Explain how data is fetched, decoded, and executed using von Neumann?
- Program Counter contains the address of the next instruction to be executed.
-This address is copied into the Memory Address Register (MAR) so that the processor knows which memory location is required.
-The control unit sends a read signal to main memory.
-The instruction at that address is transferred from memory to the Memory Data Register (MDR).
-The instruction in the MDR is copied to the Current Instruction Register (CIR).
-The Program Counter is incremented to point to the next instruction.
-The control unit decodes the instruction in the CIR.
-The instruction is split into opcode (what operation to perform) and operand(s) (data or addresses).
-Any required addresses are prepared, and additional data may be fetched from memory.
-The operation specified by the opcode is carried out by the ALU or other parts of the CPU.
-Data is processed using registers such as the Accumulator.
-Results may be stored in a register or written back to main memory.
-The Status Register is updated to reflect the outcome of the operation
List all components in the Von Neumann Architecutre.
- Buses
- Registers
- CPU
- CU
- ALU
- IAS
- System clock
What are general purpose registers?
-Hold temporary data when performing operations
- Used for any purpose
- Can be used by most instructions
-Do not have a specific function
What are special-purpose registers, and give an example.
PC, MAR, MDR
- Hold status of a program
- Specialised for specific use
- Can only be used by certain instructions
What does the program counter do?
- holds the address of the next instruction to be loaded
-is incremented once an instruction has been carried out
What does the memory address register do?
-stored address from memory location currently being read from or written to (where data is being fetched from)
What does Memory Data Register do?
Memory Data Register
- holds the data fetched from the RAM via the data bus from the location that the MAR
- data is copied to CIR which stores the data
What is the index register?
- stores a value that is added to an address to give another address
What is the function of the MDR
- holds data from MDR
- holds instruction currently being decoded/executed
What is the status register
- stores flags - from results of logic and arithmetic operations // interrupt flags
contains bits - can be individually set or cleared depending on operation
What does the control unit do?
CU
-synchronises actions of other components in the CPU based on pulses of system clock
-sends/receives control signals along the control bus
-manages the execution of instructions - decodes an instructions opcode during FE cycle
-controls communication between components of CPU
- types of signals it transfers: interrupt, timing, read & write
What is the ALU responsible for/what does it do?
-Performs arithmetic operations (addition, subtraction, etc.)
-Performs logical operations (AND, OR, NOT, comparisons)
- Uses data from registers
-stores results back into a register (often the accumulator)
What does the accumulator do?
Stores temporarily the result of the calculation from the ALU
What is the purpose of the system clock
Generates regular timing signals
Synchronises all CPU operations
Controls the speed of instruction execution
Higher clock speed → more cycles per second
What is Immediate Access Store
Another term for main memory (RAM)
Stores data and instructions currently in use
Directly accessible by the CPU
Faster than secondary storage
What is the role of the adress bus.
Carries memory addresses
One-way bus (CPU → memory / I/O devices)
Width determines maximum addressable memory
What is the role of the control bus
Control bus
Carries control signals
Signals include read, write, clock, interrupt
Coordinates actions between components
What is the role of the data bus.
Data bus
Carries data and instructions
Two-way (bidirectional)
Width affects how much data can be transferred at once
What are the factors that affect the speed of the CPU
- Clock speed – higher speed allows more instructions per second
- Number of cores – enables parallel processing
- Cache size – reduces access to slower RAM
- RAM size and speed – affects how quickly data can be accessed
- Bus width – wider buses transfer more data per cycle
- CPU architecture – efficiency of instruction processing
What is the system clock
- synchronises computer operations by creating time signals
-allows operations to be processed in the correct order / sequence - keeps track of date and time
What is immediate access store
- holds all the data/programs currently in use
-volatile memory
-has fast access times
-Also know as RAM
How is data transferred between components
- Components (CPU, memory, I/O devices) are connected by buses
- Data transfer happens using three buses together
-CPU places the address of the required memory location into the Memory Address Register (MAR)
-Address is sent along the address bus to main memory
-Control Unit sends a read or write signal along the control bus
If reading:
Memory places the data/instruction onto the data bus
-Data travels to the CPU
-Data is stored in the Memory Data Register (MDR)
If writing:
-Data is placed into the MDR
-Data is sent along the data bus to memory
How does the number of cores increase processing
-Each core processes one instruction per clock pulse
-more cores mean that more sequences of instructions can be carries out simultaneously
-multiple instructions can be executed per clock pulse
- decreased time taken to complete a task)
Why may more cores not increase processing speeds
- software may not be designed for multiple cores - one core will be left idle
-memory access speed wont match speed of cores - causes delay - may be other differences - e.g. amount of Ram
