Storage characteristics and trends

Questions

1. What does 'random access' mean?
Random access means that any storage location can be directly read or written. It is also called 'direct' access.

2. What does 'sequential access' mean?
Sequential access means that the storage locations have to be read or written in their correct order, starting with the first location.

3. What does 'volatile' mean?
Volatile refers to storage devices that will lose all thier data if the power is switched off.

4. Why is RAM the only volatile storage device?
RAM is the only storage device that can described as volatile as it needs a contant power supply to keep its data.

5. Why are sequential access devices better suited for storing backup data?
They work at a slow speed. Although access times may be poor, the rate of continuously writing or reading data can actually be very fast.

Storing and Retrieving hardware

RAM and ROM

RAM (Random Access Memory)- is needed in any system that needs to store new instructions and new data, or to change those instructions or data. RAM chips are primary storage devices found in every computer system.

The most common forms of RAM include:

• DRAM (Dynamic RAM)- cheapest and the slowest, uses transistor and capacitor, needs regualr refreshing [used in main system memory]
• SRAM (Static RAM)- faster but more expensive than DRAM, does not use capacitor [built in CPU]
• EDO (Extended Data-Out) RAM- allows more than one memory location to be accessed at the same time, speeds up data access [Main system memory]
• SDRAM (Synchronous DRAM)- most common RAM in desktop PCs [Main system memory]
• RDRAM (Rambus DRAM)- uses a high speed data bus to access memory locations, needs a specially designed motherboard [Main system memory]
• VRAM (Video RAM)- High speed RAM used to hold graphics data including 3D models and textures [3D accelerator graphics cards]
• Flash RAM

ROM (Read Only Memory)- memory whose content can be accessed and read but cannot be altered. Used for BIOS (on the ROM chip)- helps load the system

Varieties of ROM:

• ROM- Permanent data storage [can be found in singing birthday candles and musical greeting cards]
• PROM (Programmable ROM) - Permanent data storage. Data is added on to the blank ROM chips by a 'burning in' process. Easily damaged by static electric charges [used in security systems that need to prevent any alteration of data e.e. smart ID cards]
• EPROM (Erasable Programmable ROM)- Data can be erased by exposing the chip to ultraviolet light. New data can then be added
• EEPROM (Electronically Erasable Programmable ROM)- Data can easily altered without the need for special equipment or to remove the chip [used in cash registers, computer BIOS]
• Flash memory

Magnetic Storage Devices

• Common magnetic storage devices are tapes, hard disks and floppy disks.
• Magnetic tape is still commonly used for storing backup data
• Disks are organised into tracks and sectors that hold the data
• Saving a file involves filling up a disk sector and recording the location of each sector used
• Disk formatting prepares the surface to receive data

Optical storage devices

• The term 'disc' is commonly used for optical storage devicws while 'disk' is used for magnetic devices
• 'CD' stands for Compact Disc
• CD-R technology uses a coloured dye that can be changed by heating to permanently record data
• CD-RW technology uses a dye that can be swtiched between cloudy and transparent
• DVDs (Digital Versatile Discs) can store data on two separate laters on both surfaces of the disc

Assess optical storage as a possible replacement for magnetic storage devices.
Optical storage devices can store a large quantity of data and cost very little to manufacture. If low portable storage is needed, then CD-Rws are probably better value than floppy disks. However, the do not match a hard disk for speed and storage capacity. If high capacity, high speed storage is needed, then hard disks are still a better choice than optical storage devices.

Questions
1. What is the most common type of RAM used in computers?
SDRAM (Synchronous Dynamic RAM)

• uses bursts of higher speed to read the individual data bits once it has located the memory address

2. What makes EEPROM the most useful of the ROM type of memories?
The data on EEPROM can be easily altered without the need for special equipment or to remove the chip. It provides a convenient way to allow permanently stored instructions to be upgraded.

3. Optical storage devices store data as light- true or false?
False.

4. Why would PROM be an ideal storage device for security data?
Security data would usually require prevention of any alteration to the data. Having blank chips where security data can be added to and not altered makes PROM an ideal storage device for this type of data.

5. Give examples of situations where CD-R is a better choice for data storage than CD-RW

• When handing in a final assignment to a teacher, having data that cannot be altered by anyone on the disc
• When storing textbook information on a disc

6. 3.5 inch floppy disks have changed very little since they were first introduced in the mid 1980s- that makes them old technology. Give some reasons for their continued popularity.

• compact
• light
• portable
• removable data

7. Assess the advantages and disadvantages of flash storage devices as replacements for floppy disks.

Advantages

• has a higher storage capacity
• fast and does not require any power to keep data in storage
• portable (small size)

Disadvantages

• requires a small electric current to 'remember' its stored data

SSD- Solid State Drives: Instead of using a hard disk drive, it uses flash memory; permanently located inside the computer, secondary storage. Advs: fast, no moving parts to crash,