Explain the fetch phase of the machine instruction cycle using register transfer notation, describe the roles of buses, and explain the stored program concept.

a) Explain the fetch phase of the machine instruction cycle using the register transfer notation.

The fetch phase retrieves an instruction from memory.

1. The address of the next instruction is moved from the Program Counter (PC) to the Memory Address Register (MAR): $$MAR \leftarrow PC$$
2. The instruction at the memory location specified by MAR is fetched and placed into the Memory Data Register (MDR): $$MDR \leftarrow M[MAR]$$
3. The Program Counter is incremented to point to the next instruction: $$PC \leftarrow PC + 1$$
4. The instruction from the MDR is transferred to the Instruction Register (IR) for decoding: $$IR \leftarrow MDR$$

b) Describe the roles played by different buses in the machine instruction cycle.

• The Address Bus carries the memory address from the CPU (specifically the MAR) to the main memory to locate the instruction or data. It is unidirectional.
• The Data Bus carries the actual instruction or data between the CPU (MDR/IR) and the main memory. It is bidirectional.
• The Control Bus carries control signals that manage and synchronize the operations, such as read/write signals to indicate whether the CPU wants to read from or write to memory, and clock signals.

c) Describe the stored program concept in Von-Neumann machine architecture.

The stored program concept dictates that both computer instructions (the program) and data are stored together in the same main memory. This allows the computer to be reprogrammed by simply changing the contents of memory, rather than requiring physical rewiring. The CPU fetches instructions from memory, executes them, and can also treat data as instructions or vice-versa, enabling flexibility.