Systems Architecture (P1)

Examples of input devices

• keyboard

• trackpad

• microphone

• sensor

• touchscreen

Examples of output devices

• speakers

• displays / monitors

• prints

• projectors

CPU (central processing unit)

Processes and executes instructions.

Von Neumann Architecture

An architecture where program instructions and the data the programs are using are both stored in the same memory.

• CPU accesses both instructions and data from the same RAM

What are the components of the CPU?

Main two:

• CU

• ALU

and:

• Registers (very fast memory locations)

• Cache (holds frequently repeated instructions, slower than registers but faster than RAM)

What are the 4 registers?

• PC (program counter)

• MAR (memory address register)

• MDR (memory data register)

• ACC (accumulator)

Registers function

Temporary storage of data and instructions during processing

PC function:

Holds the address of the next instruction to be executed. Increments by 1 after each cycle.

MAR function:

Holds the memory address of current instruction so it can be fetched from memory

MDR function:

Holds the data instruction and data fetched from memory

ACC function:

Holds the result of an instruction or result from ALU

CU (control unit) function:

Coordinates and controls all activities taking place in the CPU.

• decodes instructions and executes them

• receives signals from system clock

• directs timing and control of other parts

• runs FDE cycle

ALU (arithmetic logic unit) function:

Where arithmetic logic operations are calculated.

• including AND, OR and NOT

FDE (fetch-decode-execute) cycle steps:

1. PCC holds the address of the next instruction to be fetched.

2. The processor sends a signal to the memory address held in the MAR.

3. The instruction/data held in the memory address is sent along to the MDR.

4. The instruction/data held in the MDR is copied to CIR.

5. The instruction/data held in the CIR is decoded and then executed by the CU.

6. Results of processing are stored in the ACC.

7. The PC is incremented by one and the cycle repeats again.

FDE cycle

Fetch:

• Fetches instructions from main memory and brings the instructions back into the CPU

Decode:

• Decodes the instruction

Execute:

• Carries out the instruction by performing calculations and storing information.

What three things affect CPU performance?

• Clock speed

• Number of processor cores

• Cache

What else can affect performance of a computer?

• RAM

• SSD

• HDD

• Graphics card

What is clock speed and how does it affect CPU performance?

Amount of FDE cycles per second.

• The higher the clock speed, the more instructions are processed per second, measured in Hertz.

What is number of processors and how does it affect CPU performance?

Number of duplicate processing units placed in one CPU.

• The more processors, the more instructions can be processed at the same time enabling multitasking.

  → Tasks can be split between processors.

→ A core of 2.5Ghz can run 2.5 billion FDE cycles per second

Why does having a quad-core processor over a dual-core processor not always result in a ‘faster computer’?

• Some software only designed to run on 1 core and not multiple

  → so some tasks cannot be split across cores.

• Clock speed, RAM and Cache also affect speed

What is cache and how does it affect CPU performance?

A small amount of very fast and expensive memory in the CPU that holds frequently repeated instructions.

• The more cache, or better type of cache, the less time the CPU has to wait for data as it doesn't need to access the RAM.

  → Data transferred and accessed faster: makes CPU more efficient.

  → RAM is accessed less frequently

Levels of Cache

Level 1 Cache: Extremely fast (2-256KB)

Level 2 Cache: Very fast (256KB-8MB)

Level 3 Cache: Slowest type - still faster than RAM (4MB-50MB)

Embedded System and characteristics

A single microprocessor that includes RAM, ROM and a CPU, typically used to control a device using simple inputs with a dedicated function.

• Limited functions

• Dedicated microprocessor

• Hard to change function

Embedded system examples

• Smart TVs

• washing machines

• pacemakers

• cameras

• fridges

• kettles

• DVD players

• microwaves

RAM (random access memory) function

Stores running programs and parts of the operating system. It is volatile, meaning all data is lost when the computer is switched off.

• Read and Write

ROM (read only memory) function

Stores boot up instructions, bootstrap and BIOS. It is non-volatile, meaning data isn’t lost when computer is switched off.

• Read only

Primary storage function and main two types

Computer component that stores data, programs and instructions currently in use.

• Fast: directly accessed by the CPU

Main two types:

• RAM

• ROM

Secondary storage function and examples

Refers to memory that stores data and programs permanently, even when computer is switched off.

• Slower: Is not directly accessed by the CPU

Examples:

• SSD

• CD

• Floppy disk

• HDD

Magnetic storage use

Used in personal computers for the storage of large quantities of data.

Magnetic storage advantages

• Cheaper than solid state

• Reliable

• Large storage capacities

• Relatively fast write speeds

Magnetic storage disadvantages

• Lots of mechanical parts

• Low durability

• Not very portable

Optical storage use

Used to store data such as songs, videos and other multi-media storage

Optical storage advantages

• Relatively Cheap

• Very portable - physically small

Optical storage disadvantages

• Low storage capacities

• Easily damaged

• Slow write speeds

Solid state storage uses

Used in high-end computers, laptops, smartphones and tablets

Solid state storage advantages

• Highly durable

• Reliable

• No moving parts, no noisy fans

• Fast write / read speeds

Solid state storage disadvantages

• More expensive than magnetic and optic storage

• Similar storage capacities to magnetic storage