lc3

a lightweight LC-3 (Little Computer 3) emulator written in C

Build

gcc -Wall -Wextra -o lc3 src/main.c
./lc3 path/to/your/program.obj

Features

• Full LC-3 instruction set
• .obj image loading
• 16-bit memory and register model
• Memory-mapped keyboard I/O
• Non-blocking input using select
• Terminal input control via termios
• Built-in trap handling (GETC, OUT, PUTS, IN, PUTSP, HALT)

Architecture Overview

Registers

• General-purpose registers: R0–R7
• Program Counter: PC
• Condition Register: COND

Memory

• 16-bit address space
• Memory-mapped registers:
  • KBSR (Keyboard Status Register)
  • KBDR (Keyboard Data Register)

Instruction Cycle

The emulator follows a standard fetch–decode–execute loop:

1. Fetch instruction from memory using PC
2. Decode opcode and operands
3. Execute instruction
4. Update condition flags
5. Advance PC

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Supported Instructions

• Arithmetic: ADD, AND, NOT
• Control flow: BR, JMP, JSR, JSRR
• Memory access: LD, LDI, LDR, LEA, ST, STI, STR
• System calls: TRAP

Trap Routines

The emulator implements LC-3 trap routines directly in the VM:

Trap	Description
GETC	Read a single character
OUT	Output a character
PUTS	Output a null-terminated string
IN	Prompt and read a character
PUTSP	Output packed characters
HALT	Stop execution

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References

• LC-3 ISA
• jmeiners LC-3 VM

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