QUESTION 54 Consider the cycles given below: (I) Bus Cycle (II) Machine Cycle (III)...

The Instruction Cycle (III) is the complete sequence of operations needed to execute one instruction, which inherently determines how many machine cycles are required. The Machine Cycle (II) is a component within the instruction cycle, and the Bus Cycle (I) is a sub-component of machine cycles. Only the instruction cycle as a whole dictates the total number of machine cycles needed.

Step-by-step Derivation:
Hierarchical relationship of CPU cycles:

1. Bus Cycle (I): The basic unit for a single bus transaction (read/write operation). Multiple bus cycles occur within one machine cycle.

2. Machine Cycle (II): A sequence of bus cycles. One or more machine cycles are needed per instruction. It handles one memory/I/O access.

3. Instruction Cycle (III): The complete execution phase of an instruction, comprising:

   • Fetch cycle (machine cycle)
   • Execute cycle (one or more machine cycles)
   • Other cycles as needed (decode, etc.)

The question asks: what determines the NUMBER of machine cycles needed to execute an instruction?

Answer: The Instruction Cycle (III) itself. Different instructions require different numbers of machine cycles (e.g., simple arithmetic might need 2-3 machine cycles, while memory operations might need 5-6). The instruction cycle encompasses all machine cycles needed, so it determines how many are required.

• (I) Bus Cycle: too granular; many bus cycles fit within one machine cycle
• (II) Machine Cycle: is a component, not a determiner; it's what gets counted
• (III) Instruction Cycle: the complete execution unit that determines the total count of machine cycles

Therefore: Only (III) determines the number of machine cycles.