An interactive simulation of the execution of mnemonically entered instructions for the Datapoint 2200 Computer has been developed which runs on this same machine. A representation of the registers, flip-flops, program counter, subroutine stack, and referenced memory cell is show on the display of the machine. Individual mnemonic instructions may be entered from the keyboard and are shown on the display in standard assembly source language except that instructions may not be labelled. Both the instructions and their operands are syntactically checked character-by-character as they are entered and all illegal entries are refused. The simulator displays the corresponding octal form of these instructions and modifies the representation of the state of the machine accordingly. Short programs of such instructions may be entered and normal sequential execution simulated, Experience indicates that such simulators can provide an easy means of learning the computer organization, mnemonic instructions, and program execution of a computer. They can be especially helpful in demonstrating the dynamics of recursive subroutine calls and push-down stacks.