BASIC-PLUS
BASIC-PLUS is an extended dialect of the BASIC programming language that was developed by Digital Equipment Corporation for use on its RSTS/E time-sharing operating system for the PDP-11 series of 16-bit minicomputers in the early 1970s through the 1980s.
BASIC-PLUS was based very closely on the original Dartmouth BASIC, although it added a number of new structures. It also included a number of features from JOSS concerning conditional statements and formatting. In turn, BASIC-PLUS was the version on which the original Microsoft BASIC was patterned.
The language was later rewritten as a true compiler as BASIC-Plus-2, and was ported to the VAX-11 platform as that machine's native BASIC implementation. This version survived several platform changes, and is today known as VSI BASIC for OpenVMS.
Operation
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, 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.As a smart terminal with cursor control could not be guaranteed, BASIC-PLUS used the common system of prefixing all source code with a line number. The code was edited by typing in the number and then changing the contents of the following code. A line of code could be removed by typing in its line number and nothing else, thereby setting it to an empty line.
Like most BASIC implementations, the interpreter would convert keywords into a more compact "tokenized" form when the program was run. This avoided the need to repeatedly decode the keywords as strings, once converted to the tokenized form the keywords were numbers that pointed to routines to run that function. BASIC-PLUS included a command, but this was not a true compiler; this tokenized the entire program into the underlying "PPCODE" so that it did not have to occur at runtime.
The system could store a user's program in two formats. One was the editable source code in text format, created using the command and normally placed in a.BAS file. The other was the PPCODE version of the program created by the command and saved to a.BAC file..BAC files were smaller and loaded and ran faster, but could not be edited.
The virtual address space of an RSTS/E user was limited to a little less than 64KB of space. Using BASIC-PLUS, about half of this virtual address space was used by the combined command interpreter and run-time library. This limited user programs to about 32 kB of memory.
Large programs were broken up into various pieces by use of the statement, and programs could chain to specific line numbers in a secondary program to indicate that a program should begin execution at a different point from its first line. 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 data to each other as needed. Disk files could also be used but were slower.
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.
Syntax and features
BASIC-PLUS is patterned closely on later versions of Dartmouth BASIC, including its powerfulMAT commands. On top of this, DEC added a number of unique flow-control structures.Line numbers were positive integers from 1 to 32767. Logical lines of code could be continued on multiple physical lines by using a line feed at the end of a line instead of the normal carriage return character. For ease of external editing of the source file, later versions of BASIC-PLUS also allowed the character as a line-continuation character.
Multiple statements could be placed on a single line using as the statement separator. The system allowed tabs to be used as inline whitespace, and was used to make loops more clear, as in modern languages. Comments used either the keyword or the character, as opposed to MS BASICs, which used and.
Variable names in the early versions of BASIC-PLUS could be a single letter or a single letter followed by a single digit. With the inclusion of "Extend mode" in later versions, variable names could be up to 29 characters long, and dot was added as a permitted character. Every variable name still had to begin with a letter, however.
As in most versions of BASIC, the keyword, for variable assignment, was optional. It did have the ability to set multiple variables to a single value, like. The language supported three data types; floating-point numbers, integers, and strings. Variables with no suffix were floating point. Integer variables were indicated with a suffix, string variables were indicated with a suffix.
Any of these data types could be used in an array of one or two dimensions, using the keyword. Additionally, the command could be used to define a "virtual DIM" whose elements resided in file. The program would first a file, which would assign it a number, and then the would refer to the data in that file and could be used like any other array. This was a very useful technique for saving memory, as data did not have to be encoded into the program itself.
The language also included a number of commands to work with the entire array. The command would fill the matrix with values in a statement, would fill the array with user-typed values, and would print out the elements in a 1D or 2D format. could also be used to set default values in a matrix using associated keywords, for instance, would fill the A array with zeros. would transform an entire matrix, and would invert it. Additionally,,, and could be used on matrixes, performing the associated matrix operation.
The command divided the screen into regions 14 spaces wide, and the comma was used to move between these locations; would output 1, 2 and 3 in a spaced out fashion, while would leave a single space and produce "1 2 3". allowed a prompt string to be specified, but used the semicolon to separate it rather than the comma;.
Strings could be delimited by single or double quotes. In addition to the and functions that converted single characters to and from string format, BASIC-PLUS also supported Dartmouth's powerful command. iterated the string and returned each character's ASCII value as a slot in a numeric array. For instance, would return an array five elements long. One could reverse the operation as well, would read the individual numbers in the X array and convert it to a string.
loops worked as in later versions of BASIC, and the command could not be used in an expression. Instead, the and keywords could be used to control early exits. For instance, continue looping until I=10, with the assumption that following code would set the value of I.
BASIC-PLUS also allowed most control structures to be placed after other commands. For instance, is the equivalent of The opposite was also provided, was the equivalent of. Using a conditional expression could make a loop, for instance, would loop until X was 100. This offered a compact format for many common loop structures.
BASIC Plus 2
A related product called Basic Plus 2, was later developed by DEC to add additional features and increased performance.It used true compilation into threaded code and wrote its output to object files compatible with the machine code object files produced by the assembler and other language systems. These object files could be kept in libraries. A linker then created executable files from object files and the libraries. TKB also supported overlays; this allowed individual routines to be swapped into the virtual address space as needed, overlaying routines not currently being used. Additionally,
BP2 programs ran under the RSX Run Time System; this RTS only occupied 8KB of the user's virtual address space, leaving 56KB for the user's program. These two factors allowed individual BP2 programs to be much larger than BASIC-PLUS programs, often reducing the need for CHAINing among multiple programs.
Unlike BASIC-PLUS, BP2 was also available for the RSX-11 operating system. BP2 programs were also more compatible with the later VAX BASIC.