GCSE Computer Science Paper 1: Computer Systems

An embedded system is a computer built into a larger device in order to carry out one specific dedicated task rather than a range of different tasks. An example is the controller inside a washing machine, which manages the wash cycle and does nothing else.

The Program Counter holds the memory address of the next instruction that is to be fetched, and during the fetch stage its value is copied into the Memory Address Register and the Program Counter is then incremented so that it already points at the following instruction. The Accumulator holds the result of any calculation carried out by the Arithmetic Logic Unit, so that the value is immediately available to be used in a further operation or written back to main memory.

When RAM becomes full there is no physical memory available for the new application, so the operating system falls back on virtual memory, which is an area of secondary storage set aside for the purpose and known as the swap file or page file. The operating system identifies data belonging to applications that have not been used recently and writes it out to that area, freeing physical RAM for the program being opened. If the user then returns to one of those applications, the data has to be read back into RAM, which usually means writing something else out to make room. Because secondary storage is thousands of times slower to access than RAM, every one of these transfers takes a substantial amount of time during which the processor has nothing useful to do, and the user experiences the machine becoming increasingly unresponsive. When the shortage is severe enough that the system spends more time moving data than executing instructions, the condition is known as disk thrashing. The most effective solution is to install more RAM so that all the open applications fit in physical memory.

The file size in bits is colour depth multiplied by width multiplied by height, which is 24 x 640 x 480 = 7,372,800 bits. Dividing by 8 converts this to 921,600 bytes. Dividing by 1000 gives 921.6 kilobytes, and dividing by 1000 again gives approximately 0.92 megabytes.

Lossy compression reduces file size by permanently discarding data that is judged least noticeable, such as very high frequencies in audio or subtle colour differences in an image, and because that data is deleted the original file can never be recovered from the compressed version. Lossless compression reduces size without discarding anything at all, instead recording the information more efficiently by identifying repetition and encoding it compactly, so decompressing the file restores the original exactly, byte for byte. A spreadsheet of financial records must use lossless compression, because every figure in it is significant and there is no redundant detail that could be discarded without changing the meaning. Applying lossy compression would alter or destroy values, producing records that are inaccurate and worthless.

In a star topology every device is connected to a single central switch, so all traffic passes through that switch, whereas in a mesh topology each device is connected to several others, creating multiple possible routes between any two points. This produces a significant difference in reliability. A star topology has a single point of failure: if the central switch fails then every device loses connectivity, although a single broken cable affects only the one device attached to it. A mesh topology has no single point of failure, because if one connection is lost the data is simply rerouted along an alternative path and the network continues to function.

A firewall sits between a network and the outside world and examines every packet of data attempting to enter or leave. Each packet is compared against a set of rules that specify which addresses, ports and types of traffic are permitted, and anything that does not meet those rules is blocked rather than being passed on. This prevents many attacks from reaching the network at all and also stops malware that is already inside from communicating with external servers.

The company should choose solid state drives. The decisive factor is durability: staff who travel frequently will carry the laptops in bags, use them on trains and inevitably knock or drop them, and a hard disk drive contains platters spinning at thousands of revolutions per minute with a read write head hovering nanometres above them, which makes it highly vulnerable to physical shock. A solid state drive stores data in flash memory with no moving parts at all, so it is far more likely to survive the same treatment, and the cost of a failed drive is not the hardware but the lost work and the time a member of staff spends unable to do their job. Solid state drives are also significantly faster to read and write, so machines boot and open files more quickly, which matters when someone has fifteen minutes on a train, and they draw less power, which extends battery life away from mains electricity. The clear trade off is cost, since solid state storage is considerably more expensive per gigabyte, but for laptops that leave the office the reliability and battery benefits justify it.

An IP address identifies a device's position on a network and is assigned by that network, which means it is not permanent and will change when the device connects somewhere else, such as moving from a home network to a school one. It is used to route data between different networks across the internet. A MAC address identifies one specific piece of network hardware, is assigned by the manufacturer when the network interface card is made and does not change. It is used by switches to deliver data to the correct device once it has arrived on the local network. Both are needed: the IP address gets the data to the right network and the MAC address gets it to the right device on that network.

One task is memory management. The operating system decides which programs and data are held in RAM and where, allocating memory to each program as it opens, keeping the areas belonging to different programs separate so that one cannot overwrite another's data and cause it to crash, and freeing that memory when the program closes. A second task is peripheral management. The operating system communicates with input and output devices through device drivers, which translate its generic instructions into the specific commands that a particular printer, scanner or graphics card understands, which is why the same operating system can support thousands of different devices.

