AQA 7517 · Paper 2 · 4.7.4

LMC
Instruction Set

Little Man Computer · Assembly language simulation · Section 4.7

WHAT YOU'LL LEARN

LMC model · Full instruction set · Tracing programs · Writing LMC code · Addressing

AQA SPEC LINK

4.7.4 — The Little Man Computer (LMC) instruction set

What is LMC?

The Little Man Computer

The Little Man Computer (LMC) is a simplified model of a CPU with a single accumulator (ACC) and 100 memory mailboxes (00–99). Used to teach how real assembly language and the FDE cycle work.

3-digit memory addresses (00–99); 3-digit data values (000–999)

Single accumulator (ACC) stores results; single PC (Program Counter)

Uses mnemonics: ADD, SUB, LDA, STA, BRA, BRZ, BRP, INP, OUT, HLT, DAT

LMC Instructions

Full LMC Instruction Set

Mnemonic	Action
ADD xx	ACC ← ACC + Memory[xx]
SUB xx	ACC ← ACC − Memory[xx]
LDA xx	ACC ← Memory[xx] (load)
STA xx	Memory[xx] ← ACC (store)
INP	ACC ← user input
OUT	Output value of ACC
BRA xx	PC ← xx (unconditional branch)
BRZ xx	If ACC=0: PC ← xx (branch if zero)
BRP xx	If ACC≥0: PC ← xx (branch if positive)
HLT	Stop execution

Worked Example 1

Add Two Numbers

INP // Input first number → ACC
STA num1 // Store ACC at num1
INP // Input second number → ACC
ADD num1 // ACC ← ACC + num1
OUT // Output ACC (sum)
HLT // Stop
num1 DAT 0 // Data store initialised to 0

DAT declares a memory location for data storage (like a variable in RAM).

Worked Example 2

Count Down to Zero

INP // Get starting number
STA count // Store it
loop LDA count
OUT // Print current count
SUB one // count ← count - 1
STA count
BRZ end // If count = 0, jump to end
BRA loop // Else repeat
end HLT
count DAT 0
one DAT 1

Tracing LMC

Trace Table

AQA exams often ask you to trace through LMC code and complete a table showing ACC, PC, and mailbox values at each step. Input: 3

PC	Instruction	ACC	count
0	INP	3	0
1	STA count	3	3
2	LDA count	3	3
4	SUB one	2	3
5	STA count	2	2

Addressing

Addressing Modes in LMC

Direct addressing — LDA 50 loads value from mailbox 50 into ACC

Immediate addressing — not in standard LMC, but in real CPUs: LDA #50 loads the literal value 50 (not what's stored at address 50)

Labels — in LMC mnemonics, labels like 'count' and 'loop' are replaced with actual addresses by the assembler

DAT — declares storage; DAT 5 initialises the location to 5

AQA Exam Style

Practice Question

AQA 7517 — Paper 2 Style

(a) State what the LMC instruction BRZ does and when it executes the branch. [2]
(b) Write LMC code to: input two numbers, output the larger one. [5]
(c) Explain the purpose of a DAT instruction in LMC. [1]

[8 marks]

2 marks

(a) BRZ branches to a specified address [1]; the branch only occurs if the value in the accumulator is zero [1]

5 marks

(b) INP / STA num1 / INP / STA num2 / SUB num1 / BRP output2 / LDA num1 / BRA done / output2 LDA num2 / done OUT / HLT / num1 DAT 0 / num2 DAT 0 [5 for logically correct program]

1 mark

(c) DAT declares a memory location to be used as a data store (variable); optionally initialises it with a value

Summary

Key Points to Remember

LMC — 100 mailboxes, one ACC, one PC; simplified CPU model

Instructions — LDA/STA (load/store), ADD/SUB (arithmetic), INP/OUT (I/O), HLT

Branches — BRA (always), BRZ (if ACC=0), BRP (if ACC≥0) — implement loops and conditions

DAT — declares a memory location (variable); optionally initialised with a value

Trace tables: show ACC and memory contents step-by-step — practice this!

🎉 Lesson complete — move to the quiz!