Little Man Computer

The Little Man Computer (LMC) is a very simply microprocessor and computer system designed for teaching beginners assembly programming. To make things simpler it is not presented as a binary computer but a decimal one. It contains 100 memory cells, numbered from 0-99. You can place program instructions and data here.

Each cell can hold a 3-digit decimal number. Here we can store instructions for the computer to perform or data to operate on. The CPU has a single accumulator which is used when performing arithmetic. There is also an input and ouput where you can read user input and write outputs to the user.

Here is a simple overview of the instructionset. Each instruction is really just a 3-digit number. But usually when programming one deals with letter abbreviations which are easier to remember. Here is a tip on how to read the description below. E.g. the Add instruction is described as having the number 1xx. What this really means is that the xx are where you put your operand (argument). So 142 is the instruction for adding the contents of memory cell 42 to the contents of the accumulator.

• ADD 1xx add content at address xx in memory to accumulator.
• SUB 2xx subtract contents of address xx from what is stored in the accumulator. Store result in accumulator.
• STA 3xx store accumulator at address xx in memory.
• LDA 5xx load accumulator with contents at address xx in memory.
• BRA 6xx jump to location xx in program.
• BRZ 7xx jump if accumulator is zero
• BRP 8xx jump if accumulator is zero or higher (positive).
• INP 901 fill accumulator with number from input.
• OUT 902 push value in accumulator into output queue.
• HLT 000 stop program

In the example folder you can find more examples of programs. Here is an example of a program which reads a number from the input and then counts down. So if it reads a 4 on input, then it will write out 4, 3, 2, 1 and 0 on the output.

     INP
     OUT
LOOP BRZ QUIT // Jump to QUIT if accumulator is 0
     SUB ONE  // Subtract from accumulator what is stored in ONE
     OUT
     BRA LOOP // Jump (unconditionally) to the memory address labeled LOOP
QUIT HLT      // Label this memory address as QUIT
ONE  DAT 1    // Store 1 in this memory address.

You can take a program written as the example store it in a file and give that filename to the assemble(file) function which will produce a list of 3-digit integers representing your program and data. You can feed this to the simulate!(program, inputs) function to run your program. It will dump output.

Alternatively you can copy paste this and put the code into one of the web based LMC simulators described below.

You can find several browser based simulators for the LMC CPU online. Where you can step through programs and watch live how the virtual computer operates.

• 101 Computing Simulator. Looks fancy in terms of appearance.
• Robo Writer Simulator. More plain and primitive looking, but I actually prefer this version.

There are several games you can play based on slight variations of the LMC idea. This is potentially a nice way of getting children involved in learning programming.

• Human Resource Machine
• TIS-100 a game by Zachtronics where you basically deal with multiple LMC computers which you got to program to communicate with each other.
• Shenzhen IO also by Zachtronics where you program small electronic gizmos that communicate with each other. Very similar to LMC but with the element of timing, which matters a lot in electronics.