Tiny BASIC

Altair BASIC The earliest microcomputers, like the MITS Altair 8800, generally had no built-in input/output (I/O) beyond front-panel switches and LED lamps. Useful work generally required the addition of an I/O expansion card and the use of some form of terminal. At the time, video-based terminals were very expensive, costing much more than the computer, so many users turned to mechanical devices like the Teletype Model 33. The Model 33, like most teleprinters of the era, included a tape punch system intended to allow operators to pre-record their messages and then play them at "high speed", faster than most individuals could type the message live. For the early microcomputers, this provided a convenient computer data storage format, allowing the users to write programs to paper tape and distribute them to other users. The Homebrew Computer Club met for the first time in March 1975, and its members soon used the meetings to swap software on punched tape. At the June meeting, a tape containing a pre-release version of Altair BASIC disappeared. The tape was given to Steve Dompier, who passed it on to Dan Sokol, who had access to a high-speed tape punch. At the next meeting, 50 copies of Altair BASIC on paper tape appeared in a cardboard box. When Ed Roberts, founder of MITS, learned of this, he stated "Anyone who is using a stolen copy of MITS BASIC should identify himself for what he is, a thief." Bill Gates made this more formal, writing "An Open Letter to Hobbyists", complaining that "As the majority of hobbyists must be aware, most of you steal your software." Tiny BASIC The complaint was not well received. Among the many responses, Bob Albrecht, another Homebrew member and founder of the People's Computer Company (PCC), felt the best response would be to produce their own BASIC that was completely free to use by anyone. He approached Dennis Allison, a member of the Computer Science faculty at Stanford University, to write a specification for a version of BASIC that would fit in 2 to 3 kilobytes of memory. To aid porting, the design was based on an intermediate language (IL), an interpreter for the interpreter, which meant only a small portion of the total code had to be ported. Allison's initial design was published in the September 1975 edition of the PCC newsletter, along with an Intel 8080 version of the IL interpreter. The article called on programmers to implement the design on their computer and send the resulting assembly language version back to the PCC. They stated their plans to publish three special newsletters containing these user-submitted versions, along with bug fixes, programs written in the new BASIC, and suggestions and enhancements. The concept gained further notice when it was republished in the January 1976 edition of the ACM Special Interest Group on Programming Languages. Submissions poured in. Among the notable early versions was Tiny BASIC Extended by Dick Whipple and John Arnold which ran in 3K of RAM, added FOR...NXT loops, and allowed a single numeric array. They avoided the use of the IL and wrote it directly in machine code, using octal. It starts with a note from Albrecht, under the penname "the dragon", suggesting that three editions would not be enough, and asked the readers if they would like to see it continue. It also reprinted the original article on Tiny BASIC from PCC, included the complete listing of Extended TB, and included a number of small BASIC programs including tips-and-tricks from Allison. Response to the first issue was so impressive that the introduction to the second issue stated they had already decided to continue publishing the new newsletter under the simplified name ''Dr. Dobb's Journal. Over the next several issues, additional versions of the language were published, and similar articles began appearing in other magazines like Interface Age''. Spread 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. A fellow Homebrew Computer Club member, Roger Rauskolb, modified and improved 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. ==Description==

Basic concepts Tiny BASIC was designed to use as little memory as possible, and this is reflected in the paucity of features as well as details of its interpreter system. Early microcomputers lacked the RAM and secondary storage for a BASIC compiler, which was more typical of timesharing systems. Like most BASICs of the era, Tiny Basic was interactive with the user typing statements into a command line. As microcomputers of the era were often used with teletype machines or "dumb" terminals, direct editing of existing text was not possible and the editor instead used takeout characters, often the backslash, to indicate where the user backed up to edit existing text. If the user typed a statement into the command line the system examined it to see if it started with a number. If it did not, the line was immediately parsed and operated on, potentially generating output via . This was known as "direct mode". If the line was entered with a leading number, the number was converted from decimal format, like "50", and converted to a 8-bit value, in this case, hexadecimal. This number was used as an index into an array-like storage area where the rest of the line was stored in exactly the format it was typed. When the user typed into the command line the system would loop over the array, convert the line number back to decimal format, and then print out the rest of the text in the line. When a program was present in memory and the user types in the command, the system enters "indirect mode". In this mode, a pointer is set to point to the first line of the program, for instance, 10 (). The original text for that line is then retrieved from the store and run as if the user had just typed it in direct mode. The pointer then advances to the next line and the process continues. Formal grammar The grammar is listed below in Backus–Naur form, almost exactly as it was specified in the Design Note. 