🏗️ Little Man Computer (LMC)

See how a CPU fetches, decodes and executes instructions — one step at a time. This is the model used in your GCSE exam to explain how processors work.

Designed with the WJEC specification in mind

CS GCSE §1.1.1CS A-Level Unit 4

📝

Program

💡 This program takes two numbers as input and adds them together.

One instruction per line. Labels before the mnemonic. Comments after //

📤

Output

Ready. Click ⚙️ Assemble, then ▶ Run to see the program execute.

📖

Instruction Set

Mnemonic	Code	Description
INP	901	Input from user → Accumulator
OUT	902	Output Accumulator value
LDA xx	5xx	Load from address xx → Accumulator
STA xx	3xx	Store Accumulator → address xx
ADD xx	1xx	Add value at address xx to Accumulator
SUB xx	2xx	Subtract value at address xx from Accumulator
BRA xx	6xx	Branch always to address xx
BRZ xx	7xx	Branch to xx if Accumulator = 0
BRP xx	8xx	Branch to xx if Accumulator ≥ 0
HLT	000	Halt the program
DAT xx	xx	Reserve memory with optional initial value

Fetch
Get instruction from memory

Decode
Work out what it means

Execute
Carry out the instruction

🔧

Registers

PC
Program Counter

ACC
Accumulator

MAR
Memory Address Register

MDR
Memory Data Register

CIR
Current Instruction Register

Flag
Status indicator

📦

Memory (100 mailboxes)