Computer systems: free AQA GCSE Computer Science practice questions

Mistakes to avoid

• Confusing main memory with secondary storage. RAM is volatile main memory for the programs and data in use now; secondary storage is non-volatile and keeps data when the power is off.
• Saying a compiler is faster than an interpreter. The compiled program runs faster, but compiling takes time up front.
• Getting the truth table row count wrong. It is 2 to the power of the number of inputs, so 3 inputs give 8 rows.
• Saying more cores always means proportionally more speed. Many programs cannot be split effectively across cores.
• Saying the CPU does everything in one step. Each instruction is fetched, then decoded, then executed, and the cycle repeats.
• Describing an embedded system as a small computer. The point is that it is dedicated to one specific task inside a larger device.

Hardware, software and the operating system

Hardware is the physical parts of a computer system. Software is the programs that run on it. Neither is useful without the other. System software runs the machine itself and includes the operating system and utility programs. Application software is what the user runs to get a job done, such as a word processor or a browser. The operating system manages the processor, memory, input and output devices, applications and security. Editing the text of a document is not one of its jobs. That is application software.

Boolean logic and truth tables

You need four gates: NOT, AND, OR and XOR. AND gives 1 only when both inputs are 1, so a single 0 forces the output to 0. OR gives 1 when at least one input is 1. XOR gives 1 when exactly one input is 1. NOT gives the opposite of its single input. A truth table lists every possible combination of inputs and the output for each. A circuit with 2 inputs has 4 rows and one with 3 inputs has 8, because the row count is 2 to the power of the number of inputs.

Languages and translators

Machine code is binary instructions the CPU runs directly. Assembly language uses short mnemonics in place of those binary patterns and is specific to one processor family. A high level language is closer to English, is easier and quicker to write and is portable between machines. A low level language gives finer control of the hardware and can be more memory efficient. A compiler translates the whole program into machine code in one go and produces a file that can be run again without the compiler. An interpreter does not produce machine code directly: it carries out the program one statement at a time by calling machine code routines within its own code, which makes running slower. An assembler translates assembly language into machine code.

The fetch execute cycle and what makes a CPU faster

The CPU repeats one cycle endlessly: fetch the next instruction from memory, decode it to work out what it means, then execute it. The control unit coordinates everything. The arithmetic logic unit does calculations and comparisons. Registers hold small amounts of data very briefly. A higher clock speed means more cycles per second. More cores let the CPU work on more than one instruction stream at a time. More cache means fewer slow trips to main memory. Doubling the cores does not always double real world speed, because not every program can be split across cores.

Memory, storage, cloud and embedded systems

Cache is a small amount of very fast memory close to the CPU holding frequently used data and instructions. RAM is volatile, so it loses its contents when the power goes off, and it holds the programs and data currently in use. ROM is non-volatile and holds the instructions the computer needs to start up. Secondary storage is non-volatile and keeps data permanently. The common types are magnetic hard disks and solid state drives with no moving parts. Cloud storage keeps your files on someone else's servers, reached over the internet. It is easy to access anywhere and to share, but it needs a connection and puts your data in someone else's hands. An embedded system is a computer built into a larger device to do one specific job, such as a washing machine controller.