LC3

LC-3, short for Little Computer 3, is a 16-bit educational instruction set architecture used to teach computer organization and assembly language programming. Developed for undergraduate coursework, it provides a small, well-documented platform that is easy to simulate and analyze while capturing essential concepts of real processors.

Architectural overview

The LC-3 uses 16-bit words and a 16-bit address space, yielding up to 65,536 memory locations. It features eight general-purpose registers labeled R0 through R7, a program counter (PC), and condition codes that indicate negative, zero, or positive results after operations. Memory is accessed with a simple load/store model, and input/output is handled through software-visible traps that invoke routines provided by the operating environment. The architecture emphasizes straightforward data paths and a minimal yet functional set of building blocks for teaching.

Instruction set and addressing

LC-3 instructions are 16 bits wide and begin with a 4-bit opcode. The ISA includes arithmetic and logical operations (such as add and and), memory access instructions (load and store, including indirect and base-plus-offset forms), and control flow instructions (various branches, jumps, and subroutine calls). It also supports immediate operands for certain instructions, enabling simple constant use without additional instructions. Indirect addressing variants and PC-relative addressing aid in teaching memory hierarchies and program structure. A trap mechanism provides a controlled channel to system routines, enabling input/output and other services without exposing low-level hardware details.

Educational use

LC-3 is widely used in university courses and textbooks to illustrate core concepts of computer organization, machine-level programming, and the interaction between software and hardware. Simulators and reference implementations accompany many curricula, reinforcing topics such as instruction encoding, memory addressing, condition codes, and the role of operating system interfaces in a simple, comprehensible environment.