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 string. 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) | > ( Note that string wasn't defined in the Design Note. This syntax, as simple as it was, added one innovation: and could take an expression rather than just a line number, providing an assigned GOTO rather than the switch statement of the , a structure then supported in HP Time-Shared BASIC and predating . The syntax allowing statement (as opposed to just a line number to branch to) was not yet supported in Dartmouth BASIC at this time but had been introduced by Digital and copied by Microsoft. Implementation in a virtual machine The Design Note specified a virtual machine, in which the Tiny BASIC interpreter is itself run on a virtual machine interpreter. The designer's idea to use an application virtual machine goes back to Val Schorre (with META II, 1964) and Glennie (Syntax Machine). The choice of a virtual machine approach economized on memory space and implementation effort, although the BASIC programs run thereon were executed somewhat slowly. Dialects that used the virtual machine included Tiny BASIC Extended, Tom Pittman's Tiny BASIC and NIBL. Other dialects such as Denver Tiny BASIC (DTB) and Palo Alto Tiny BASIC were direct interpreters. Some programmers, such as Fred Greeb with DTB, treated the IL (Interpretive Language) program as pseudocode for the algorithm to implement in assembly language; Denver Tiny BASIC did not use a virtual machine, but it did closely follow the IL program. This is a representative excerpt from the 120-line IL program: S1: TST S3,'GO' ;GOTO OR GOSUB? TST S2,'TO' ;YES...TO, OR...SUB CALL EXPR ;GET LABEL DONE ;ERROR IF CR NOT NEXT XFER ;SET UP AND JUMP S3: TST S8,'PRINT' ;PRINT. A common pattern in the program is to test for a keyword or part of a keyword, then act on that information. Each test is an assertion as to what is next in the line buffer. If the assertion fails, control jumps to a subsequent label (usually looking for a new keyword or token). Here the system advances its buffer cursor over any spaces and tests for and if it fails to find it then jumps to line . If it finds it, execution continues with the next IL command. In this case, the system next tests for , skipping to line if it fails (a test for , to see if this is instead a command). If it passes, control continues; in this case, calling an IL subroutine that starts at label , which parses an expression. In Tiny BASIC, (a computed GO TO) is as legal as and is the alternative to the ON-GOTO of larger BASIC implementations. The subroutine pushes the result of the expression onto the arithmetic stack (in this case, the line number). verifies no other text follows the expression and gives an error if it does. pops the number from the stack and transfers execution (GOes TO) the corresponding line number, if it exists. The following table gives a partial list of the 32 commands of the virtual machine in which the first Tiny BASIC interpreter was written. ; : If string matches the BASIC line, advance cursor over and execute the next IL instruction; if the test fails, execute the IL instruction at the label lbl ; : Execute the IL subroutine starting at ; save the IL address following the CALL on the control stack ; : Report a syntax error if after deleting leading blanks the cursor is not positioned to reach a carriage return ; : Test value at the top of the AE stack to be within range. If not, report an error. If so, attempt to position cursor at that line. If it exists, begin interpretation there; if not, report an error. ; : Continue execution of the IL at the label specified ; : Return to the IL location specified at the top of the control stack ; : 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 Tom Pittman, discussing the IL, says: "The TINY BASIC interpreter was designed by Dennis Allison as a recursive descent parser. Some of the elegant simplicity of this design was lost in the addition of syntactical sugar to the language but the basic form remains. The IL is especially suited to Recursive Descent parsing of TINY BASIC because of the general recursive nature of its procedures and the simplicity of the TINY BASIC tokens. The IL language is effectively optimized for the interpretation of TINY. Experience has shown that the difficulty of adding new features to the language is all out of proportion with the nature of the features. Usually it is necessary to add additional machine language subroutines