Tiny BASIC

See BASIC interpreters

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 PRINT. 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, $32 hexidecimal. 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 LIST 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 RUN 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 ($0A hex). 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.

The grammar is listed below in Backus-Naur form, almost exactly as it was specified in the Design Note.[8] 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) | =

    string ::= " ( |!|#|$ ... -|.|/|digit|: ... @|A|B|C ... |X|Y|Z)* "

Note that string wasn't defined in the Design Note.

This syntax, as simple as it was, added one innovation: GOTO and GOSUB could take an expression rather than just a line number, providing an assigned GOTO[9] rather than the switch statement of the GOTO/GOSUB ... OF ..., a structure then supported in HP Time-Shared BASIC and predating ON ... GOTO. The syntax allowing IF-THEN statement (as opposed to just a line number to branch to) was not yet supported in Dartmouth BASIC as this time but had been introduced by Digital[10] and copied by Microsoft.

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.[11]

Dialects that used the virtual machine included Tiny BASIC Extended, Tom Pittman's Tiny BASIC[12] 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 GO and if it fails to find it then jumps to line S3. If it finds it, execution continues with the next IL command. In this case, the system next tests for TO, skipping to line S2 if it fails (a test for SUB, to see if this is instead a GOSUB command). If it passes, control continues; in this case, calling an IL subroutine that starts at label EXPR, which parses an expression. In Tiny BASIC, GOTO X*10+100 (a computed GO TO) is as legal as GOTO 100 and is the alternative to the ON-GOTO of larger BASIC implementations. The subroutine EXPR pushes the result of the expression onto the arithmetic stack (in this case, the line number). DONE verifies no other text follows the expression and gives an error if it does. XFER 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.[13]

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

DONE

Report a syntax error if after deleting leading blanks the cursor is not positioned to reach a carriage return

XFER

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.

JUMP lbl

Continue execution of the IL at the label specified

RTN

Return to the IL location specified at the top of the control stack

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[14] 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."[15]

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."[16] 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 RND(). Though not included in the specification, a newsletter article prior to the Design Note for Tiny BASIC requested only this function.
• All enabled LET 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 IF statements, all but MINOL removed the need for expressions to contain relational operators (e.g., IF X THEN LET Y=X was valid). Implementations removed THEN altogether or made it optional or supported it only for implied GOTO.
• Many modified PRINT to support print zones, using , to go to the next zone and ; to not advance the cursor.
• All but 6800TB and DTB added NEW.
• All but 6800TB and MINOL added a function to return memory size: TBX had SZE, DTB and PATB had SIZE, L1B had MEM, and NIBL had TOP.
• Four implementations added arrays, whether a single, undimensioned array in PATB and L1B or DIMensionable arrays in TBX and DTB.
• Four implementations added the REMark statement.
• Four implementations added the FOR loop: PATB, NIBL, and L1B offered FOR-TO-STEP/NEXT, while TBX did not support STEP and used the keyword NXT to end a loop.
• Only NIBL had any nod towards structured programming, with DO/UNTIL, despite Allison's lament in Issue 2 about problems with BASIC.

As an alternative to tokenization, to save RAM, TBX,[17] DTB,[18] and MINOL[19] truncated keywords: PR for PRINT, IN for INPUT, RET for RETURN. 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, PRINT could be typed P., although PR. 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.[20]

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.[29]

Both SCELBAL[30] and 6800 Tiny BASIC were announced in the magazine but did not publish their source code.

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 assembler language with non-standard mnemonics. This led to further ports that worked with conventional assemblers on the 8080.[20] The first version of the interpreter occupied 1.77 kilobytes of memory and assumed the use of a Teletype Machine (TTY) for user input/output. An erratum to the original article appeared in the June/July issue of Dr. Dobb's (Vol. 1, No 6). This article also included information on adding additional I/O devices, using code for the VDM video display by Processor Technology as an example.

Wang was one of the first to use word copyleft. In Palo Alto Tiny BASIC's distribution notice, he had written "@COPYLEFT ALL WRONGS RESERVED".[31] 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.[17] He encouraged others to adapt his source code and publish their adaptions, as with Roger Rauskolb's version of PATB published in Interface Age.[1] He himself published a third version in PCC's Reference Book of Personal and Home Computing.[32]

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 IF and GOTO. 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 STEP modifier was optional, as in MS.[20]

Another significant change was the ability to place several statements on a single line. For reasons not explained, PATB used the semicolon ; to separate statements, rather than the already common colon :.

Other changes include the addition of a single numeric array, with the variable name @, STOP in addition to END, and the use of # for not-equals in comparisons, as opposed to <>.[20][lower-alpha 1]

PATB used words for error messages instead of numbers. To reduce the amount of memory required, there were only three messages and they consisted of single words. The system would respond with WHAT? for syntax errors, HOW? for run-time errors like GOTOs to a line that didn't exist or numeric overflows, and SORRY for out-of-memory problems.[20]

Wang also wrote a STARTREK program in his Tiny BASIC that appeared in the July 1976 issue of the People's Computer Company Newsletter.[33][34]

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).[35]

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).[36]

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 relops (<, =, #), omitting GOSUB and RETURN. It only supported unsigned 8-bit precision (in contrast to signed 16-bit precision for every other implementation) and line numbers from 0 to 254.

