 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.