Tiny BASIC

Tiny BASIC is a dialect of the BASIC programming language that can fit into as little as 2 or 3 KB of memory. This small size made it invaluable in the early days of microcomputers (the mid-1970s), when typical memory size was only 4–8 KB. The prevalence of BASIC on the first generation of home computers is an outcome of Tiny BASIC.

An early free software project

The use of "Copyleft; All Wrongs Reserved" in 1976

Tiny BASIC is an example of a free software project that existed before the free software movement. It started in the newsletter of the People's Computer Company in 1975. Dennis Allison, a member of the Computer Science faculty at Stanford University, wrote a specification for a simple version of the BASIC programming language. He was urged to create the standard by Bob Albrecht of the Homebrew Computer Club. He had seen BASIC on minicomputers and felt it would be the perfect match for new machines like the MITS Altair 8800, which had been released in January 1975. This design did not support text strings and only used integer arithmetic. The goal was for the program to fit in 2 to 3 kilobytes of memory.

The Tiny BASIC contents of the newsletter soon became Dr. Dobb's Journal of Tiny BASIC with a subtitle of "Calisthenics & Orthodontia, Running Light Without Overbyte." Hobbyists began writing BASIC language interpreters for their microprocessor-based home computers and sending the source code to Dr. Dobb's Journal and other magazines to be published. Dick Whipple and John Arnold wrote an interpreter that required only 3K of RAM. By the middle of 1976, Tiny BASIC interpreters were available for the Intel 8080, the Motorola 6800 and MOS Technology 6502 processors. This was a forerunner of the free software community's collaborative development before the internet allowed easy transfer of files, and was an example of a free software project before the free software movement. Computer hobbyists would exchange paper tapes, cassettes or even retype the files from the printed listings.

Jim Warren, editor of Dr. Dobb's Journal, wrote in the July 1976 ACM Programming Language newsletter about the motivations and methods of this successful project. He started with this: "There is a viable alternative to the problems raised by Bill Gates in his irate letter to computer hobbyists concerning 'ripping off' software. When software is free, or so inexpensive that it's easier to pay for it than to duplicate it, then it won't be 'stolen'." The Bill Gates letter was written to make software into products. The alternative method was to have an experienced professional do the overall design and then outline an implementation strategy. Knowledgeable amateurs would implement the design for a variety of computer systems. Warren predicted this strategy would be continued and expanded.

The May 1976 issue of Dr. Dobbs Journal had Li-Chen Wang's Palo Alto Tiny BASIC for the Intel 8080 microprocessor. The listing began with the usual title, author's name and date but it also had "@COPYLEFT ALL WRONGS RESERVED". A fellow Homebrew Computer Club member, Roger Rauskolb, modified and improved Li-Chen Wang's program and this was published in the December 1976 issue of Interface Age magazine. Roger added his name and preserved the COPYLEFT Notice.

Tiny BASIC grammar

The grammar is listed below in Backus-Naur form. In the listing, an asterisk ("*") denotes zero or more of the object to its left — except for the first asterisk in the definition of "term", which is the multiplication operator; parentheses group objects; and an epsilon ("ε") signifies the empty set. As is common in computer language grammar notation, the vertical bar ("|") distinguishes alternatives, as does their being listed on separate lines. The symbol "CR" denotes a carriage return (usually generated by a keyboard's "Enter" key). A BREAK from the console will interrupt execution of the program.

   line ::= number statement CR | statement CR

   statement ::= PRINT expr-list
                 IF expression relop expression THEN statement
                 GOTO expression
                 INPUT var-list
                 LET var = expression
                 GOSUB expression
                 RETURN
                 CLEAR
                 LIST
                 RUN
                 END

   expr-list ::= (string|expression) (, (string|expression) )*

   var-list ::= var (, var)*

   expression ::= (+|-|ε) term ((+|-) term)*

   term ::= factor ((*|/) factor)*

   factor ::= var | number | (expression)

   var ::= A | B | C ... | Y | Z

   number ::= digit digit*

   digit ::= 0 | 1 | 2 | 3 | ... | 8 | 9

   relop ::= < (>|=|ε) | > (<|=|ε) | =

Implementation in virtual machine

For some implementations, a virtual machine was used, others such as Palo Alto Tiny BASIC and 6800 Tiny BASIC, were direct interpreters. In a virtual machine implementation, Tiny BASIC interpreter is run on an application virtual machine interpreter. The idea to use an application virtual machine goes back to Val Schorre (with META II, 1964) and Glennie (Syntax Machine).

The following table gives a partial list of the commands of the virtual machine in which the Tiny BASIC interpreter was written. The length of the whole interpreter program was only 120 virtual machine operations. Thus the choice of a virtual machine approach economized on memory space and implementation effort, although the BASIC programs run thereon were executed somewhat slowly.

TST lbl, string	If string matches the BASIC line, advance cursor over string and execute the next IL instruction; if the test fails, execute the IL instruction at the label lbl
CALL lbl	Execute the IL subroutine starting at lbl; save the IL address following the CALL on the control stack
RTN	Return to the IL location specified at the top of the control stack
DONE	Report a syntax error if after deleting leading blanks the cursor is not positioned to reach a carriage return
JUMP lbl	Continue execution of the IL at the label specified
PRS	Print characters from the BASIC text up to but not including the closing quotation mark
PRN	Print number obtained by popping the top of the expression stack
SPC	Insert spaces to move the print head to next zone
NLINE	Output a CRLF to the printer

See also

• Copyleft
• Dartmouth BASIC
• Palo Alto Tiny BASIC, by Li-Chen Wang

External links

• Tiny Basic User Manual and Experimenter's Kit – by Tom Pittman
• Robert Uiterwyk's BASIC and Robert Uiterwyk's Micro Basic – A MC6800 tiny BASIC later sold with the SWTPC 6800 computer
• MINOL – Erik Mueller's MINOL – Tiny BASIC with strings for Intel 8080
• TBL – A similar IL to Tiny BASIC's, used in the first successful compilers for PL/I, Fortran, etc., on limited memory minicomputers
• 68000 Tiny BASIC – Based on Li-Chen Wang's Palo Alto Tiny BASIC, and published in the 100th edition of Dr. Dobb's
• Tiny BASIC – A version for the curses character screen handling library
• tinyBasic – An implementation written in iziBasic
• Tiny BASIC – A live web version, ported to Run BASIC from iziBasic
• TinyBasic – A port of Tom Pittman's TinyBasic C interpreter to Java, C# and Adobe Flex. Includes live web versions.