RSTS/E

RSTS/E (an acronym for Resource Sharing Time Sharing Extended) was a multi-user time-shared operating system developed by Digital Equipment Corporation ("DEC") for the PDP-11 series of 16-bit minicomputers, and used primarily during the 1970s and 1980s, although some installations were still being upgraded well into the 1990s. RSTS/E is usually referred to just as as "RSTS" and this article will generally use the shorter form. The acronym was usually pronounced as "RIST-ess" or "RIST-uhs".

The RSTS Operating System was primarily programmed using an extended version of the BASIC programming language which DEC called "BASIC-PLUS." Essentially all of the system software for the operating system, including the programs for resource accounting, login, logout and managing the system, were written using BASIC-PLUS.

The RSTS system operates strictly in text mode only as it used non-graphical terminals (and modems) connected to it by the equivalent of serial ports, and uses a command interpreter whose primary command language is BASIC-PLUS, although special "CCL" commands which can be installed by the System Manager may be used.

As many as 63 terminals could be connected to a RSTS/E system, depending on the processor being used, the amount of memory and disk space, and the system load. Most RSTS systems had nowhere near that many terminals. Users could also submit jobs to be run in batch mode.

Users connected to the system by typing the LOGIN command (or HELLO) at a logged-out terminal and pressing return. Actually, typing any command at a logged-out terminal simply started the LOGIN program which then interpreted the command. If it was one of the commands which were allowed to be used by a user that is not yet logged in ("Logged Out"), then the associated program for that command was CHAINed to, otherwise the message "Please say HELLO" was printed on the terminal.

One could determine the status of a terminal by the prompt printed by the command interpreter, usually the BASIC-PLUS run-time system. A logged-in user was given the prompt "Ready" and a user who is logged out is given the prompt "Bye".

A user would log in by supplying their user number and password. User numbers consisted of a project number (this would be the equivalent of a group number in Linux), a comma, and a programmer number. Both numbers were in the range of 0 to 254, with special exceptions. When specifying an account, the project and programmer number were enclosed in brackets. A typical user number could be [10,5] (project 10, programmer 5), [2,146], [254,31], or [200,220], etc. When a user was running a system program while logged out (because the system manager had enabled it) their user number would appear as [0,0]. Thus that is not a valid account number.

In every project, the programmer number 0 was usually reserved as a group account, as it could be referenced by the special symbol #. If one's user number were [20,103], a reference to a file name beginning with "#" would refer to a file stored in the account of the user number [20,0].

Two special classes of project numbers existed. Project numbers of 0 were generally reserved for system software. Project numbers of 1 are privileged accounts, equivalent to the single account "root" on Linux systems, except that the account numbers [1,0] through [1,254] are all privileged accounts.

There are also certain special accounts on the system. The account [0,1] is used to store the operating system file itself, all run-time library systems, and certain system files relating to booting the system (author's comments appear on the right in bold):

Ready

DIR [0,1]
 Name .Ext    Size   Prot    Date       SY:[0,1]
BADB  .SYS       0P  < 63> 06-Jun-98         List of bad blocks
SATT  .SYS       3CP < 63> 06-Jun-98              
INIT  .SYS     419P  < 40> 06-Jun-98         Startup commands
ERR   .ERR      16CP < 40> 06-Jun-98         System error messages
RSTS  .SIL     307CP < 60> 06-Jun-98         Operating system itself
BASIC .RTS      73CP < 60> 06-Jun-98         BASIC-PLUS run time system
RT11  .RTS      20C  < 60> 06-Jun-98         RT-11 run time system
SWAP  .SYS    1024CP < 63> 06-Jun-98         System swap file   
CRASH .SYS      35CP < 63> 06-Jun-98         System crash dump
RSX   .RTS      16C  < 60> 23-Sep-79         RSX-11 run-time system
TECO  .RTS      39C  < 60> 24-Sep-79         TECO text editor

Total of 1952 blocks in 11 files in SY:[0,1]

Ready

The numbers shown after each file represent its size in disk blocks, a block being 512 bytes. "C" indicates the file is contiguous. The numbers in brackets (like "< 40>") represent the protections for the file, which is stored internally as an octal number but displayed here in decimal. Protections indicate if the file may be seen by any other user, by other users with the same programmer number, if the file is read only or if it may be altered by another user, and whether the file may be executed by an ordimary user giving them additional privileges.

Certain other system files are kept in account [1,1]. The account [1,2] is the system-wide public library, and any file in that account could be referenced by prefixing its name with a dollar sign ($). "!" is used for account [1,3] and & is used for account [1,5]. The account [1,1] also had the special privilege of being the only account permitted to execute the POKE system call to put values into any memory in the system.

When logged in, a user may either type language statements in the BASIC-PLUS programming language, issue the RUN command to run a program, or issue a special command called a CCL to execute a program with command options.

One of the features of RSTS is the means for the execution of programs and the environment used to run them. The various environments allowed for programming in BASIC-PLUS, the enhanced BASIC Plus 2, and in more traditional programming languages such as COBOL and FORTRAN. These environments were separate from each other such that one could start a program from one environment and switch to a different environment to run a different program. These environments were referred to as "Run-Time systems".

One of the most powerful text editors ever developed, the TECO editor, was itself implemented as a run-time system due to the amount of memory it used, (i.e. if it had run within another run-time system there probably would have been no memory available to edit user documents.)

Programs written in BASIC-PLUS ran under the BASIC run-time system, which allowed them perhaps 32Kbytes of memory. If one wrote programs in a language that permitted true binary executables such as FORTRAN or the Macro Assembler or others that ran under the RT11 run-time system, the amount of memory available would be about 56Kbytes.

Programs written for the RSX run time system such as COBOL, the Macro Assembler, or the later releases of BASIC Plus 2, the amount of memory available to such a binary program would still be about 56Kbytes due to the requirements of the RSX Run-Time System being allowed the top 8Kbytes to use for itself. A later release of RSTS allowed part of the RSX Run-Time system to be included in the kernel of RSTS, and permitting most of the RSX Run-Time system to "disappear", reducing its space to 1Kbyte, and thus allowing about 63Kbytes of memory for programs running under the RSX operating system.

Programs got around the limitations of the amount of available memory by using libraries (when permissible), by complicated overlay strategies, or by calling other programs ("Chaining") and passing them commands in a shared memory area called "Core Common," among other practices.

The cost of micro computers dropped so drastically, and their performance rose so radically, that minicomputers such as the PDP-11 were no longer cost effective. In the late 1990s DEC sold its PDP-11 software business to a company named Mentec, and DEC then essentially discontinued releases of RSTS in order to concentrate on their VAX series of mainframe computers.

Mentec would later publicly grant a no-cost license to anyone for non-commercial hobby purpose use of the operating system software developed for the PDP-11, and as the result of the release (for non-commercial use) of a PDP-11 emulator for the IBM-PC, as well as image copies of disk drives of RSTS which exist on the Internet, it is possible for a hobbyist to run RSTS/E on a personal computer in software emulation as fast or faster than it originally ran on the real hardware.

The last release of RSTS/E was version 10. DEC would later be purchased by Compaq, which itself later merged with Hewlett Packard.