Copyright © 2020 Maury Markowitz
RetroBASIC is an interpreter for programs written using 1970s/80s variations of BASIC. It is patterned mostly on MS-BASIC as seen on Commodore machines and Apple II (and many others). It also includes the alternate string manipulation methods from Dartmouth BASIC v4 and HP Timeshared BASIC, the later of which was widely used in early microcomputer interpreters. The ultimate goal is to support almost any dialect from the era, including Dartmouth BASIC, Tiny BASIC (including Palo Alto), MS-BASIC (Altair, Commodore, etc.), HP Timeshared BASIC (Northstar, Apple, Atari, Sinclair, etc.) and others.
RetroBASIC can redirect the output from PRINT statements and INPUT prompts to a file, and read the responses to INPUT statements from a file. This can be used to provide the same input to a program multiple times, and then the output can be diffed to look for changes. This is aided by setting the random seed value, which can be done on the command line to avoid editing the original BASIC code.
RetroBASIC also includes a simple static analyzer that (optionally) prints statistics for the program after it completes. This includes the length of the program and its line number range, the number and types of variables used, and similar details. The original impetus for RetroBASIC was to collect statistics on common programs to provide hints to the retrocomputing field, where new BASICs continue to be developed.
RetroBASIC is based on gnbasic by James Bowman.
RetroBASIC is meant to be used with an existing program source file, not interactively. To run it, use:
./retrobasic program.bas
It will accept any text file as input and report (cryptic) errors if it cannot properly parse it. If parsing succeeds, the program (normally) begins running immediately.
Command-line options include:
--help, -h: print usage notes
--version, -v: print version info
--upper-case, -u: force input to upper-case, basically like using caps lock
--array-base, -a: set base index for arrays, normally 1 but some dialects use 0
--trace, -t: turn on line number tracing
--tabs: set the number of spaces for comma-separated items, defaults to 10
--goto-next, -g: if a branch is to a non-existent line, should it go to the next line or return an error?
--random, -r: seed the random number generator
--slicing, -s: enable string slicing like HP, Integer, Atari, etc.
--output-file, -o: redirect PRINT to the named file
--input-file, -i: redirect INPUT from the named file, one INPUT value per line
--no_run, -n: do not run the BASIC program, simply read and parse it and then exit
--print_statistics, -p: send a selection of statistics to the console
--write_statistics, -w: write the statistics to the named file in a machine readable format
If you wish to use RetroBASIC to simply check syntax or collect statistics, use the -n and -p switches.
Short options with no parameters can be ganged, for instance, -unp.
The RetroBASIC interpreter is written for lex/yacc/c and is normally compiled with flex/bison. It has no external dependancies, although cygwin is required for compiling on Windows. A makefile is included that should run on almost any Unix-like system, including macOS.
An Xcode project is also included, which is the primary building method during testing. It has one drawback (currently), the bison build rule in Xcode does not properly build parse.h into the src folder, but follows yacc-like rules and builds y.tab.h file in the ../DerivedSources folder. If you make changes to parse.y, be sure to copy the new y.tab.h to /src/parse.h for those changes to be visible.
If anyone would like to contribute a VS.net project, it would be greatly appreciated.
A complete list is maintained in the TODO file, but here are some important limitations:
- Variable names are currently limited to two characters, which is needed to support "crunched" statements without spaces in MS style.
- The system does not support "immediate mode" (command line) input. It is not supposed to, but could potentially do so.
- LIST, LOAD and SAVE are currently not implemented, in keeping with the use-case.
- File I/O is not supported, commands like OPEN will be parsed but have no effect.
- POKE does nothing, PEEK returns zero.