P13 - 18 Flashcards
Von Neumann Architecture
Every computer is made up of:
A processing unit that contains an ALU and processor registers.
A control unit that contains an instruction register and program counter register,
Memory that stores both data and instructions.
External data storage.
Input and output mechanisms.
Fetch
CPU fetches data and instructions from RAM.
CPU stores in its own memory called registers.
CPU uses address bus.
The memory address of the next item in the CPU wants is put onto the address bus.
Data from this memory travels from RAM to CPU on the data bus.
decode
The CPU works out what the instruction it just fetched actually means.
The control unit decodes the instruction and gets things ready for the next step.
It does this by looking up the instructions from the instruction set.
This is the full list of operations that a microprocessor can carry out.
execute
Data processing happens.
Instructions are carried out on the data,
Some instructions are carried out by the ALU.
Fetch decode execute
Fetch
The CPU fetches data and instructions from the RAM.
Decode
CPU working out what the instructions it fetched actually means.
Execute
Data is processed
Central Processing Unit (CPU)
The CPU is the brain of the computer.
It is the hardware component responsible for all the processing that the computer carries out.
Its job is to process data.
Processing means: searching, sorting, calculating and decision-making.
Control Unit
The control unit receives signals from other parts of the computer system and sends signals to them.
It is responsible for handling hardware interrupts.
It is responsible for fetching, decoding and executing instructions.
It has the most responsibility.
It is the powerhouse of the CPU.
Arithmetic Logic Unit (ALU)
Performs actual operations on data. Arithmetic: + / = * Logic: AND, OR, NOT It is able to compare numbers against 0. It can test if two numbers are equal. It uses logic gates in combination to perform operations.
Registers
Also known as immediate access storage.
Registers are a type of memory that is extremely fast.
Each type of processor has different registers that are designed to hold different information.
Most processors have:
An instruction register - holds instructions currently being executed by the CPU.
An accumulator - holds the accumulated result of operations carried out by the ALU.
A program counter - holds the memory address of the next instruction to be executed.
clock
A clock chip determines the speed of the CPU.
This chip uses vibrating crystals that maintain a constant rate of vibration.
The speed of the clock is measured in Hertz (Hz), which is the amount of cycles per second.
Current computers have a clock speed of 3GHz, which means 3 billion cycles per second.
Each ‘tick’ means that one part of the fetch,decode,execute cycle can be carried out.
pipelining
Pipeline is a method of keeping all the components busy to improve overall CPU performance.
Pipelining attempts to keep every component busy at all times.
Instructions flow through the CPU in stages.
This means that an instruction can be completed every clock cycle.
This increases CPU throughput allowing overall performance to be outputted.
A CPU is fully ‘pipelined’ if an instruction can be fetched every clock cycle.
a) clock
b) pipelining
c) instructions
d) fetch decode execute
e) control unit
f) ALU
g) RAM
a) clock
An electronic device that ‘ticks’ at regular intervals and is used to synchronise the actions of the other components of the CPU.
b) pipelining
A way of improving the performance of the CPU. The stages of the fetch-decode-execute cycle are overlapped. While the CPU is executing one instruction, it is decoding a second and fetching a third one from memory.
c) instructions
An operation that the CPU carries out.1
e) control unit
The unit that decodes the instruction and coordinates all the actions of the other components.
f) ALU
Carries out arithmetic operations (add, shift) and Boolean logic operations (AND, OR, NOT)
g) RAM
The memory where the instructions and data are stored for programs.
Instructions and data
In a computer, everything is represented in binary.
This includes the instructions that the CPU needs to carry out.
Instructions and data are stored in RAM until they are needed by the CPU.
Each instruction and item of data is a location in memory.
Each element of the memory has a unique address.
This means that each instruction has an address.
The first memory element that contains the instructions is the address of the instruction.
data bus
Carries binary data from component to component.
For example an instruction from RAM that is being transferred to the CPU.
Bidirectional
Two way data transfer is required when reading and writing data to memory.
control bus
Carries signals that control the CPU components.
For example a signal to start fetching the next instruction.
Bidirectional
It needs to carry status information to devices and to carry the status information signals that devices send back.
address bus
Carries the address of a memory location.
For example the address of an instruction being fetched from memory.
Unidirectionality
The CPU can point to the addresses in memory or to other devices, however those hardware components cannot point to an address in the CPU.
Magnetic
Magnetic media are coated with a substance that can be magnetised.
Magnetic storage technology works by magnetising parts of this substance as north and south poles to represent binary 1s and 0s.
Examples include: hard disk drive, tape drive and cassette.
Optical
Optical media consists of a platter with a flat reflective surface.
Optical storage technology works by using a laser to burn ‘pits’ into the flat surface.
The reflective areas between pits are known as ‘lands’.
A laser is directed onto the medium’s surface.
When it hits land, a beam reflects (this is read as binary 1).
When it hits a pit, the beam does not reflect (this is read as binary 0).
Examples: CD and DVD.
solid state / flash
Solid state media consists of silicon chips that feature a special kind of transistor called NAND flash.
This type of media is called ‘solid state’ because it has no moving parts.
Solid state technology works by using a large electric current to force electrons through a barrier trapping them in positions called pools.
A full pool - 0.
An empty pool - 1.
Examples include: SSD card, USB, SD memory card and flash storage.
optical advantages + disadvantages
+ Cheap to produce
Thin and lightweight
Portable
- Slow to access
Low capacity
Prone to scratches
magnetic advantages + disadvantages
+ High capacity
Quick to access,
Providing the disk is defragmented periodically
- Has moving parts that will eventually fail
solid state advantage + disadvantage
\+ Quick to access No moving parts so very reliable. Low power requirements Quite No need to defragment
- Expensive.
Has a limited number of read write cycles
