RSTS (pronounced russ-tuss, or rastus to those with an affection for it) was an operating system provided by Digital Equipment Corporation that ran on PDP-11 hardware. The acronym stands for "Resource Sharing Time Sharing". This operating system was released in 1970, and nearly all applications have been moved off this platform by 1990 apart from some diehard enthusiasts2.

It was intended to be a general purpose operating system with a shallow learning curve for new users. This is in contrast with the other two main core operating systems1 on the PDP-11. RT11 was a single user real time operating system, chiefly used for control applications such as running machinery. RSX-11 was a multi-tasking operating system, used mainly inside the scientific and academic community, with an arcane command language of three letter commands and three letter program names.

Once you log in to RSTS (with the command "hello"), you are in a Basic-Plus interpreter. This takes basic-plus statements, if the first thing is a line number, immediate mode expressions (assignments and print statements), and commands.

Run time systems

If you didn't want to use Basic-Plus, there was a selection of alternative environments available, including emulators for RT11 and RSX-11. This meant you can run binaries from another PDP-11 that was running RSX or RT11, or build binaries for that other machine. These environments were called run time systems, and were a hybrid of a shared library, a command interpreter and a kernel emulator.


The most irksome of these was that of the 64K limit on the per-process address space (the PDP-11 is a 16 bit machine). The machine itself can address upto 2M by means of memory extension windowing techniques (similar to MS-Dos expanded memory). This meant that programs above a certain size have to resort to overlay techniques.

File names on this system were 6.3, e.g. foobar.txt, and directories were of the form [mmm,nnn] where mmm and nnn are octal numbers, together called the project programmer number (PPN). Named directories forming trees came in with VMS.

The operating system was also quite insecure. The equivalent of the password file (the accounting file) held account details and passwords in clear text, and there was no high watermarking of the disk. This meant that a savvy person could allocate a huge data file, and pip it out (pip was the rough equivalent of cat), looking for fragments of the accounting file with user passwords. With a bit of luck, there would be a [1,*] account, which would have all the privileges.

The O/S was also vulnerable to resource hogging. I inadvertantly caused a DoS on my school's PDP-11 by leaving something running when I went home. The poor physics teacher who was on duty to shut it down couldn't get any response on the system console. This did get me banned from the school's computer for six months, but taught a valuable lesson to me as a teenage hacker (it would have been much more harmful to my career if I had tried something like this at university, or in employment :).


1I do not count as core operating systems the likes of IAS (which ran on top of RSX) or TSX+ which ran on top of RT-11. There were other, specialised, core operating systems, such as Mumps, which was used in medical environments. Note also that the PDP-11 was one of the original platforms for AT&T Unix.

2See for a time line.