GCSE Computer Science Key Words
Key words for use at GCSE level and above computer science.
The job of the ALU is to carry out calculations, adding, subtracting, multiplying and dividing. The ALU also carries out comparisons which answers questions such as, 'is the number 'A' greater, less than or equal to the number 'B'.
Assembly language consists of a set of mnemonics that can be used to program a CPU. There are a number of mnemonics that together make up the complete instruction set of the CPU. They can be grouped according to the kind of processing they cover.
Binary logic is much the same as general logic, except it works on 1's and 0's. Given one or more binary inputs a logical output will result.
It is common practice to consider binary 1 to be 'True' and binary '0' to be 'False'.
There are only a few simple logic operations but they do lead to incredibly complex devices such as a CPU.
It is used to speed up the way applications work. For example, Internet Explorer uses a cache to store all the web pages that you have visited recently including all of the pictures, sounds, videos and so on. So, next time you visit that page it will check to see if it is available in the cache. If it is, then it will instantly load the page from the cache rather than having to download it again from the Internet. This makes it much faster when you browse the Internet as it doesn't have to fetch every single file every time.
Inside the Central Processing Unit (CPU) there is a 'clock' - the CPU clock 'ticks' by switching from high to low and then back again in a very precise time:
Every time the clock pulses an instruction is carried out: Clock speed is measured in pulses per second. Since "1 pulse per second" is also known as 1 Hertz computer speed is often specified in 'Gigahertz' (GHz). The CPU clock speed on personal computers runs from less than 1GHz to about 3.8GHz
Effectively a dual core is two computers built into one device. The downside of multi-core chips is that whatever software is running has to make full use of the cores - and right now, there isn't much software around that does that.
A standard process describes the steps needed for processing to take place. It is called the Fetch - Decode - Execute cycle or sometimes simply called the Fetch-Execute Cycle.
The first step the CPU carries out is to fetch some data and instructions (program) from main memory then store them in its own internal temporary memory areas. These memory areas are called 'registers'.
The next step is for the CPU to make sense of the instruction it has just fetched.
This process is called 'decode'. The CPU is designed to understand a specific set of commands. These are called the 'instruction set' of the CPU. Each make of CPU has a different instruction set. The CPU decodes the instruction and prepares various areas within the chip in readiness of the next step.
This is the part of the cycle when data processing actually takes place. The instruction is carried out upon the data (executed). The result of this processing is stored in yet another register. Once the execute stage is complete, the CPU sets itself up to begin another cycle once more.
Grafical User Interface
The operating system is part of the system software. All computers have an operating system, they cannot function without one. The operating system is a program that allows applications software to communicate with the hardware.
This is the same as if you are not allowed to have a decimal number greater than 255 because of a computer limit but the software is trying to add 249 + 21. This is 270 but since the system cannot deal with numbers larger than 255 an overflow occurs.
Overflow is such a problem in a CPU that a special 'flag' inside the CPU is set when it happens. The software carrying out the maths should read this flag to see if an overflow has occured. If it has then it has to deal with the problem.
Pseudo-code does not follow any particular computer language. It lays out the algorithm as a series of statements written in English (or any local language). Some statements will test for some condition and branch to different parts of the algorithm.
A 1 byte scheme can only represent 256 symbols. This is fine for many individual languages which is why ASCII is so popular.