Worksheet

A9.2 The processor and its registers

Computer architecture · A level · OCR H446 1.1.1, AQA 7517 4.7.3.1, Eduqas A500QS 2.1 · about 20 min

What this lesson is about

The ALU, control unit and clock, general-purpose and dedicated registers, and the status flags an 8-bit ALU sets.

• The components of the processor
• General-purpose and dedicated registers
• The status register's flags
• How flags make decisions
• Registers, cache and memory

Questions 5 marks in all

1. [1 mark]Which register holds the instruction currently being decoded and executed?

   1. ACurrent instruction register
   2. BProgram counter
   3. CMemory address register
   4. DStatus register

2. [1 mark]What does the control unit do?

   1. ADecodes instructions and sends control signals to coordinate the other components
   2. BPerforms arithmetic and logic operations
   3. CStores the result of calculations
   4. DGenerates the clock pulses

3. [1 mark]An 8-bit ALU adds 200 and 100. Which flag is set?

   1. ACarry
   2. BZero
   3. CNegative
   4. DOverflow

4. [1 mark]What does this program print?

   def flags(a, b):
       r = (a + b) & 255
       return r, r >> 7, int(r == 0)
   
   print(flags(100, 50))
   print(flags(128, 128))

5. [1 mark]What is the difference between a general-purpose register and a dedicated register?

   1. AA general-purpose register holds whatever the program chooses; a dedicated register has one fixed job, such as the PC
   2. BGeneral-purpose registers are in main memory; dedicated ones are in the processor
   3. CDedicated registers can only hold addresses
   4. DThere is no difference

The task: the flags

Write alu_add(a, b) yourself for an 8-bit ALU. Its inputs a and b are whole numbers from 0 to 255 (bytes). It works out the 8-bit result (0 to 255, anything that does not fit is lost) and the four flags N, Z, C and V, each 0 or 1, exactly as the table above defines them. For each pair in pairs, print one line in the form 200 + 100 = 44 N=0 Z=0 C=1 V=0: the two inputs, the 8-bit result, then the flags in the order N, Z, C, V. The robot does not move.

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

pairs = [(20, 30), (100, 50), (200, 100), (128, 128), (255, 1)]

def alu_add(a, b):
    return a + b

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/a9-2-the-processor-and-its-registers/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Write alu_sub(a, b) that works out a - b as an 8-bit subtraction and sets N, Z and V.
2. Find a pair of bytes that sets C but not V, and a pair that sets V but not C, without running the program. Then check.
3. Why does a processor need a CIR as well as an MDR, when both can hold the same instruction?