The Little Man Computer

Programming.Posted April 12, 2011. 321 words.

The Little Man Computer (LMC) is an instructional model of a computer, created by Dr. Stuart Madnick in 1965. The LMC is generally used to teach students, because it models a simple von Neumann architecture computer - which has all of the basic features of a modern computer. It can be programmed in machine (albeit usually in decimal) or assembly code.

Screenshot of the Program

For reference, here is some example code that takes numbers as input, and squares the result. Credit Wikipedia.

START   LDA ZERO     // Initialize for multiple program run
        STA RESULT
        STA COUNT
        INP          // User provided input
        BRZ END      // Branch to program END if input = 0
        STA VALUE    // Store input as VALUE
LOOP    LDA RESULT   // Load the RESULT
        ADD VALUE    // Add VALUE, the user provided input, to RESULT
        STA RESULT   // Store the new RESULT
        LDA COUNT    // Load the COUNT
        ADD ONE      // Add ONE to the COUNT
        STA COUNT    // Store the new COUNT
        SUB VALUE    // Subtract the user provided input VALUE from COUNT
        BRZ ENDLOOP  // If zero (VALUE has been added to RESULT by VALUE times), branch to ENDLOOP
        BRA LOOP     // Branch to LOOP to continue adding VALUE to RESULT
ENDLOOP LDA RESULT   // Load RESULT
        OUT          // Output RESULT
        BRA START    // Branch to the START to initialize and get another input VALUE
END     HLT          // HALT - a zero was entered so done!
RESULT  DAT          // Computed result (defaults to 0)
COUNT   DAT          // Counter (defaults to 0)
ONE     DAT 1        // Constant, value of 1
VALUE   DAT          // User provided input, the value to be squared (defaults to 0)
ZERO    DAT          // Constant, value of 0 (defaults to 0)

In the absence of any Windows desktop emulator, I created my own in .NET and here it is. View and download on GitHub.