to support the new features. Often the difficulty outweighs the advantages." Deviations from the design Defining Tiny BASIC for the Homebrew Computer Club, Pittman wrote, "Tiny BASIC is a proper subset of Dartmouth BASIC, consisting of the following statement types only: LET, PRINT, INPUT, IF, GOTO, GOSUB, RETURN, END, CLEAR, LIST, RUN. Arithmetic is in 16-bit integers only with the operators + - * / and nested parentheses. There are only the 26 single letter variable names A, B, ...Z, and no functions. There are no strings or arrays... Tiny BASIC specifies line numbers less than 256." He then went on to describe his implementation: "This language has been augmented to include the functions RND, USR, and PEEK and POKE, giving the user access to all his system components in the 6800 from the BASIC program." Many implementers brought their own experiences with HP Time-Shared BASIC or DEC BASIC-PLUS to their designs and relaxed the formal Tiny BASIC language specification. Of the seven prominent implementations published by 1977: • All added some sort of random number function, typically . Though not included in the specification, a newsletter article prior to the Design Note for Tiny BASIC requested only this function. • All enabled to be optional and most let expressions in assignment statements contain relational operators. • All but 6800TB supported statement delimiters in lines, typically although TBX used and PATB used . • In statements, all but MINOL removed the need for expressions to contain relational operators (e.g., was valid). Implementations removed altogether or made it optional or supported it only for implied . None supported clauses. • Many modified to support print zones, using to go to the next zone and to not advance the cursor. • All but 6800TB and DTB added . • All but 6800TB and MINOL added a function to return memory size: TBX had , DTB and PATB had , L1B had , and NIBL had . • Four implementations added arrays, whether a single, undimensioned array in PATB and L1B or ensionable arrays in TBX and DTB. • Four implementations added the ark statement. • Four implementations added the loop: PATB, NIBL, and L1B offered , while TBX did not support and used the keyword to end a loop. • Only NIBL had any nod towards structured programming, with , despite Allison's lament in Issue 2 about problems with BASIC. As an alternative to tokenization, to save RAM, TBX, DTB, and MINOL truncated keywords: for , for , for . The full, traditional keywords were not accepted. In contrast, PATB allowed accepted traditional keywords but also allowed any keyword to be abbreviated to its minimal unique string, with a trailing period. For instance, could be typed , although and other variations also worked. This system was retained in Level I BASIC for the TRS-80, which used PATB, and was also later found in Atari BASIC and the BASIC of various Sharp Pocket Computers. ==Dialects==

The most prominent dialects of Tiny BASIC were the original Design Note, Tiny BASIC Extended, Palo Alto Tiny BASIC, and 6800 Tiny BASIC. However, many other versions of Tiny BASIC existed. List of prominent dialects Tiny BASIC was first published in a newsletter offshoot of the People's Computer Company, a newsletter which became ''Dr. Dobb's Journal'', a long-lived computing magazine. About ten versions were published in the magazine. TBX was also known as Texas Tiny BASIC. Both SCELBAL and 6800 Tiny BASIC were announced in the magazine but did not publish their source code. Palo Alto Tiny BASIC One of the most popular of the many versions of Tiny BASIC was Palo Alto Tiny BASIC, or PATB for short, by Li-Chen Wang. PATB first appeared in the May 1976 edition of Dr. Dobbs, written in a custom assembly language with non-standard mnemonics. This led to further ports that worked with conventional assemblers on the 8080. Tiny BASIC was not distributed under any formal form of copyleft distribution terms, but was presented in a context where source code was being shared and modified. In fact, Wang had earlier contributed edits to Tiny BASIC Extended before writing his own interpreter. One of the most notable changes in PATB is the addition of the FOR...NEXT loop. In the original TB, loops could only be implemented using and . As in Microsoft BASIC, the upper and lower bounds of the loop were set on loop entry, and did not change during the loop, so if one of the bounds was based on a variable expression; changing the variable did not change the bound. The modifier was optional, as in MS. He later adapted the language into 3K Control Basic for Cromemco, adding variable names of the form letter-digit (e.g., A0 to Z9), logic functions (AND(), OR(), XOR()), a CALL command to execute machine language routines, more PRINT-formatting options, and others (GET() and PUT() instead of PEEK and POKE; I/O port functions). Palo Alto Tiny BASIC was adapted for many other implementations, including Level I BASIC (1977), BASIC for the Sharp PC-1211 pocket computer (1980), and Astro BASIC (1982, by Jamie Fenton). MINOL Written by a junior in high school, MINOL was the only implementation that didn't support the full Design Note, lacking operator precedence, having only three relational operators ( ==See also==