GCSE Computer Science Paper 1 B: Computer Systems

The Program Counter (PC) holds the address of the next instruction. It is copied into the MAR at the start of the fetch.

The ALU calculates, the CU decodes and controls, the MAR holds an address and the MDR holds data.

Cache is a small amount of very fast memory built into or next to the CPU. It holds the instructions and data used most often, so the CPU can read them far more quickly than it could from RAM.

More cores only help if the work can be split up. Many programs are written to use only one core, and some tasks depend on the result of the step before, so they cannot run in parallel. Even when work is split, the cores share the cache, RAM and the bus and must coordinate, which costs time, so the speed up is less than double.

The clock speed is the number of fetch-decode-execute cycles the CPU performs each second, measured in hertz. A higher clock speed means more instructions are carried out every second, so programs run faster.

An embedded system is a computer built into a larger device to do one dedicated job.

The von Neumann architecture.

RAM is volatile working memory whose contents change all the time. ROM is non-volatile and holds the boot up instructions.

When RAM is full, the operating system uses part of secondary storage as virtual memory. Data that is not needed straight away is moved out of RAM to make room, and swapped back in when it is needed.

1101 0110 = 128 + 64 + 16 + 4 + 2 = 214. 167 = 128 + 32 + 4 + 2 + 1 = 1010 0111. 1001 is 9 and 1110 is E: 9E. 4D = 4 x 16 + 13 = 77.

85 + 62 = 147, which is 1001 0011.

Every bit moves two places left and zeros fill the gap: 0101 1000.

Each left shift doubles the number, so two shifts multiply it by 4: 22 becomes 88.

1000 x 16 x 10 = 160,000 bits. 160,000 / 8 = 20,000 bytes. 20,000 / 1000 = 20 KB (or 19.53 KB using 1024).

Colour depth is the number of bits stored for each pixel. Increasing it adds bits to every pixel in the image, so the total number of bits, and the file size, increases.

2 to the power 8 is 256.

Every bit of a program matters, so only lossless compression can be used.

Lossy compression makes the files much smaller, so they upload and send faster and fit within email limits. But it permanently removes data, so the photos lose some quality and the original detail can never be restored.

A router forwards data between networks, such as the LAN and the internet.

Bandwidth is the amount of data that can be transmitted each second; the lower it is, the longer every transfer takes. The number of devices on the network also matters, because they all share the same bandwidth, so when many are transmitting at once each one gets less and collisions and delays increase.

SMTP sends mail, HTTPS is secure web traffic, IMAP keeps mail on the server, and FTP transfers files.

Layers divide networking into self-contained parts, each dealing with one job, which makes it simpler to design, develop and fix. One layer can be changed without affecting the others, and agreed standards for each layer mean equipment from different manufacturers works together.

Many connections give alternative routes, which is reliable but costly.

The MAC address.

Flooding is denial of service, commands in an input box are SQL injection, guessing every password is brute force, and fake bank emails are phishing.

The business pays testers to attack its own network in the way a criminal would. Any weaknesses they find can then be fixed before a real attacker discovers them.

Lock the account after three failed attempts: an automated attack can then only try a handful of passwords before it is stopped. Require strong passwords, at least twelve characters mixing letters, digits and symbols, because that raises the number of possible combinations so far that trying them all would take years.

Drivers are peripheral management, RAM and virtual memory are memory management, access rights are user management and folders are file management.

As files are saved, deleted and resized, a hard disk stores parts of each file wherever there is space, so files become fragmented. Defragmentation rearranges the disk so that each file's parts are stored together and the free space is in one block. Files can then be read faster, because the read/write head does not have to keep moving.

Compression software makes files smaller.

Keeping data too long breaks data protection, sharing the e-book breaks copyright, and using another account is computer misuse.

Open source software comes with its source code, which anyone may view, modify and share under the licence. Proprietary software keeps its source code secret; users buy a licence to use it but cannot see or change the code.

Open source means the source code is open to view and change.

The most direct impact is on the staff: hundreds of checkout workers would lose their jobs, which hits towns where the supermarket is a major employer. For the company the case is financial: machines cost less than wages, can run around the clock and may shorten queues. Culturally, not every customer benefits. Older and disabled customers can find self-service difficult, and for some people the conversation at the till is the only social contact of their day. The AI cameras raise serious ethical and legal questions. Filming every shopper and analysing their behaviour is surveillance, and the footage is personal data, so under the Data Protection Act 2018 it must be processed fairly, kept secure and deleted when no longer needed. AI theft detection can also be wrong, and if it has been trained on unrepresentative data it may flag some groups more often than others, leading to innocent people being accused. Environmentally, every machine and camera has to be manufactured from mined materials, powered continuously and eventually disposed of as e-waste. On balance the plan saves money but shifts real costs onto staff and vulnerable customers; a fairer approach would keep some staffed tills, retrain staff to support customers, and use cameras only with clear signage, strict retention limits and a human checking every alert.