<P> 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 . </P> <P> 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). </P> <P> As specified by the von Neumann architecture, each mailbox (signifying a unique memory location) contains both instructions and 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 . </P>

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