Table of Contents

SD-8516 Stellar Basic V1.0

Dennis Allison’s 1975 article in Dr. Dobb’s Journal was a key moment in the history of Computer Science. It contained a formal specification of Tiny BASIC, a BASIC that could be implemented in less than 4 KB.

Stellar BASIC is very much in the same vein as Tiny BASIC, and is intended to evolve over time. However, Tiny BASIC itself is only a specification. One way is to compile a list of IL (intermediate language) statements. Once you have the IL interpretation you compile it and interpret it from there. On a machine with limited ram this approach can require up to 50% of the memory of a standard BASIC program. This is problematic. So machines like the C64 tokenized the statements on entry and executed them that way. The other issue with IL is that you have this compiled version and you also have the text lying around. That's two copies of the program. Which one is the 'real' program? If you remove the original text you can't necessarily LIST your code. Therefore, tokenization was settled on as a standard.

Therefore, when it came time to compile the IL into a program we instead chose a C64 style tokenization, and added $99 as PRINT as a subtle homage to the adventure, the love, and the magic of the C64 era.

Core features

Some versions stored programs as text, some as tokenized program code to save space.

Example BASIC program

10 LET A = 1
20 PRINT A
30 A = A + 1
40 IF A <= 10 THEN GOTO 20
50 END

Stellar BASIC will allow this even shorter form: 

10 A=1
20 ?A
30 A=A+1
40 IF A<=10 GOTO 20

(? is shorthand for PRINT.)

More Information

Notable Tiny BASIC implementations

Appendix I: Pao Alto Tiny Basic

Stellar Basic is based on Pao Alto Tiny Basic, written by Dennis Allison in 1975.

Implementation

Please see: https://www.helloneo.ca/wiki/doku.php?id=vc-3_system_interrupt_table#int_05h_-_pao_alto_tiny_basic

I implemented it in this order:

Stack-based design

Everything in PATB is stack-based. This is a core design principle. There are three stacks:

  1. Expression Stack - Arithmetic evaluation
    • “5 + 3 * 2” is “push 5”, “push 3”, “push 2”, “multiply” (pop 2 values, push result), “add” (pop 2 values, push result).
  2. GOSUB Stack - Subroutine calls
    • “GOSUB 1000” – push current line number, jump to 1000
    • “RETURN” – pop line number, jump back
  3. FOR Stack - Loop context
    • “FOR I=1 TO 10” – push (I, 10, 1)
    • “NEXT I” – peek stack, increment I, check if done, pop if finished

Immediate Modifications to PATB 1.0

During the implementation phase I made several by-the-way changes to PATB in order to support a more advanced integration with the SD-8516. I added LINE_FIND_REVERSE and LINE_PREV as well as LINE_MAKE_SPACE and LINE_REMOVE_SPACE for line management. I am not sure how PATB does those things. I worked from example code and the specs but I did not really understand what I was doing. After the tests for the IL implementation passed I added string helpers. Some of these are in PATB (about 4 or 5 of them IIRC) but I added enough to support proper string handling, as it is the first thing I plan to add. After the basic port, of course.

I'm really excited to start working on these because I feel I understand them and they will be easy. LINE_INSERT is not easy It is long and hard. I do not understand it… YET!