No spaces were permitted except in strings; ! returns a random number, $ before an expression loads a string at that address; OS returns to operating system. Memory was addressable as if it were a two-dimensioned array of high and low bytes (e.g., "(0,0)" to "(255,255)"); CALL executes a machine language subroutine.[19]

Many dialects appeared in various other publications.

The May 1977 issue featured a Floppy ROM containing MICRO-BASIC.

Inspired by PCC's call for Tiny BASICs, Robert Uiterwyk wrote MICRO BASIC 1.3 for the SWTPC (a 6800 system), which SWTPC published in the June 1976 issue of the SWTPC newsletter. Uiterwyk had handwritten the language on a legal tablet. He later expanded the language to 4K, adding support for floating point; this implementation was unique among BASIC interpreters by using Binary Coded Decimal to 9 digits of precision, with a range up to 10⁹⁹, and by being published for free as a "Floppy ROM" magazine insert. An 8K version added string variables and trigonometry functions. Both the 4K and 8K versions were sold by SWTPC. In January, 1978, Uiterwyk sold the rights of the source code to Motorola.[37][38]

Thomas F. Waitman wrote a Tiny BASIC in 1976 for the Hewlett-Packard HP-2640 and HP-2645 terminals (which used the Intel 8008 and 8080 processors), which was published in the Hewlett-Packard Journal.

Published in the December 1976 issue of Interface Age was LLL (Lawrence Livermore Laboratory) BASIC, the first draft of which was developed by Steve Leininger from Allison's specification before Leininger left National Semiconductor for Tandy Corporation. The final interpreter was developed by John Dickenson, Jerry Barber, and John Teeter at the University of Idaho on a contract with LLL. Taking 5K, it included a floating point package, developed by David Mead, Hal Brand, and Frank Olken. The program was placed into the public domain by LLL, which developed the system under the auspices of the U.S. Energy Research and Development Administration.[39]

Altair BASIC, 4K BASIC, could run within a 4kB RAM machine, leaving only about 790 bytes free for program code.[40][41] The Tiny BASIC initiative started in response to the $150 charge for Altair 4K BASIC.

In 1975, Steve Wozniak joined the newly formed Homebrew Computer Club, which had fellow members Li-Chen Wang (Palo Alto Tiny BASIC) and Tom Pittman (6800 Tiny BASIC). Wozniak concluded that his machine would have to have a BASIC of its own, which would, hopefully, be the first for the MOS Technology 6502 processor. As the language needed 4 kB RAM, he made that the minimum memory for the design.[42] Integer BASIC was originally published on Compact Cassette in 1976.

In 1977, Radio Shack (as it was known then) released their first computer, the TRS-80, a Z80 system with Level I BASIC in a 4kB ROM. Tandy-employee Steve Leininger had written the first draft of the NIBL (National Industrial Basic Language) interpreter for the SC/MP while employed at National Semiconductor. [25] Unable to take that source code with him, he adapted Li-Chen Wang's Palo Alto Tiny BASIC for the original prototype of the TRS-80 Model I. He extensively revised the interpreter, adding floating-point support, simple black-and-white graphics, and READ/DATA/RESTORE statements.[43]

Originally developed in 1979, Sinclair 4K BASIC, written by John Grant, used as its language definition the 1978 American National Standards Institute (ANSI) Minimal BASIC standard, but was itself an incomplete 4Kb implementation with integer arithmetic only.[44]

Tiny BASIC implementations have been adapted for processor control and for microcontrollers such as the Arduino:

• Stephen A. Ness wrote XYBASIC for the Mark Williams Company in 1977, a 4K integer implementation. The language was often used for process control applications.[45]
• Arduino BASIC - Adapted from Gordon Brandly's 68000 Tiny BASIC, ported to C by Mike Field.
• Tiny Basic Plus - Adapted from Arduino BASIC by Scott Lawrence.[46]
• Half-Byte Tiny Basic - Adapted from Arduino BASIC.[47]
• Tiny Basic on the Micro: Bit - Adapted from Palo Alto Tiny BASIC.[48]

In 2002, Emmanuel Chailloux, Pascal Manoury and Bruno Pagano published a Tiny BASIC (lacking GOSUB/RETURN) in Developing Applications with Objective Caml as an example Objective Caml application.[49]

In 2013, Alex Yang published an implementation in Python.[50]

In 2019, Sergey Kuznetsov published a version in Ruby.[51]

The following table compares the language feature of Tiny BASIC implementations against other prominent BASICs that preceded them.