This 'used to be' the way to write OS code -- well, not quite this well-organized, but, yeah, the MACRO facility was used to clean up the rough edges, and funct

This 'used to be' the way to write OS code -- well, not quite this well-organized, but, yeah, the MACRO facility was used to clean up the rough edges, and functions (subroutines, methods, ...) provided big chunks of, well, functionality. The TOPS-10 OS did this. As did TOP-20.