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 4–8 KB.
The language was first developed solely as a standards document, written primarily by Dennis Allison, a member of the Computer Science faculty at Stanford University. 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.
Bob and Dennis published the design document in the newsletter of the People's Computer Company. In December 1975, Dick Whipple and John Arnold created an interpreter for the language that required only 3K of RAM. Bob and Dennis decided to publish this version and corrections to the original design documents in a newsletter dedicated to Tiny BASIC, which they called "Dr. Dobb's Journal of Tiny BASIC Calisthenics and Orthodontia". In the 1976 issues several versions of Tiny BASIC, including design descriptions and full source code, were published.
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).
line ::= number statement CR | statement CR
statement ::= PRINT expr-list
IF expression relop expression THEN statement
LET var = expression
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) | =
A BREAK from the console will interrupt execution of the program
Source: Dr. Dobb's Journal of Computer Calisthenics & Orthodontia, Volume 1, Number 1, 1976, p.9.
 Implementation in interpreted language
For the implementation, an interpreted language (IL) is used. An interpreter written in IL interprets a line of Tiny Basic code and executes it. The IL is run on an abstract machine, which interprets IL code. The idea to use an interpreted language goes back to Val Schorre (with META II, 1964) and Glennie (Syntax Machine). See also virtual machine, CLI.
The following table gives a partial list of the commands of the interpreted language in which the Tiny BASIC interpreter is written. The length of the whole interpreter program is only 120 IL operations. Thus the choice of an interpretive approach economized on memory space and implementation effort, although the BASIC programs run thereon were executed somewhat slowly. The CRLF in the last line symbolizes a carriage return followed by a line feed.
|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
||Execute the IL subroutine starting at lbl; save the IL address following the CALL on the control stack
||Return to the IL location specified at the top of the control stack
||Report a syntax error if after deleting leading blanks the cursor is not positioned to reach a carriage return
||Continue execution of the IL at the label specified
||Print characters from the BASIC text up to but not including the closing quotation mark
||Print number obtained by popping the top of the expression stack
||Insert spaces to move the print head to next zone
||Output a CRLF to the printer
Source: Dr. Dobb's Journal, Volume 1, Number 1, 1976, p.12.
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