern printers are equipped with buffers that store the information or data to be printed. The speed of printing tends to be slow when compared to the speed at which data is sent to the printer. Information or data is sent to the printer buffer, freeing memory to concentrate on other important tasks.
Secondary/Backing Storage
Backing storage is used for long-term data storage, usually on magnetic tape; magnetic disk or compact disk (CD) The storage is non-volatile.
Backing storage is needed for two main reasons:
-
Computer memory is volatile.
For example, when you are working on a word processing document it is stored in memory. If you do not finish it by the end of the lesson, and the computer is then switched off, the document will be lost. You would need to store it on some form of backing store (eg floppy disk) before the power is switched off. The document will stay on the disk so that you can load it into memory and carry on with it next lesson.
- Computer memory is often not large enough to store large data files.
The data would be stored on backing storage and loaded into memory a small amount at a time as required.
The school database of pupil’s grades and reports is approximately 80 million bytes of information. The memory in your computer is approximately 4 million bytes. You could not load the whole database into memory.
Points to note
- A storage medium is the material on which the data is stored eg magnetic tape, floppy disk, CD etc.
- A storage device is the piece of equipment, which uses the storage medium. e.g. Floppy disk drive, hard drive, tape unit, CD drive....
- The amount of data that can be stored on a disk or tape is measured in the same way as the size of memory - using bytes.
- One byte is the amount of storage needed to store one character.
So, for example to store the word CUCUMBER would need 8 bytes.
Magnetic Disks
Magnetic disks are the most common form of backing storage for all sizes of computers.
A typical magnetic disk has two surfaces although sometimes disk packs on mainframe computers are made up of a number of disks).
Data can be written to or read from the disk.
Access on magnetic disks is direct access.
There are two types of access to a backing storage medium...
Serial Access - all data before the required item has to be read first before the required data can be accessed (e.g. on magnetic tape)
Direct Access - Any data item can be accessed without having to read other data first (e.g. on Magnetic disk, CD)
If you want to play a music track on a CD you can do so straight away (direct access), but on a tape you have to play through other tracks first to find it (serial access).
Used for storage on microcomputers.
Usually 3.5 inches in diameter, they can store 720K of data. High density disks can store 1.44Mb.
Access to data is slower than from a hard disk.
The disks are protected by a stiff plastic cover. This has a hole for the read/write heads which is protected by a sprung metal cover.
The data can be protected by opening a small write-protect tab that prevents the contents of the disk being changed.
Floppy disks are portable and can be used for
- transferring data or programs from one microcomputer to another.
- backing up data from a hard disk.
Floppy disks (and hard disks) have to be formatted before they can be used.
Care of Floppy Disks
Do NOT....
- open the sliding metal cover and touch the disk.
- put disk in heat or damp.
- put a disk near a magnetic source.
- put the disk in a dirty or dusty place.
One or more hard disks are found in the hard drive. Each drive is built into a sealed unit to prevent contamination by dust and moisture.
Access to data is faster than from floppy disks.
Hard disks store more data than floppy disks. 3 Gb drives are now common.
They are more reliable than floppy disks as they have more protection from dirt etc.
(Don't try to open yours!!!)
All magnetic disks are similarly formatted, or divided into areas, called tracks, sectors and cylinders
The formatting process sets up a method of assigning addresses to the different areas. It also sets up an area for keeping the list of addresses. Without formatting there would be no way to know what data went with what. It would be like a library where the pages were not in books, but were scattered around on the shelves and tables and floors. You'd have a hard time getting a book together. A formatting method allows you to efficiently use the space while still being able to find things
TRACKS
A track is a circular ring on one side of the disk. Each track has a number.
The diagram shows 3 tracks.
SECTORS
A disk sector is a wedge-shape piece of the disk, shown in yellow. Each sector is numbered.
On a 5¼" disk there are 40 tracks with 9 sectors each.
On a 3½" disk there are 80 tracks with 9 sectors each.
So a 3½" disk has twice as many named places on it as a 5¼" disk.
A track sector is the area of intersection of a track and a sector, shown in yellow.
CLUSTERS
A cluster is a set of track sectors, ranging from 2 to 32 or more, depending on the formatting scheme in use.
The most common formatting scheme for PCs sets the number of track sectors in a cluster based on the capacity of the disk. A 1.2 gig hard drive will have clusters twice as large as a 500 MB hard drive.
1 cluster is the minimum space used by any read or write. So there is often a lot of slack space, unused space, in the cluster beyond the data stored there.
There are some new schemes out that reduce this problem, but it will never go away entirely.
The only way to reduce the amount of slack space is to reduce the size of a cluster by changing the method of formatting. You could have more tracks on the disk, or else more sectors on a track, or you could reduce the number of track sectors in a cluster.
CYLINDERS
A cylinder is a set of matched tracks.
On a double-sided floppy, a track from the top surface and the same # track from the bottom surface of the disk make up a cylinder. The concept is not particularly useful for floppies.
Compact Disks
CD-ROM (Compact Disk Read Only Memory) is a form of CD used for the storage of computer data.
Data is read from the disk using a low-power laser to detect the patterns in the surface of the disk.
A CD-ROM stores over 600Mb of data, which may involve sounds, video clips, animated sequences, text and graphics.
Usually data is written onto the disk before it is sold. Data can be read from the disk but
not written onto it.
New types of CD are being produced which can be written to by the user. These include the WORM (Write Once Read Many) - this can have data written onto it once only but other erasable CDs are now available where CDs (CD-RW Compact disk Read Write) can be written onto many times like a floppy disk.
Magnetic Tapes
Magnetic Tape is now mainly used as a backup medium. It is a cheap medium for backing up hard disks on both microcomputers and mainframes.
Conventional tape comes in reels up to 4600 feet long.
Magnetic tapes stored in racks.
Tape is also available in cartridges. Tape cartridges are frequently used for backing up hard disks on microcomputers. Tape streamers can be scheduled to make backups at night.
Advantages of Hard Disk over Floppy Disk
- Storage capacity is significantly greater.
- Hard disk allows faster access to data than floppy disks because a hard disk spins several times faster.
Disadvantages of Hard Disk over Floppy Disk
- Head crash can occur. Ahead crash happens when the surface of the read/write head or particles on its surface come into contact with the disk surface causing the loss of some or all the data on the disk.
- Unlike a floppy, the hard disk cannot be easily moved from place to place without much complication.
Advantages of Optical Disks
- It is much more sturdier. It is physically hard to break or melt or warp.
- It is not sensitive to being touched, though it can get dirty or scratched.
- It is entirely unaffected by magnetic fields.
Disadvantages of Optical Disks
- Optical disk technology is costly but prices are falling.
- It is not easy to copy an optical disk.
Advantages of Magnetic Tape Storage
- Inexpensive, reusable and easy to handle.
- High data density, making it easy to access data fast.
- Variable record length.
Disadvantages of Magnetic Tape Storage
- Only sequential access.
- Cannot be read by humans
- Vulnerable to dust, heat, and humidity.
