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F9.4 The CPU and fetch-execute

Logic and computer systems · GCSE · OCR J277 1.1.1, AQA 8525 3.4.5, Edexcel 1CP2 3.1.1 · about 20 min

What this lesson is about

The ALU, control unit, registers and buses, and tracing the fetch-decode-execute cycle.

• Inside the CPU
• Buses
• The fetch-decode-execute cycle
• Instructions are numbers too

Questions 6 marks in all

1. [1 mark]Which register holds the address of the next instruction to fetch?

   1. AProgram counter
   2. BMemory data register
   3. CAccumulator
   4. DMemory address register
   Answer: A. The PC points at the next instruction.

2. [1 mark]What does the ALU do?

   1. ACarries out arithmetic and logic operations
   2. BDecodes instructions
   3. CHolds the address to read from
   4. DSends the clock pulse
   Answer: A. Arithmetic logic unit: adding, subtracting, comparing.

3. [1 mark]Put these steps of the cycle in order.

   Number the lines 1 to 4 to put them in the right order.

   1. The instruction is executed
   2. The address in the PC is copied to the MAR
   3. The instruction at that address is copied to the MDR
   4. The control unit decodes the instruction
   Answer:

   The address in the PC is copied to the MAR
   The instruction at that address is copied to the MDR
   The control unit decodes the instruction
   The instruction is executed

   Fetch, decode, execute, then back to fetch.

4. [1 mark]Which bus carries addresses from the CPU to memory?

   1. AAddress bus
   2. BData bus
   3. CControl bus
   4. DUSB
   Answer: A. The address bus goes one way: CPU to memory.

5. [1 mark]What does the memory data register hold?

   1. AThe data or instruction just read from memory, or about to be written
   2. BThe address of the next instruction
   3. CThe result of the last calculation only
   4. DThe clock speed
   Answer: A. The MDR is the CPU's end of the data bus.

6. [1 mark]What does this program print?

   pc = 4
   mar = pc
   pc = pc + 1
   print(mar, pc)

   Answer:

   4 5

   The MAR gets the old PC value, then the PC moves on.

The task: trace the cycle

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

memory = ["LOAD 6", "SUB 7", "ADD 8", "STORE 9", "OUT 9", "HALT", 40, 12, 7, 0]
pc = mar = acc = 0
mdr = None

from bugbot import *
connect()
memory = ["LOAD 6", "SUB 7", "ADD 8", "STORE 9", "OUT 9", "HALT", 40, 12, 7, 0]
pc = mar = acc = 0
mdr = None
while True:
    mar = pc
    mdr = memory[mar]
    print(f"fetch {mar}: {mdr}")
    pc = pc + 1
    op, *arg = mdr.split()
    if op == "LOAD":
        acc = memory[int(arg[0])]
    elif op == "ADD":
        acc = acc + memory[int(arg[0])]
    elif op == "SUB":
        acc = acc - memory[int(arg[0])]
    elif op == "STORE":
        memory[int(arg[0])] = acc
    elif op == "OUT":
        print("output:", memory[int(arg[0])])
    elif op == "HALT":
        break