OCR GCSE Computer Science - Unit 1 Revision

Calculate how many megabytes there are in 6GB.

1 GB = 1000 MB 6 GB = 6 X 1000 = 6000 MB

Give two reasons why a secondary storage device is needed in most computer systems.

1. Data that is stored in RAM is lost when the power is turned off because RAM is volatile. 2. A secondary storage device, such as a hard drive, provides permanent storage for data that would otherwise be lost when the power is turned off. 3. Data can be moved between computers using secondary storage devices.

Noah has 200 photos that he wants to put on a CD or DVD. Each photo is 8MB in size. Calculate whether all his photos can be saved on a single CD or on a single DVD.

200 photos at 8MB each needs a total of 200 X 8 MB = 1600 MB = 1.6 GB. A single CD can store 700 MB, so Noah's photos cannot be saved on a single CD. A single DVD can store 4.7 GB, so his photos can be saved on a single DVD.

Define the term "stored program concept"

A computer in which the program instructions and the data are stored in memory.

Describe what is meant by a register.

A register is a memory location within the CPU used to store program instructions and/or data.

Define what is meant by an 'embedded system'.

An embedded system is a computer system built into another device in order to control it.

Explain why low-level languages such as assembly language are used for writing the programs for embedded systems.

Assembly language is used so that the hardware can be directly controlled by the programs written. This is far more efficient than using a high-level language that needs an interpreter or compiler.

John's computer is using virtual memory. Explain how the use of virtual memory affects the performance of the computer.

By using virtual memory, the computer will be able to continue to process instructions without having to close down any programs. There will be a significant drop in performance if the computer system relies too heavily on using virtual memory because the read/write speed of a magnetic disk drive is a lot slower than that of RAM.

Ollie gets a free DVD containing images and programs attached to a computer magazine. Give two reasons why a DVD is suitable for this purpose.

DVDs are very cheap and store up to 4.7 GB of data. DVDs are very light and easy to distribute.

Explain why magnetic devices are used in preference to optical ones as the main storage devices in most computer systems.

Data can be written to and read from magnetic devices far more rapidly than from optical devices. This improves the performance of the data processing.

Explain the difference between 'general purpose machines' and 'embedded systems'.

Desktop computers are designed to run a range of different applications e.g. word processors, spreadsheets etc. while each embedded system is designed to perform a small, specific range of functions linked to the device it is embedded into.

Explain how a computer's OS manages the use of virtual memory.

If there is no free memory, the OS will 'swap out' some of the data stored in RAM to the swap area on the hard disk drive and 'swap in' the requested data to the now free area. Usually the least recently used stored data is swapped out and, if it is needed again, it is then swapped back in at the expense of other data.

An SD card is an example of a solid-state storage device. Give a reason why it is called a 'solid state' storage device.

It is called solid-state storage devices because it has no moving parts.

Gloria has removed the hard disk drive from her laptop and replaced it with a solid-state drive. Give two reasons why this is a suitable upgrade.

Lighter and more portable. No moving parts that can be damaged if dropped. Data access speeds are faster than for magnetic storage.

Describe how data is stored on magnetic storage devices.

Magnetic storage devices use electromagnets in their read/write heads to read and write the data, which is encoded as opposing magnetic polarities on the surface of the disk or tape.

List two components with which the CPU works to execute program instructions.

Memory Input and Output Devices

State two components of Von Neumann's design in addition to the CPU.

Memory to store program data and instructions. Input and output devices.

Describe how data is stored on optical storage devices.

Optical disks use a laser to read and write data. The data is encoded as a series of pits in a spiral track running from the inside to the outside of the disk.

The performance of a CPU can be improved by increasing its clock speed. Explain why performance cannot be increased indefinitely by increasing the clock speed.

Performance cannot be improved indefinitely because the rate at which the transistors process the instructions is limited. There can also be a problem with the amount of heat generated by high clock speeds, which may cause a malfunction if it cannot be dissipated.

Identify three components of an embedded system.

Processor Memory Input and output interfaces.

A computer's main memory consists of both RAM and ROM. Compare RAM and ROM.

RAM is volatile but ROM is non-volatile. RAM can be written to and read from, but ROM can only be read from. RAM stores program instructions and data, but ROM is used to store the sets of instruction needed for a computer to start.

State the role of RAM in the fetch-decode-execute cycle.

RAM is where the program instructions and data are stored until they are needed.

State one example of data which is stored in RAM.

Sets of instructions needed for the computer to start. Data used by a program when it is executing.

Apart from SD cards, state two other uses of solid-state storage.

Solid state drive USB drives

Describe how data is stored on a solid-state drive.

Solid state memory is made of flash memory which uses arrays of transistors working as switches that are set to represent the digits 1 or 0. These retain their state when the power is switched off.

State the function of the Memory Data Register (MDR).

The MDR is temporary store(buffer) for anything copied from memory.

State the function of the Accumulator.

The accumulator is a temporary store for the results of calculations carried out by the ALU.

Explain why cache memory is used in the CPU.

The cache consists of very fast memory and it is used to store frequently used commands and data. These can be accessed more quickly from the cache than from the slower RAM when they are needed.

Describe the role of the Control Unit in the fetch-decode-execute cycle.

The control unit coordinates the actions of the computer. It sends out control signals to other parts of the CPU and to other components of the computer.

Ollie's desktop computer has both a magnetic hard disk drive and a solid-state drive. Explain why a desktop computer would have both instead of just having a solid-state drive.

The solid-state drive can be accessed more rapidly and used for system files, programs and application data that need to be accessed rapidly and frequently. The slower, hard disk drive can be used for long-term storage of files that don't need to be accessed as quickly.

State the function of the Program Counter.

This holds the address of the next instruction to be fetched.

Give one reason why cache memory is not used for RAM.

Too expensive.

Explain what is meant by 'virtual memory'.

Virtual memory is an area of the hard disk drive or solid-state drive used as temporary RAM when the actual RAM is full. Instructions that haven't been used recently are swapped out to virtual memory to free up space in main memory for other instructions and data that the processor needs immediately.

Define what is meant by 'volatile memory'.

Volatile memory provides temporary storage for program instructions and data. It loses its content when the power is switched off.

Explain why a computer sometimes needs to make use of virtual memory.

When a computer is running the operating system and several applications at the same time, the RAM often becomes full. The OS will use virtual memory to store some of the data, usually on the hard disk drive.