← IGCSE Computer Science Paper 1

Instruction set

The list of all the commands (such as load, add, store and jump) that a CPU design can process. The control unit decodes each instruction using it.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 29

Understanding Instruction set in IGCSE Computer Science Paper 1

What it does: LOAD copies data from memory into the CPU and STORE copies it back. Both are common commands in an instruction set. The control unit reads the instruction in the CIR and uses the instruction set to work out which command it is, such as ADD. Different CPU designs have different instruction sets, so machine code written for one design may not run on another. ADD is one of the commands in a CPU's instruction set. It tells the ALU to carry out an addition. JUMP is an instruction in the instruction set that changes which instruction the CPU runs next, instead of following in order.

Common mistakes: The closing bracket of print() is missing. It should be print(len(iset)), which prints 4, the number of commands in the instruction set. A single command cannot equal a whole list. To check that the command is in the instruction set, use in: if opcode in iset:.

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This module's key terms are also in a printable crossword (Modules 228–239) and word search (Modules 218–239).

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Learning
Duo
What is an instruction set?
The list of all the programs stored on a hard driveThe list of all the commands a CPU can process
Matches
A program written for one CPU always runs on any other CPU because all CPUs share the same instruction set.
↓
Each CPU design has its own instruction set, so a program using one design's commands may not work on another.
Applying
Meanings
Term
instruction set
Meaning
The list of all the commands (such as load, add, store and jump) that a CPU design can process. The control unit decodes each instruction using it.
Practice
LOAD 10
Testing
Gaps
10 // copy memory 10 into ACC
Check Answer
Spots
3iset.append("JUMP")
4print(len(iset)

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Quick Questions

1. What is an instruction set?
A. The list of all the commands a CPU can process ✓
B. The list of all the programs stored on a hard drive
An instruction set is every command a CPU design can understand and carry out, such as load, add, store and jump.
2. Which of these is a typical instruction in a CPU's instruction set?
A. ADD ✓
B. PRINTER
ADD is a basic instruction that tells the CPU to add numbers. Other examples are LOAD, STORE and JUMP.
3. Do all CPU designs use exactly the same instruction set?
A. No, each CPU design has its own instruction set ✓
B. Yes, every CPU uses the same instruction set
Different CPU designs have different instruction sets, so machine code written for one design may not run on another.
4. Which part of the CPU decodes an instruction using the instruction set?
A. The program counter (PC)
B. The control unit (CU) ✓
The control unit decodes each instruction by looking it up in the instruction set, then sends signals to carry it out. The PC only holds the address of the next instruction.
5. What does the instruction STORE usually do?
A. Copies a value from memory into the CPU
B. Copies a value from the CPU into memory ✓
STORE saves a value into memory. LOAD does the opposite and brings a value from memory into the CPU.

Match the Pairs

ADD 5 → An instruction from the instruction set that tells the CPU to add the value 5 to the accumulator
JUMP 20 → An instruction from the instruction set that makes the CPU continue from the instruction at address 20
An instruction set includes commands to move data between registers and memory → LOAD 10 STORE 11
The control unit decodes each instruction by matching it to a command in the instruction set → CIR: ADD 5 -> CU decodes: ADD
A program written for one CPU always runs on any other CPU because all CPUs share the same instruction set. → Each CPU design has its own instruction set, so a program using one design's commands may not work on another.
An instruction set is the list of data values stored in RAM. → An instruction set is the list of commands a CPU can process, not the data stored in RAM.

Key Term

instruction set
The list of all the commands (such as load, add, store and jump) that a CPU design can process. The control unit decodes each instruction using it.

Fill in the Blanks

1. LOAD 10 // copy memory 10 into ACC ADD 11 // add memory 11 to ACC STORE 12 // save ACC into address 12
Hint: Type the instruction name that copies a value from memory into the accumulator.
2. LOAD 10 ADD 11 STORE 12 // save ACC into address 12
Hint: Type the instruction that writes the accumulator value back into memory.
3. ADD 1 JUMP 1 // go back to the first line
Hint: Type the instruction that moves execution to a different address instead of the next one.
4. LOAD 10 ADD 11 // add the value at address 11 STORE 12
Hint: Type the memory address of the value being added, as given in the comment.
5. // the CU decodes using the instruction set decoded = lookup(opcode, instruction_set)
Hint: Type the variable name for the list of all commands the CPU design can process.

Spot the Error

Question 1
A iset = ["LOAD", "ADD"]
B iset.append("STORE")
C iset.append("JUMP")
D print(len(iset)
Line D — The closing bracket of print() is missing. It should be print(len(iset)), which prints 4, the number of commands in the instruction set.
Question 2
A ins = "ADD 5"
B iset = ["LOAD", "ADD", "JUMP"]
C opcode = ins.split()[0]
D if opcode == iset:
E print("CU can decode it")
Line D — A single command cannot equal a whole list. To check that the command is in the instruction set, use in: if opcode in iset:
Question 3
A LOAD 20 # copy RAM to ACC
B ADD 21 # add value to ACC
C STORE 22 # copy RAM to ACC
D HALT
Line C — STORE copies the value from the accumulator into RAM, not the other way round. The comment should say: copy ACC to RAM.
Question 4
A # CPU A: LOAD, ADD, STORE
B # CPU B: LOAD, SUB, JUMP
C # All designs share one set
D # so code for A may fail on B
Line C — Each CPU design has its own instruction set, so different CPUs can understand different commands. That is why code for A may fail on B.
Question 5
A iset = ["LOAD" "ADD", "JUMP"]
B for cmd in iset:
C print(cmd)
Line A — A comma is missing between "LOAD" and "ADD". Each command in the list must be separated by a comma.

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