Basic Plus (or Basic-Plus) was an extended dialect of the BASIC programming language developed by Digital Equipment Corporation ("DEC") for use on its RSTS/E time_shared operating system for the PDP-11 series of 16-bit minicomputers in the early 1970s through the 1980s.

Users would sit at a terminal and type in programming language statements. The statements could either be entered into the system's command interpreter directly, or entered into a text editor, then saved to a file and loaded into the command interpreter from the file. Errors in source code were reported to the user immediately after the line was typed.

The language system provided two modes of use. A program could be stored on the system in editable source code, or pseudo-compiled into a non-editable binary interpreted by the run time system.

Source programs could be edited and run, similar to the BASIC interpreter which years later was included with the IBM PC in ROM and under MS-DOS, or a Basic Plus program could be translated into a non-editable binary application using the "compile" command.

While the command interpreter had a "compile" command, it was not a true machine language compiler. The language was a semi-compiled language, in which a source program was "compiled" into byte code called "tokens," and the tokens interpretively executed, in a manner similar today to the execution of programs using the Java programming language.

Programs were entered into the command interpreter starting with line numbers, integers from 1 to 32767, and were continued on multiple lines by using a line feed at the end of a line instead of the return (enter) key. (This feature carries on today in Visual Basic's use of the underline (_) at the end of a line to indicate a continuation. Later version of Basic plus also used the & character.) Multiple statements could be placed on a line using as a separator.

Memory on the PDP-11 was limited to about 64KB of space for any particular user at a terminal, of which a large portion was used by the command interpreter and run-time library. This limited user programs to about 16KB of memory. Large programs were broken up into various pieces by use of the "CHAIN" instruction, and programs could chain to specific line numbers in a secondary program to indicate that program should begin execution at a different point than its start. This feature of chaining to a certain line number allowed programs to signal to each other that they were being called from another program. The use of a shared memory section called "core common" also allowed programs to pass commands to each other as needed.

To conserve memory the interpreter included a garbage collecting memory manager, used for both string data and byte-code.

A running program could be interrupted, have variables examined and modified, and then be resumed.

Many of the control structures used in other high-level languages existed in Basic Plus, including WHILE, and the language also supported the use of modifiers on a line to control execution such as

 PRINT I UNLESS I < 10 

BASIC Plus 2

An enhanced version of Basic Plus, called Basic Plus 2 ("BP2" or BASIC-Plus-2), was later developed by DEC to add additional features. It used true compilation to threaded code, output to object files, compatible with the machine code object files produced by the assember. These object files could be kept in libraries. TKB (TasK Builder), a linker, created executable files from object files and the libraries. TKB also supported overlays in which individual routines could be swapped in from disk as needed, overlaying routines no longer used. (TKB was ported from RSX_11, another DEC PDP_11 OS.)

Overlays and the fact that BP2 didn't need the run_time library allowed for much larger programs without the use of CHAINing.