1
Q
Computer architecture
A
1- The central processing unit (CPU)
2- Von Neumann architecture
3- Cores, cache and internal clock
2
Q
1- The central processing unit (CPU)
A
- it is also known as a microprocessor or processor
- it is central to all modern computer systems (including tablets and smartphones).
- The CPU has the responsibility for the execution or processing of all the instructions and data in a computer application
3
Q
2- Von Neumann architecture
A
- with early computers it wasn’t possible to store programs or data.
- In the mid-1940s, John von Neumann developed the concept of the ‘stored program computer’, which has been the basis of computer architecture ever since.
4
Q
The main novel features of the von Neumann architecture
A
- the concept of a central processing unit (CPU or processor)
- the CPU was able to access the memory directly
- computer memories could store programs as well as data
- stored programs were made up of instructions which could be executed in
sequential order.
5
Q
Components of the central processing unit (CPU)
A
- Arithmetic & Logic Unit (ALU)
- Control Unit (CU)
- Registers
6
Q
The Arithmetic & Logic Unit (ALU)
A
- It allows the required arithmetic (e.g. +, - and shifting) or logic (e.g. AND, OR) operations to be carried out while a program is being run; it is possible for a computer to have more than one ALU to carry out specific functions.
- Multiplication and division are carried out by a sequence of addition, subtraction and left or right logical shift operations.
7
Q
The control unit(CU)
A
- The control unit reads instructions from memory.
- Signals are generated during the Fetch-Decode-Execute cycle to control all components of the computer.
- The control unit synchronises data flow and program instructions throughout the computer.
8
Q
Registers
A
- One of the most fundamental components of the von Neumann system are the registers.
- Registers can be general or special purpose.
9
Q
Types of Register
A
- current instruction register (CIR) : this register stores the current instruction being decoded and executed
- accumulator (ACC) : this register is used when carrying out ALU calculations; it stores data temporarily during the calculations
- memory address register (MAR) : this register stores the address of the memory location currently being read from or written to.
- memory data/ buffer register (MDR) : this register stores data which has just been read from memory or data which is about to be written to memory.
- program counter (PC) : this register stores the address where the next instruction to be read can be found
10
Q
System buses and memory
A
buses are used to connect the CPU to the memory and to input/ output devices.
11
Q
Memory
A
- The computer memory is made up of a number of partitions.
- Each partition consists of an address and its contents.
- The address will uniquely identify every location in the memory and the contents will be the binary value stored in each location.
- The random access memory (RAM) holds data, programs and the part of the operating system currently in use.
- RAM is often known as the Immediate Access Store (IAS).
- All data being used in programs needs to be transferred to the RAM.
- Primary memory also includes read-only memory (ROM) which cannot be altered; ROM contains start-up routines for the computer.
12
Q
(System) buses
A
- Buses transfer data and control signals throughout a computer.
- They use parallel data transfer and each wire in the bus transmits one bit of data.
13
Q
The three common buses used in the von Neumann architecture
A
- address bus
- data bus
- control bus
14
Q
address bus
A
- the address bus carries addresses throughout the computer system. Between the CPU and memory.
- the address bus is unidirectional (bits can travel in one direction only). this prevents addresses being carried back to the CPU.
- The width of a bus is very important. The wider the bus, the more memory locations that can be directly addressed at any given time.
15
Q
Data bus
A
- The data bus is bidirectional (allowing data to be sent in both directions along the bus).
- This means data can be carried to/from CPU / memory and to/from input- output.
- As with the address bus, the wider the bus the larger the word length that can be transported.
16
Q
Control bus
A
- The control bus is also bidirectional. It carries signals from the control unit (CU) to all the other computer components.
- It is usually 8-bits wide.
- There is no real need for it to be any wider since it only carries control signals.
17
Q
Fetch–Decode–Execute cycle
A
- To carry out a set of instructions, the CPU first of all fetches some data and instructions from memory and stores them in suitable registers.
- Both the address bus and data bus are used in this process.
- Once this is done, each instruction needs to be decoded before finally being executed
18
Q
The process of Fetch–Decode–Execute cycle
A
- Fetch
- Decode
- Execute
19
Q
- Fetch
A
- Both data and instruction can be stored in MDR.
- The next instruction is fetched from the memory address currently stored in the MAR and the instruction is stored in the MDR.
- The contents of the MDR are then copied to the Current Instruction Register (CIR). - The PC is then incremented (increased by 1) so that the next instruction can be then be processed.
20
Q
- Decode
A
The instruction is then decoded so that it can be interpreted in the next part of the cycle.
21
Q
- Execute
A
- The CPU passes the decoded instruction as a set of control signals to the appropriate components within the computer system.
- This allows each instruction to be carried out in its logical sequence.
