Worksheet

A9.4 Instruction sets and addressing modes

Computer architecture · A level · OCR H446 1.2.4, AQA 7517 4.7.3.3, Eduqas A500QS 2.1 · about 20 min

What this lesson is about

Opcodes, operands and instruction formats; immediate, direct, indirect and indexed addressing.

• The instruction set
• Opcode and operand
• Addressing modes
• Indexed addressing walks an array

Questions 6 marks in all

1. [1 mark]Address 30 holds 45 and address 45 holds 8. The operand is 30. What value does indirect addressing use?

   1. A8
   2. B45
   3. C30
   4. D75

2. [1 mark]Which addressing mode is best suited to stepping through an array in a loop?

   1. AIndexed
   2. BImmediate
   3. CDirect
   4. DIndirect

3. [1 mark]In immediate addressing, the operand is...

   1. Athe actual value to be used
   2. Bthe address of the value
   3. Cthe address of the address of the value
   4. Dadded to the index register

4. [1 mark]An instruction is 16 bits: 5 bits of opcode and 11 bits of operand. What is the largest address direct addressing can reach?

5. [1 mark]Why will a program compiled for an x86 processor not run on an ARM processor?

   1. AThe instruction set is processor specific, so the machine code means something different
   2. BARM processors have no memory
   3. Cx86 programs are written in assembly
   4. DARM processors only run Python

6. [1 mark]An 8-bit instruction has a 4-bit opcode then a 4-bit operand. What does this print?

   instruction = 0b10110110
   print(instruction >> 4, instruction & 0b1111)

The task: four ways to read an operand

Memory is the list memory (addresses 0 to 9) and the index register ix holds 2. Write operand_value(mode, operand): - mode is one of the strings "immediate", "direct", "indirect" or "indexed"; - operand is a whole number from 0 to 9; - it returns the value an instruction would use in that mode, as defined in the table above. For each operand in [2, 5], and for each mode in the order immediate, direct, indirect, indexed, print a line in the form direct 2 -> 1: the mode, the operand, ->, then the value. That is eight lines. The robot does not move.

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

memory = [4, 7, 1, 9, 5, 3, 8, 0, 6, 2]
ix = 2

def operand_value(mode, operand):
    return operand

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

Do it on the robot
www.bugbotlab.com/learn/a9-4-instruction-sets-and-addressing-modes/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. An instruction format has 16 bits: 6 for the operation and 10 for the operand. How many operations can it have, and what is the largest address direct addressing can reach?
2. Using the task's memory, find an operand for which indirect and immediate give the same value.
3. Explain why indirect addressing is slower than direct addressing, in terms of the fetch-decode-execute cycle.