Computer Science
Name 4 components in the CPU
- Control Unit
- Arithmetic Logic Unit
- Dedicated registers
- Buses
Give 2 functions of the Control Unit?
- Uses control signals to enable data to be read from/ written to memory
- It decodes every instruction that the processor will execute to know what needs to be done
What are buses?
A communication system that transfers data between components, components are connecteb using buses
What is the system bus and what is it made up of?
- A set of parallel instructions that allow internal components to communicate with each other and exchange data, it connect the CPU, main memory and I/O controllers
Made of:
- Address bus
- Data bus
- Control bus
What are External buses?
External buses - used to connect peripherals to the processor. Either series or parallel
What's different about the control and data bus to the address bus
The control and data bus are bidirectional, so data can be both written to/read from a memory location
what does the control bus do?
- Send control signals between internal and external components (bidirectional)
Name each control signal and it's function
- Memory read - Places data from a certain memory location onto the data bus
- Memory write - Stores data from the data bus onto a certain memory location
- Bus request - Signifies that a component needs to access a bus
- Interrupt request - Signifies that an error or exception has occurred that requires the attention of the CPU
- Clock signals - The control bus supplies the components with clock pulses generated by the system clock
Under what condition can Memory read and Memory write take place?
The address must be on the data bus
what does the data bus do?
Transfers data/instructions between components (bidirectional)
What does the width refer to on a data bus and what does it determine
Width refers to the no of parallel lines, this the determines the no of bits that can be transferred
What does the address bus do?
- Transmits memory addresses specifying where data should be sent/retrieved from
Memory is internally divided into units, what are these units called and how many bits can they hold
Words(16, 32 or 64 bits)
What is the purpose of the ALU
Performs arithmetic, logical and shift operations on data
What is the puropse of the accumulator?
Stores results from the ALU
State all the registers and their functions
PC - Holds the memory address of the next instruction to be executed
CIR - Holds the address in memory which is split into opcode and operand
MAR - Holds the address in memory that is meant to be read from/written to
MDR - Temporarily holds data that has been read or that needs to be written to
Accumulator - Holds the results of an instruction
Explain the fetch step of the fetch execute cycle
- The address of the next instruction is copied from
the PC to the MAR
- The instruction held at the MAR is then sent to RAM via the address bus & waits for a control signal to know what needs to be done
- The control unit sends a control signal to RAM
- This now allows the data to be sent to the MDR from RAM through the data bus
- The data is then copied from the MDR to the CIR
- Finally the contents of the PC are incremented
Explain the decode step of the fetch execute cycle
- The instruction in the CIR is decoded
- It's then split into operand and opcode to determine the type of instruction it is
- It's then passed on to the accumulator
Explain the execute stage in the fetch exeute cycle
- The opcode is executed on the operand
Give 3 factors that can affect the CPU perormance
- Cache size - Larger the cache, the more frequently used data it can store
- No of cores - The more cores, the more instructions the computer can execute at the same time(Not all programs can utilise this as they haven't been designed to do so)
- Clock speed - The greater the clock speed, the faster instructions can be carried out
What is pipelining
The process of carrying out multiple instructions concurrently
What does the von neumann architecture consist of?
- A control unit
- An ALU
- Special registers
- RAM
Give an advantage of shared buses
Reduces the chances of a bottleneck occuring
Give advantages of Von Neumann Architecture
- It simplifies the design of the control unit and is cheaper to develop
- Programs can be optimised to use less memory. This can be done by reusing code, loops & branches.
What does the harvard architecture do?
Keeps data/instruction in separate memory units
Give advantages of using Harvard architecture
- Instructions are exectued quicker as they're fetched in parallel
- Instructions/Data are held in separate memory units which have different word lengths, which can make more efficient use of space
Explain the different levels of cache
L1 cache - Has very fast memory cells, but a small capacity
L2 cache - Has relatively fast memory cells, with a medium sized capacity
L3 cache - Has a large capacity, but slow memory cells
What are contemporary processors and explain how they work
They're a combination of von neumann and harvard architectures
The von neumann architecture works on the data/instruction in main memory, & the harvard architecture divides cache into instruction cache and data cache