22
Q
3- Cores, cache and internal clock
A
- Caches: is temporary memory using static RAM to hold frequently used data/instructions by the CPU thereby increasing CPU performance. More generally, cache means any area of storage used to quickly access frequently-used data - other examples include web cache, database cache, DNS cache
23
Q
Cores
A
- core : a unit on a CPU made up of an ALU, control unit and registers; a CPU may contain a number of cores.
- dual core : a CPU containing two cores
- quad core : a CPU containing four cores
24
Q
system clock
A
produces timing signals on the control bus to ensure synchronisation takes place
25
clock cycle
clock speeds are measured in terms of GHz; this is the vibrational frequency of the system clock which sends out pulses along the control bus; for example, a
3.5 GHZ clock cycle means 3.5 billion clock cycles a second
26
factors that determine the performance of a CPU
- By increasing the system clock speed, the processing speed of the computer is also increased.
- The width of the address bus and data bus increases the processing speed of the CPU.
- Caches, which store frequently used instructions and data, can speed up CPU performance. The larger the cache memory size the better the CPU performance.
- Using a different number of cores can also improve CPU performance. Each core communicates with the other three cores using six channels.
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How to speed a computer performance?
- increasing bus width (data and address buses)
- increasing clock speed will potentially increase the speed of a computer
- use of multi-core CPUs
- use large of cache memories.
28
4- Instruction set
- In a computer system, instructions are a set of operations which are decoded in sequence.
- Each operation will instruct the ALU and CU (which are part of the CPU). An operation is made up of an opcode and an operand.
- Since the computer needs to understand the operation to be carried out, there
is actually a limited number of opcodes that can be used; this is known as the instruction set.
- All software running on a computer will contain a set of instructions (which need to be converted into binary).
- The Fetch–Decode–Execute cycle is the sequence of steps used by the CPU to process each instruction in sequence.
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opcode
The opcode informs the CPU what operation needs to be done
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operand
The operand is the data which needs to be acted on or it can refer to a register in the memory
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5- Embedded systems
- An embedded system is a combination of hardware and software which is designed to carry out a specific set of functions.
- The hardware is electronic, electrical or electro-mechanical.
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Embedded systems can be based on :
- microcontrollers : this has a CPU in addition to some RAM and ROM and other peripherals all embedded onto one single chip (together they carry out a specific task).
- microprocessor : integrated circuit which only has a CPU on the chip (there is no RAM, ROM or peripherals – these need to be added).
- system on chips (SoC) : this may contain a microcontroller as one of its components (they almost always will include CPU, memory, input/output (I/O) ports and secondary storage on a single microchip).
33
how embedded systems work ?
- When installed in a device, either an operator can input data manually or the data will come from an automatic source, such as a sensor.
- This sensor input will be analogue or digital in nature, for example, inputs such as oxygen levels or fuel pressure in a car’s engine management system.
- The output will then carry out the function of the embedded system by sending signals to the components that are being controlled (for example, increase the power to the heating elements in an oven or reduce fuel levels in the engine).
- Depending on the device, embedded systems are either programmable or non- programmable.
- Non-programmable devices need, in general, to be replaced if they require a software upgrade.
- Programmable devices permit upgrading by two methods:
1- connecting the device to a computer and allowing the download of updates to the software (for example, this is used to update the maps on a GPS system used in a vehicle)
2- automatic updates via a Wi-Fi, satellite or cellular (mobile phone network) link (for example, many modern cars allow updates to engine management systems and other components via satellite link).
34
A set of tasks that An embedded system will have :
- Analogue or digital input.
- Embedded systems can contain
sensor input, mechanical components, actuators and software.
- Some of the latest embedded systems use very powerful dual and quad CPUs and a variety of I/O connections leading to a number of applications.
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Benefits of using embedded systems
- they are small in size and therefore easy to fit into devices
- compared to other systems, they are relatively low cost to make
- they are usually dedicated to one task allowing simple interfaces and often no requirement for an operating system
- they consume very little power
- they can be controlled remotely using a mobile phone, for example
- very fast reaction to changing input (operate in real time and are feedback orientated)
- with mass production comes reliability
36
Drawbacks of using embedded systems
- it can be difficult to upgrade some devices to take advantage of new technology
- troubleshooting faults in the device becomes a specialist task
- although the interface can appear to be more simple (e.g. a single knob) in reality it can be more confusing (e.g. changing the time on a cooker clock can require several steps!)
- any device that can be accessed over the internet is also open to hackers, viruses, etc
- due to the difficulty in upgrading and fault finding, devices are often just thrown away rather than being repaired (very wasteful)
- can lead to an increase in the ‘throw away’ society if devices are discarded just because they have become out-of-date
IGCSE Cambridge Computer Science
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