JTTB - Jason's Tiny-ish BASIC

This is a small BASIC interpreter that started out as a straight-forward implementation of the original Tiny BASIC "specification" using the Tiny BASIC virtual machine, inspired by a mild case of boredom while I was on a plane flight. Much to my delight, my boredom was cured and it's now a fun little plaything where I can channel my nostalgia.

This dialect of Tiny BASIC is written in very portable C, and leverages the C run-time to do some of the heavy lifting (memory management and floating point math support, specifically). And while the code is reasonably small, it doens't go to any great effort to squeeze out every last byte of memory efficiency possible; The program store, for example, is a sparsely populated fixed-size array of pointers.

Jason's Tiny-ish BASIC also takes some syntactic inspiration from the classic Microsoft BASIC of the late 70s and early 80s, specifically the TRS-80 Extended Color BASIC variant, since that's what I grew up on. This includes the PRINT and FOR loop syntax, as well as several of the built-in functions found in that (and other) flavors of Microsoft BASIC.

Jason's Tiny-ish BASIC also supports string variables (A$ ... Z$) and treats strings as first-class objects, unlike the original Tiny BASIC which only supported immediate (static) strings and only for printing. Strings can be included in expressions (and adding two strings with the + operator creates a new string object whose value is the concatenation of the two strings); type mis-matches are detected at run-time.

Staying true to the original Tiny BASIC specification, Jason's Tiny-ish BASIC is built using a virtual machine, and the virtual machine is capable of executing the original example Tiny BASIC VM program published in the original article (well, with the well-known minor corrections, anyway).

Above all, Jason's Tiny-ish BASIC is a work-in-progress, is not at all meant to be serious software, and is purely a vehicle for my own amusement. So just keep that in mind when you're looking at the stuff that's here! :-)

Components of Jason's Tiny-ish BASIC

There are 3 main components of Jason's Tiny-ish BASIC:

• The JTTB virtual machine, tbvm, which implements the program store, and the other supporting functions, as well as implements the VM opcodes of the Tiny BASIC VM assembly language that the heart and soul of the interpreter is written in.
• An assembler for the JTTB virtual machine assembly language, tbasm, the output of which is executed by the Tiny BASIC virtual machine.
• The driver program, jttb, which is a small wrapper around the tbvm library and provides the I/O interface to the outside world.

There were a couple of ideas behind separating things out this way:

• By making the virtual machine a library, it would be relatively easy to provide additional inputs/outputs to the virtual machine that would enable embedding Jason's Tiny-ish BASIC as an extension language into something else, if anyone were to be crazy enough to do such a silly thing.
• By separating the VM program from the back-end, it provides a simple mechanism to play alternate syntaxes or even translating Jason's Tiny BASIC into something not based on English.

The JTTB Virtual Machine

At its core, the JTTB VM is relatively simple. It is structured around three stacks:

• The control stack
• The subroutine stack
• The expression stack

The control stack tracks return addresses for subroutines within the VM program itself.

The subroutine stack tracks return locations for subroutines within the BASIC program, used to support the GOSUB statement. It also stores the state needed for FOR loops. For each loop, it records the following:

• The variable involved in the loop
• The line number of the first statement in the loop body
• The starting value to assign to the variable
• The terminating value of the variable
• The loop stepping (the value to add to the variable each time through the loop)

The expression stack is used to hold:

• The left and right values used with arithmetic operators
• Arguments to functions
• Results of arithmetic operations and functions

The expression stack is an integral part of the typing system in JTTB.

In addition to these four stacks, there are a few other supporting characters that support the core functionality of the BASIC language. Those are:

• The variable store
• The program store
• The line buffer
• The line cursor
• The string store

And finally, there are a few other odds and ends:

• I/O routines
• Exception handling (BASIC errors, VM internal errors)
• VM program state

The tbvm_exec() function implements the VM's main execution loop. When called, it first extracts two special VM program addresses from the VM program, the collector PC and the executor PC. It then initializes everything, including two setjmp() environments for BASIC errors and VM internal errors. Finally, it enters the main loop which performs the following tasks in order:

• Garbage-collects unreferenced strings
• Checks for a BREAK from the console
• Fetches the next opcode from the VM program
• Calls the function that implements that opcode

If a VM internal error occurs (signalled by the vm_abort() function), the longjmp() target marks the VM as no longer running and tbvm_exec() returns.

If a BASIC error occurs (signalled by the basic_error() function), the longjmp() target forces the interpreter back into direct mode and re-enters the main loop.

(I will write some more notes here about how it works, eventually. Promise!)

The JTTB Virual Machine Assembler

(I will write some notes here about how it works, eventually. Promise!)

The JTTB Driver Program

(I will write some notes here about how it works, eventually. Promise!)

How to build it

(I will write some notes here about how to built it, eventually. Promise!)

The Stuff At The Bottom Of The Page

If you have any questions or feedback, I'd love to hear it! You can reach out to me on Twitter (@thorpej) or Mastodon (@[email protected]). You can also check out my YouTube channel, which has this and other retrocomputing related content.