Little Man Computer - System Architecture

System Architecture

The LMC model is based on the concept of a little man locked in a small room or 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. 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 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, memory contains both instructions and data. Care therefore needs to be taken to stop the Program Counter reaching a memory address containing data or the Little Man will attempt to treat it as an instruction. 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.