The Blit debugger

This paper describes the evolution of a debugger for C programs on the Blit, a multiprocessing bitmap terminal. The debugger is of interest for the following reasons: 1.(1) it is assisted by the terminal software's elegant separation of the debugger process from its subject process; 2.(2) it resides autonomously in the terminal and is bound dynamically to arbitrary subject processes; 3.(3) it executes asynchronously with its subject; 4.(4) its implementation is distributed as a small process in the terminal and a large process in the host timesharing system; 5.(5) its user interface uses graphics and a mouse. The first three sections of the paper introduce the Blit terminal and explain how the debugging of processes running in the terminal is influenced by its architecture. The next two sections present some details about the distributed implementation and the graphics user interface. Finally, an opinion about the most fruitful direction for further application of graphics is offered.