CS-iGCSE-CIE-Notes
Central Processing Unit
The CPU executes and processes the instructions and data in a computer application.
The CPU is responsible for executing and processing instructions and data from an application so that a system can complete certain operations.
The CPU contains the:
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Control unit (CU)
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Arithmetic Logic Unit (ALU)
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A number of registers
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A number of buses to connect the components
A microprocessor (also CPU) is an electrical device that works on an integrated system on a single device and contains the circuitry necessary for a computer to perform logical operations. Memory is connected externally, so there is only a CPU on the chip.
Main functions:
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Receives input data
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Processes data
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Provides output data
**
Von Neumann Model:
Fetch → Decode → Execute
Random Access Memory
A volatile storage unit; volatile meaning dependent on a power source. It is called random because any storage location can be accessed at any time. When running, it stores data, instructions, and the operating system.
The information in the RAM can be swapped and rewritten at any time. It is also known as the immediate access store as read/write operations can be completed considerably faster than a hard disk/ssd due to the way it functions.
Stores a value/instruction and its address in binary code.
Opcode and Operand
Opcode: instruction
Operand: data to be processed
| Address | Value |
| 1101 | 0101 |
| 0110 | 0010 |
| 1011 | 1101 |
The stored program concept refers to instructions and data stored on the same RAM memory and both are fetched sequentially one at a time.
ROM
ROM (Read only memory) and contains the BIOS and Bootstrap. It stores data (instructions and memory) that is necessary for the computer to run and startup. This is non-volatile..
The BIOS (basic input/output system) is a component in the computer that is used whenever a computer is booted up;
checks initialisation settings for the computer;
checks if the hardware is running correctly;
Bootstrap loads the operating system into the RAM.
CPU Registers
These are high-speed storage units within the CPU
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MAR - Memory address register
- Address of location in RAM with data being read to or location being rewritten
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MDR - Memory data register
- Data being used (data being read or data about to be written to)
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ACC - Accumulator built into the ALU
- Temporarily stores data from calculations from the ALU
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PC - Program counter
- Stores the address of the next instruction to be executed, a counter
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CIR - Current instruction register
- Current instruction being processed (decoded and executed)
Units
ALU - Arithmetic logic unit
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Numbers for simple calculations
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Allows for logical and arithmetic operations to be carried out while a program is running
CU - Control unit
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Control unit receives instruction from RAM via control bus
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Instruction is stored in PC (program counter)
Buses
Paths/wires for data to be transferred between the components of a computer through parallel data transmission.
CPU <-> RAM (bi-directional)
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Control bus
- Carries signals to coordinate computer’s activities (CU to RAM, RAM to CU)
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Data bus
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Exchanges data between processor, memory, and input/output devices (between cache and RAM)
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So that data can either be read (ram to cache) or written (cache to ram)
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Can also be between the MDR and RAM
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CPU -> RAM (uni-directional)
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Address bus
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Carries signals correlated to memory addresses (MAR to RAM) between processors
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Addresses cannot be carried back to the CPU
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Fetch-Decode-Execute
Fetch
The address of the next instruction is stored in the PC and stores the address in the MAR.
Data from the address in MAR is fetched from the RAM.
Data is read and placed into the MDR (memory data register) via the data bus.
The current instruction is stored in the CIR
Decode
The CU reads the data in the CIR and checks if it is a valid instruction in its instruction set (each CPU has its own instruction set which may vary between different computers). It will then decode the instruction so that the computer understands it.
Execute
ALU performs a logical operation and the result is stored in the ACC. Additional data may be fetched from the RAM to complete the instruction.
Data in ACC is then transferred to the MDR and then written into RAM.
Fill in the blanks:
PC, CIR, MDR, MAR, ACC
The address of the next instruction to be fetched is stored in the ____. This address is then moved to the ____. Data in that address is moved into the ____ and placed in the ____. The ____ then increments by one to prepare for the next instruction.
PC, MAR, MDR, CIR, PC
| Stage | Address | |
LOAD |
10 |
Fetches value in [10] |
ADD |
11 |
Adds value in [11] |
STORE |
12 |
Stores value in [12] |
In the example above, lets say:
| Instruction | Address |
LOAD 10 |
10 |
ADD 2 |
11 |
ADD 3 |
12 |
SUBTRACT 2 |
13 |
PC: [10] - address of memory
MAR: [10] - copies memory address in PC
MDR: LOAD 10 - fetches data in address at MAR
CIR: LOAD 10 - copies data in MDR
Cores, cache, clock
Clock speed/clock cycle: The number of times the CPU’s clock ticks per second // measured in hertz. Whenever this clock ticks, the CPU processes an instruction. A common clock speed is 3.5 to 4.0 GHz.
When this clock speed changes, the settings within the BIOS are altered.
A higher clock speed typically means that the computer is faster as is processes instructions at a faster rate.
Overclocking
Changing the clock speed in the bios to a value higher than the manufacturer’s settings. This will allow the CPU to execute more instructions at a time however this will generate much more heat than it was designed to handle.
The cache is very high performance memory stored in the CPU. This has the fastest access rate within the system which improves the CPU performance. The RAM is not able to pass data to the CPU fast enough. The cache allows it to operate at its optimum speed by feeding it data to the MDR. Data buses.
Cores are the CPU’s processors. Each of these cores read and execute instructions. Multi-core processors can execute multiple instructions simultaneously, meaning that the more cores a CPU has the more instructions it can execute at a time, and the more efficient it is.
Instruction sets
These are a set of simple, low-level instructions that the CPU can understand (ADD, JMP). These are also referred to as opcodes. Opcodes inform the CPU what needs to be done. Operands are the data required to complete the task. This is what an operation is made up from.
These instruction sets can be read during the fetch-decode-execute, fetched in the CU and then decoded in the CIR.
Embedded systems
A combination of software and electrical/electro-mechanical hardware to perform a specific task.
Feedback oriented:
Analogue sensor where ADC (analogue to digital converter) converts these signals to digital values
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Transmits the digital signals to input
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Decision is made in processor - input value compared to stored or decision value
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Output - sends a signal to actuator
It then resamples after a given amount of time - receives and test the value from the input again
Ambient value: referring to the immediate surroundings
Embedded systems only do specific sets of functions, unlike a PC. They may be on microcontrollers, microprocessors, or microchips. It is always embedded into a single chip. The system will take an input either automatically through a sensor or manually by an operator, process it, then produce an output which will be the function of the system.
Rom drive - mb
Ram - gb
Hard disk drive - tb
Perfect sensor response:
The {sensor} will take in/record/measure the {what the sensor records} of a/the {area/person/target} to/trying to {goal}, for example {>3 examples}, this analogue data is then sent to an ADC (analogue to digital converter) which turns it into machine code/digital format/binary code so the microprocessor can understand. The signal is sent to the microprocessor which {what is done with values} so {goal} can be achieved. {IF CONDITIONS} Depending on if {condition}, do {goal} otherwise do {other goal}. To do these goals, a signal is sent to a DAC, then to a {device} that {does goal}.
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