3.1 Computer architecture

## Computer Architecture & Von Neumann architecture

The central processing unit (CPU) (also known as a microprocessor or processor) is central to all modern computer systems
##### the purpose of a core in the CPU
- To perform a fetch-decode-execute cycle
- To process / execute an instruction
##### the purpose of  the CPU

- It processes data
- It processes/executes instructions
- It carries out calculations
- It carries out logical operations
- perform fetch-decode-execute cycles

The CPU consists of the following architecture:
##### Control Unit:
A component in the CPU that manages the flow of data around the CPU 
- Fetches each instruction in turn and manages their execution
- Handles all processor control signals.
- It directs all input and output flow, fetches code for instructions and directs them

Sends control signals to control the flow of data through the CPU // manages the execution of instructions in the CPU
##### Arithmetic Logic Unit:

- Carries out calculations
- Carries out logical operations
- Holds temporary values during calculations in a register called the accumulator (ACC)
##### Accumulator:
Stores the interim result of a calculation
##### System Clock:
It controls the number of fetch-decode-execute cycles that are performed per second.
- produce timing signals/clock pulses on the control bus to ensure the synchronization take place 
- to keep track of the date and time / timestamp files 
- to process operations in the correct order / sequence

##### Busses:

-  To connect the internal components of CPU
-  Pathway for transmitting data and instructions

###### Address bus

- Transmit / carries addresses between components in the CPU
- Data travels in only one direction (unidirectional)

###### Control Bus

- Transmits control signals from the control unit to other components in the CPU
- can be unidirectional or bidirectional

###### Data Bus

- Transmit / carries data between components in the CPU
- data can travel in both directions

##### Immediate Access Store:

- holds all the data and programs that the CPU needs to access 
- CPU takes data and programs held in backing store and puts them in IAS temporarily 
- This is done because read/write using IAS is faster compared to the backing store 
- The IAS is another name for primary (RAM) memory.

##### Registers

- A component in the central processing unit (CPU) that is used to temporarily store data.

###### Memory Data Register(MDR)

- holds the data/instructions that in use which has been read from or is to be written to the address in the MAR

###### Memory Address Register(MAR)

- holds the address of the data/instruction that needs to be fetched/processed // holds the address of where the data needs to be stored.

###### Program Counter (PC)

- to store the address / location / memory location of the next instruction to be fetched 
###### Index Register (IX)

- to store a value that is added to an address to give another address

###### Status Register (SR)

- to store flags which are set by events // from the results of arithmetic and logic operations and interrupt flags 
###### Current instructions Register(CIR)

- Data gets executed from here by sending to bios or processed by sending to ALU

###### Accumulator

- stores the result of interim calculations // during calculations data is temporarily held in it

![left-50](https://znotes-static.s3.amazonaws.com/cie/igcse/computer-science-0478/642d85f52a6da93764e8d77f16841b046d030515572edc6f.png)

## The Fetch-Execute Cycle

###### Three stages

- Fetch
- Decode
- Execute

![image.png](https://doc.tiegg.com/media/202603/bc925bc28e574299b18fbe7d5133ce378287.png)

![image.png](https://doc.tiegg.com/media/202603/070254939cb64f08858659c169fa9ad61037.png)

###### Opreation

1. PC contains address of the next instruction to be fetched
2. This address is copied to the MAR via the address bus
3. The instruction of the address is copied into the MDR temporarily
4. The instruction in the MDR is then placed in the CIR
5. The value in the PC is incremented by 1, pointing the next instruction to be fetched
6. The instruction is finally decoded and then executed

### Stored program concept:

- Instructions and data stored in the main memory 
- Instructions fetched and executed in order

## Memory Concept

- A computer’s memory is divided in partitions : Each partition consists of an address and its contents e.g.

| MEMORY LOCATION | CONTENT  |
| --------------- | -------- |
| 10101010        | 01010110 |

##### Instruction Set
- An instruction set is a list of all the commands that can be processed by a CPU and the commands are machine code

An operation is made up of an opcode and an operand.
The opcode informs the CPU what operation needs to be done.
The operand is the data which needs to be acted on or it can refer to a register in the memory.
Load 10
Add 11
Store 12
Load, Add, Store are opcode. 10, 11, 12 are operand.
#### Performance of Computer System Factors

##### Bus Width

- Determines number of bits that can be simultaneously transferred 
- Refers to number of lines in a bus 
- **Increasing bus width increases number of bits transferred at one time, hence increasing processing speed and performance since there**

##### Clock speed

- Each instruction is executed on a clock pulse // one F-E cycle is run on each clock pulse 
- …. so the clock speed dictates the number of instructions that can be run per second 
- **The faster the clock speed the more instructions(F-E cycles) can be run per second**

##### Number of cores
This is a processing unit within the CPU that can fetch, decode and execute instructions.
- Each core processes one instruction per clock pulse 
- **More/multiple cores mean that sequences of instructions can be split between them** 
- **so more sequences of instructions can be run at the same time** 
- More cores decreases the time taken to complete task

##### Cache Memory
It stores frequently used data/instructions
- Commonly used instructions are stored in the cache memory area of the CPU. 
- **the higher capacity the more frequently used instructions it can store for fast access**

## Embedded System
- An embedded system is a combination of hardware and software which is designed to a perform dedicated/single function
### Characteristics
- Performs a single/limited/dedicated function/task
- It has a microprocessor
- It has dedicated hardware
- Uses firmware
- It is normally built into a larger device/system
- User normally cannot reprogram
- It does not require much power
- It is cheap to manufacture
- Works automatically // works without human intervention
- It is small (in size)
- It is a real-time system
### Examples:
- security light system
- freezer
- vending machine
- Set-top box
- Motor vehicles

Explain why the computer is not an embedded system
- An embedded system is designed to perform a dedicated/limited/single function // computer can be used to perform many different functions.
- An embedded system has **dedicated** hardware // computer has hardware that can be used by other devices.
- An embedded system has software that is not easily updated/reprogrammed // software can easily be updated/reprogrammed on the computer.
- An embedded system has a microprocessor // A computer has a CPU.
- An embedded system can be part of/built into a larger device // A computer is normally standalone

A room has an automatic lighting system. Electric lights are automatically turned on when a person enters the room and the natural light level in the room is 10 or less.
Explain how sensors and a microprocessor are used to control the electric lights in the room.
- Uses light sensor and Infrared / Motion / Pressure sensor
- Sensors send data to the microprocessor
- Data is converted from analogue to digital (using ADC)
- Microprocessor compares both values to stored values
- If motion value is out of range/in range, light value is checked // If light value is <= 10, motion value is checked If light value is <= 10 lights are turned on // If motion value is out of range/in range lights are turned on ...
- .. by sending a signal to actuator
- Lights remain on for set period (and then turn off) // If motion is in range/out of range or light is > 10 then signal sent to turn lights off
- Process repeats / is continuous