Any command or program can be interrupted during execution by pressing the active level pushing key, (usually the BREAK or ESC key). When a command or program is interrupted, the system stops execution and saves all parameters so that execution can be resumed exactly where it was interrupted. When a process is interrupted at the normal system level, the system prompts with two colons. At this point the command or program is said to be pushed one level.

Up to 16 levels can be pushed. The number of colons in the prompt indicates the number of levels pushed. (The normal system level is considered level 1).

Pressing ENTER at the higher-level TCL prompt returns to the previous level and continues execution of the process at that level.

The end command terminates the current process and returns control to the previous level.

Levels are a function of the D3 virtual machine and should not be confused with spawning a child process (as would occur in the UNIX engine). The current virtual task is suspended, a new virtual workspace is established, and TCL is activated as the current user.

In D3, each new level grabs a minimum of 4 frames (user control blocks). Since workspace is floating, new workspace is allocated as required. When a level wraps up and returns, all workspace associated with that level is returned to overflow.