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 code (albeit in decimal rather than binary) or assembly code.
The LMC model is based on the concept of a little man shut in a closed mail room (analogous to a computer in this scenario). At one end of the room, there are 100 mailboxes (memory), numbered 0 to 99, that can each contain a 3 digit instruction or data (ranging from 000 to 999). Furthermore, there are two mailboxes at the other end labeled INBOX and OUTBOX which are used for receiving and outputting data. In the center of the room, there is a work area containing a simple two function (addition and subtraction) calculator known as the Accumulator and a resettable counter known as the Program Counter. The Program Counter holds the address of the next instruction the Little Man will carry out. This Program Counter is normally incremented by 1 after each instruction is executed, allowing the Little Man to work through a program sequentially. Branch instructions allow iteration (loops) and conditional programming structures to be incorporated into a program. The latter is achieved by setting the Program Counter to a non-sequential memory address if a particular condition is met (typically the value stored in the accumulator being zero or positive).
As specified by the von Neumann architecture, any mailbox (signifying a unique memory location) can contain either an instruction or data. Care therefore needs to be taken to stop the Program Counter from reaching a memory address containing data - or the Little Man will attempt to treat it as an instruction. One can take advantage of this by writing instructions into mailboxes that are meant to be interpreted as code, to create self-modifying code. To use the LMC, the user loads data into the mailboxes and then signals the Little Man to begin execution, starting with the instruction stored at memory address zero. Resetting the Program Counter to zero effectively restarts the program, albeit in a potentially different state.
Execution cycle
To execute a program, the little man performs these steps:
Check the Program Counter for the mailbox number that contains a program instruction (i.e. zero at the start of the program) Fetch the instruction from the mailbox with that number. Each instruction contains two fields: An opcode (indicating the operation to perform) and the address field (indicating where to find the data to perform the operation on). Increment the Program Counter (so that it contains the mailbox number of the next instruction) Decode the instruction. If the instruction uses data stored in another mailbox then use the address field to find the mailbox number for the data it will work on, e.g. 'get data from mailbox 42') Fetch the data (from the input, accumulator, or mailbox with the address determined in step 4) Execute the instruction based on the opcode given Branch or store the result (in the output, accumulator, or mailbox with the address determined in step 4) Return to the Program Counter to repeat the cycle or halt
Commands
While the LMC does reflect the actual workings of binary processors, the simplicity of decimal numbers was chosen to minimize the complexity for students who may not be comfortable working in binary/hexadecimal.
Instructions
Some LMC simulators are programmed directly using 3-digit numeric instructions and some use 3-letter mnemonic codes and labels. In either case, the instruction set is deliberately very limited (typically about ten instructions) to simplify understanding. If the LMC uses mnemonic codes and labels then these are converted into 3-digit numeric instructions when the program is assembled.
The table below shows a typical numeric instruction set and the equivalent mnemonic codes.
Examples
Using numeric instruction codes
This program (instruction 901 to instruction 000) is written just using numeric codes. The program takes two numbers as input and outputs the difference. Notice that execution starts at Mailbox 00 and finishes at Mailbox 07. The disadvantages of programming the LMC using numeric instruction codes are discussed below.