Concept
RT-11 (Real-time 11) is a discontinued small, low-end, single-user real-time operating system for the full line of Digital Equipment Corporation PDP-11 16-bit computers. RT-11 was first implemented in 1970. It was widely used for real-time computing systems, process control, and data acquisition across all PDP-11s. It was also used for low-cost general-use computing. RT-11 was written in assembly language. Heavy use of the conditional assembly and macro programming features of the MACRO-11 assembler allowed a significant degree of configurability and allowed programmers to specify high-level instructions otherwise unprovided for in machine code. RT-11 distributions included the source code of the operating system and its device drivers with all the comments removed and a program named "SYSGEN" which would build the operating system and drivers according to a user-specified configuration. Developer's documentation included a kernel listing that included comments. In RT-11, device drivers were loadable, except that prior to V4.0 the device driver for the system device (boot device) was built into the kernel at configuration time. Because RT-11 was commonly used for device control and data acquisition, it was common for developers to write or enhance device drivers. DEC encouraged such driver development by making their hardware subsystems (from bus structure to code) open, documenting the internals of the operating system, encouraging third-party hardware and software vendors, and by fostering the development of the Digital Equipment Computer Users Society. RT-11 systems did not support preemptive multitasking, but most versions could run multiple simultaneous applications. All variants of the monitors provided a background job. The FB, XM, and ZM monitors also provided a foreground job, and six system jobs if selected via the SYSGEN system generation program. These tasks had fixed priorities, with the background job lowest and the foreground job highest.