code provided by the supplier which would be mapped into the high end of a 32K, 16-bit word address space that a user program would use to interface with the operating system. Only one copy of an RTS would be loaded into RAM, but would be mapped into the address space of any user program that required it. In essence, shared, re-entrant code, to reduce RAM requirements, by sharing the code between any programs that required it. RTSS (Resource Time Sharing System – never marketed) – The second name for RSTS SATT (Storage Allocation Truth Table) a series of 512KB blocks on every disk that indicated if the block, or cluster, on the whole disk was allocated on the disk. Bitwise, a 1 indicated a cluster was in use; a 0 indicated it was not in use. SIL (Save Image Library) – The OS container file format after the RSTS system generation procedure was switched from one using DOS-11 to one running natively on RSTS/E, called that by analogy to the CIL it replaced. SILUS (Save Image Library Update and Save) – The program to create SIL files, called that by analogy to the (unrelated) DOS-11 CILUS program it replaced. UFD (User File Directory) – A user's home directory. Root directory of a file system. XRB (Transfer Request Block) – A data structure containing information about other types of system requests that do not use FIRQBs to convey the information Development 1970s The kernel of RSTS was programmed in the assembly language MACRO-11, compiled and installed to a disk using the CILUS program, running on a DOS-11 operating system. RSTS booted into an extended version of the BASIC programming language which DEC called "BASIC-PLUS". All of the system software CUSPS for the operating system, including the programs for resource accounting, login, logout, and managing the system, were written in BASIC-PLUS. From 1970 to 1973, RSTS ran in only 56K bytes of magnetic core memory (64 kilobytes including the memory-mapped I/O space). This would allow a system to have up to 16 terminals with a maximum of 17 jobs. The maximum program size was 16K bytes. By the end of 1973 DEC estimated there were 150 licensed systems running RSTS. In 1973 memory management support was included in RSTS (now RSTS/E) for the newer DEC PDP-11/40 and PDP-11/45 minicomputers (the PDP-11/20 was only supported under RSTS-11). The introduction of memory management