Which of these is an example of system software?
Based on: AQA 8525 · 3.4.1
AQA GCSE Computer Science (8525) · Systems
Computer systems: free AQA GCSE Computer Science practice questions
Section 3.4 asks what a computer is made of and what each part does. The truth table, the fetch execute cycle and the difference between RAM and storage are the parts of it that the topic notes flag as most often got wrong.
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Free practice questions
What is the output of an AND gate when its two inputs are 1 and 0?
Based on: AQA 8525 · 3.4.2
What does an assembler do?
Based on: AQA 8525 · 3.4.3
What are the three stages of the fetch execute cycle, in order?
Based on: AQA 8525 · 3.4.4
The mistakes that cost the most marks
These are the answers that sound right and are not.
- Calling RAM storage. RAM is volatile memory for what is in use now. Secondary storage keeps data permanently.
- Saying a compiler is faster than an interpreter. The compiled program runs faster, but compiling takes time up front and errors are harder to locate.
- 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.
- Confusing the memory address register with the memory data register. One holds the address. The other holds the data at it.
- 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 processes, memory, hardware through device drivers, files, and users and their permissions, and it provides the user interface. Editing the text of a document is not one of its jobs. That is application software.
Boolean logic and truth tables
There are three basic gates. 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. 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 translates and runs one line at a time, which makes finding errors easier and 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. The program counter holds the address of the next instruction. The memory address register holds the address being read from or written to. The memory data register holds the data itself. The accumulator holds the result of calculations. 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 double real world speed, because not every program can be split across cores and the cores may still compete for memory.
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, solid state drives with no moving parts, and optical discs. 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.
| Compiler | Interpreter |
|---|---|
| Translates the whole program into machine code in one go. | Translates and runs one line at a time. |
| Produces a file that runs again without the compiler. | Needs the interpreter present every time the program runs. |
| The finished program runs faster. | Running is slower. |
| Errors are reported after the whole program is translated, so they are harder to locate. | Errors are found at the line that fails, which makes them easier to find. |
FAQ
Frequently asked questions
What is the difference between RAM and secondary storage?
RAM is volatile. It loses its contents when the power goes off and holds the programs and data in use right now. Secondary storage is non-volatile and keeps data permanently, on a hard disk, a solid state drive or an optical disc.
How many rows does a truth table have?
2 to the power of the number of inputs. Two inputs give 4 rows and three inputs give 8.
Is a compiler faster than an interpreter?
The compiled program runs faster once it exists. Compiling takes time up front and errors are harder to locate. An interpreter runs one line at a time, so running is slower and finding errors is easier.
Does doubling the number of cores double the speed?
No. Not every program can be split across cores, and the cores may still compete for memory. More cores help most when the work can run as more than one instruction stream at a time.
What is an embedded system?
A computer built into a larger device to do one specific job, such as a washing machine controller or a car engine management system. Being dedicated to one task is the defining feature, not being small.
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