Editing RSX-11 (section)

==Operation==
RSX-11 was often used for general-purpose timeshare computing, even though this was the target market for the competing [[RSTS/E]] operating system. RSX-11 provided features to ensure better than a maximum necessary response time to peripheral device input (i.e. real-time processing), its original intended use. These features included the ability to lock a process (called a ''task'' under RSX) into memory as part of system boot up and to assign a process a higher priority so that it would execute before any processes with a lower priority.

In order to support large programs within the PDP-11's relatively small [[virtual address]] space of 64&nbsp;KB, a sophisticated semi-automatic [[Overlay (programming)|overlay]] system was used; for any given program, this overlay scheme was produced by RSX's ''taskbuilder'' program (called ''TKB''). If the overlay scheme was especially complex, taskbuilding could take a rather long time (hours to days).

The standard RSX prompt is ">" or "MCR>", (for the "Monitor Console Routine". All commands can be shortened to their first three characters when entered and correspondingly all commands are unique in their first three characters. Only the login command of "HELLO" can be executed by a user not yet logged in. "HELLO" was chosen as the login command because only the first three characters, "HEL", are relevant and this allows a non-logged in user to execute a "[[help (command)|HELP]]" command.

When run on certain PDP-11 processors, each DEC operating system displays a characteristic light pattern on the processor console panel when the system is idle. These patterns are created by an idle task running at the lowest level. The RSX-11M light pattern is two sets of lights that sweep outwards to the left and right from the center of the console (inwards if the IND indirect command file processor program was currently running on older versions of RSX). <!-- this last part does not seem right?--> By contrast, the IAS light pattern was a single bar of lights that swept leftwards. Correspondingly, a jumbled light pattern (reflecting memory fetches) is a visible indication that the computer is under load (and the idle task is not being executed). Other PDP-11 operating systems such as RSTS/E have their own distinctive patterns in the console lights.