3) Debugger

It should be possible to construct a user level program (debugger) which can intimately control other user level programs, even if those other programs use sophisticated system facilities. This control should include the ability to start a user's program at a specified location, run it at full machine speed, and regain control under some specified condition. These conditions should include the detection of an instruction error on the part of the user program, or a command from the user at a teletype. Once the debugger has regained control, it should be able to inspect the internal state of the user's program in detail, and make modifications.