Detailed summary on Unit 1.1 Systems Architecture from OCR GCSE Computer Science. Covers the function of the CPU, Von Neumann Architecture, Factors affecting Speed and Embedded Systems.
Buses - The communication channels linking the CPU with the RAM and I/O devices.
Execute - The action of a CPU performing an instruction.
Decode - Making sense of an instruction.
What is a computer?
- An electronic device which takes input, processes data and delivers output.
In order to do this computers follow a set of instructions known as a computer
program.
Two important parts of a computer are the CPU and the RAM.
CPU - Central Processing Unit
RAM - Random Access Memory
What is the purpose of the CPU?
The CPU is often known as the ‘brain of the computer’.
Its job is to process data. And by processing we mean things like searching, sorting,
calculating and decision making.
Whenever you are working on your computer, it is the CPU which is at the heart of
everything.
The Fetch-Decode-Execute Cycle
The CPU follows three steps in order to process data:
It is known as the Fetch - Decode - Execute cycle (aka Fetch-Execute Cycle).
To begin with, whenever you open and work with a program, its data and instructions
are loaded onto the RAM.
As the RAM is accessed directly by the CPU, the CPU can get to work!
The Fetch Stage
In this step the CPU fetches some data and instructions from main memory (RAM)
and then stores them in its own temporary memory called 'registers'.
For this to happen, the CPU uses a piece of hardware path called the 'address bus'.
, The address of the next item that the CPU wants is put onto the ‘address bus’.
Data from this area then travels from the RAM to the CPU on another piece of
hardware called the ‘data bus’.
The Decode Stage
The decode step is where the CPU understands / works out what the instruction it
has just fetched actually means.
The CPU ‘decodes’ the next instruction and gets things ready for the next step.
The Execute Stage
The execute stage is where data processing happens.
Instructions are carried out on the data.
Once a cycle has been completed, another begins.
Von Neumann Architecture
In 1945 John von Neumann designed computers with stored programs.
Key characteristics:
- Central Processing Unit (CPU)
- Single Control Unit (CU)
- Arithmetic Log Unit (ALU)
- Onboard Cache
- Internal clock
Each instruction is stored in a location in the memory which each has its own
address.
It is based on the concept of the stored-program. Both instruction data and program
data are stored in the same memory in binary form. There is no way to know if the
pure binary held in memory is representing instructions or data by simply looking at
it.
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