Exam Tips - FDE Cycle

Exam Tips - FDE Cycle

The FETCH sequence is a guaranteed 4-mark question! Learn this by heart:

• Line 1: "PC is copied to MAR" (1 mark)
• Line 2: "Address sent via Address Bus to memory" (1 mark)
• Line 3: "Instruction sent via Data Bus to MDR, then copied to CIR" (1 mark)
• Line 4: "PC is incremented to point to next instruction" (1 mark)

Common exam questions:

• "Describe the FETCH stage" → Use the 4 steps above, one per line
• "What happens in DECODE?" → Control Unit examines instruction in CIR, works out what operation to perform
• "When does PC increment?" → During FETCH (NOT execute!)
• "When does ACC change?" → During EXECUTE (when ALU completes calculation)

Common mistakes to avoid:

• Saying PC increments during EXECUTE - it's during FETCH!
• Missing the MAR and MDR registers in the FETCH sequence
• Forgetting to mention CIR (Current Instruction Register)
• Saying data changes during DECODE - nothing changes in DECODE!