Replacing computers every two years has significant environmental costs. Each replacement cycle sends the previous machines into the waste stream, and electronic equipment contains toxic substances including lead, mercury and cadmium, much of which is exported to countries with weaker environmental regulation where informal recycling exposes workers and contaminates land and water. Manufacturing the replacements is equally damaging, since it consumes rare earth metals extracted by mining that destroys habitats, and doubling the replacement rate doubles that impact. There is also a clear financial cost. A school budget is fixed, so spending more than twice as often on hardware means spending less on something else, whether that is teaching staff, textbooks or building maintenance, and computers replaced after two years are typically still perfectly capable of running the software a school actually uses. Against this there are real benefits. Newer machines are faster and will run current software that older hardware may struggle with, which matters in subjects such as computing and design where students need to use professional tools. More importantly, manufacturers and operating system vendors eventually stop issuing security updates for older hardware, and a school network full of unsupported machines holding student data is a genuine security risk rather than a theoretical one. Newer machines are also generally more energy efficient, which partially offsets the manufacturing cost over their lifetime. On balance the school should not move to a two year cycle. A four or five year cycle keeps machines within their supported lifetime while producing far less waste and freeing budget for other purposes, and a better approach would be to replace machines when they genuinely become inadequate or unsupported rather than on a fixed schedule, and to ensure that retired machines are refurbished and donated or properly recycled rather than discarded.

A brute force attack works by systematically trying every possible combination of characters until the correct password is found. It is carried out automatically by software capable of making millions of attempts per second, which is why short or simple passwords fall very quickly. The most effective prevention is to limit the number of failed login attempts allowed, locking the account or imposing a delay after a small number of failures, because this makes an attack requiring millions of attempts completely impractical however fast the attacker's computer is. This should be combined with requiring long passwords, since each additional character multiplies the number of combinations that must be tried, and with two factor authentication, which means that even a correctly guessed password is not sufficient to gain access.

The Data Protection Act 2018 controls how organisations collect, store and use personal data. It requires that data is used lawfully and fairly, collected only for a stated purpose, kept accurate and secure, and not retained longer than necessary, and it gives individuals the right to see the data held about them. The Computer Misuse Act 1990 makes it a criminal offence to gain unauthorised access to a computer system or the data on it, to do so with the intention of committing a further offence, and to make unauthorised modifications such as deleting files or spreading malware. It covers hacking even where nothing is changed or taken.

RAM is volatile, which means everything in it is lost the moment the power is removed, so when a computer is switched on its RAM contains nothing at all and it has no instructions to follow. ROM is non volatile, so the small boot program stored in it survives being powered down and is available immediately at switch on. That program checks the hardware and then loads the operating system from secondary storage into RAM. ROM alone would not be sufficient, because it is read only in normal operation and running programs constantly need to write and change data, which requires the read and write capability that RAM provides. The two are therefore complementary rather than alternatives.

The first way is to configure every machine to enter a low power sleep state after a short period of inactivity and to shut down automatically at the end of the day. A school may have several hundred computers, and each one left running overnight and at weekends consumes electricity continuously for no benefit, so the saving across the whole site is substantial both financially and in terms of carbon emissions. The second way is to keep hardware in use for longer and to dispose of it responsibly when it is finally replaced. Manufacturing a computer consumes rare earth metals extracted through environmentally damaging mining and accounts for a large share of the device's total lifetime environmental impact, so extending a machine's working life from three years to five reduces that impact considerably. When machines are finally retired, refurbishing and donating those that still work, and using a certified recycler for those that do not, keeps toxic materials out of landfill and out of the informal export trade.

Phishing is a form of social engineering in which an attacker sends a fraudulent message, usually by email or text, that appears to come from a legitimate organisation such as a bank, a delivery company or a school, in order to trick the recipient into revealing personal information such as a password or bank details, or into clicking a link that installs malware. A user can identify a phishing message by several signs. It will typically create a sense of urgency, warning that an account will be closed or a payment has failed unless action is taken immediately, because panic stops people checking carefully. The link in the message will point to an address that does not belong to the real organisation, often with an extra word or a substituted character that is easy to miss. It will frequently use a generic greeting such as Dear Customer rather than the recipient's actual name, and above all it will ask for information that no legitimate organisation ever requests by email, particularly a password. The safest response is never to click the link but to go to the organisation's website by typing the address directly.

A core is effectively a complete processing unit with its own arithmetic logic unit, control unit and registers, so each core can fetch, decode and execute instructions independently of the others. This means a quad core processor can genuinely work on four sequences of instructions at the same time rather than switching between them, and for suitable work it completes far more instructions per second than a single core processor at the same clock speed. However the benefit depends entirely on the software. A program has to be written as multiple threads for its work to be divided between cores, and a single threaded program will run on one core while the other three sit idle, gaining nothing at all. Even in multi threaded programs some parts must run sequentially because they depend on earlier results, and coordinating the cores adds overhead, so doubling the cores never quite doubles the performance.

First, the charity must process the data fairly and lawfully and use it only for the purpose it stated when the data was collected. If donors gave their details in order to make a donation, the charity cannot then sell those details to another organisation or use them for an unrelated purpose without consent. Second, it must collect only data that is adequate and relevant for that purpose and not excessive, so it should not ask for a donor's date of birth or occupation unless there is a genuine need for it, and it must keep the data accurate and up to date, correcting it when told of a change. Third, it must keep the data secure, protecting it with appropriate measures such as encryption, access controls and backups so that it cannot be lost, stolen or accessed by anyone who should not see it, and it must not retain it for longer than necessary, deleting records when they are no longer needed. Donors also have the right to request a copy of the data held about them and to have inaccuracies corrected, and a serious breach must be reported to the Information Commissioner within seventy two hours.