I set out to write this interpreter with the single goal of being able to play the Swedish text adventure game Stuga ("Cottage") written in the late 70's in some version of DEC BASIC.
The source code of Stuga is published with permission from the original authors on this site:
http://microheaven.com/svenska/stuga.shtml
One of the big problems was to find exactly what dialect was used, as it has changed through the year and it's quite hard to find a reference for older versions. The references available on the Internet are typically scanned PDF's of the original paper references, so text search rarely works.
One hint in finding the dialect used by the game is the usage of
IF as a modifier:
10 GOTO 20 IF X > YThis syntax is not that common in BASIC dialects, but it's described in this VAX BASIC reference from 1990:
VAX Basic User Manual (1990)
I implemented substantial parts of the interpreter based on that,
but when I found how functions are defined in the game, I had to look
further. The reference above supports defining functions with the
DEF statement. The DEF statement should be followed by a code
block eventually ending with a RETURN statement followed by a
FNEND statement to define the end of the function. But the function
declarations in the game instead looks like this:
90800 DEF FNL$(X1$,X)
90805 IF X<=0 THEN FNL$="" \ GOTO 90815
90810 IF X>LEN(X1$) THEN FNL$=X1$ ELSE FNL$=LEFT$(X1$,X)
90815 FNENDSo instead of a RETURN statement, FNL$ is assigned a value
and the function returns when FNEND is reached, which is forbidden
in the 1990's reference.
I found this reference of "BASIC 4.0 for the HP 9000 series 200/300
computers" which includes the DEF FN statement mostly as used in
the game (except that the reference says it's an error if FNEND
is reached):
DEC was bought by Compaq in 1997/98, which in turn was merged with HP in 2001/02, so there is a path from DEC to HP. But as this happened long after the reference was written (1985) it's most likely not the exact version used when the game was written.
To conclude: I've mostly given up on doing a strict implementation of a fixed dialect of BASIC. I will make sure all statements in the game works as expected, but I may add statements that does not follow any of the references listed, and I will also not implement all functionality of any reference.
There are still some statements and functions I've not found any documentation of.
A function likely related to the display in one way or another. Used once in the game:
90064 X=CRT(1)Used when saving game state to disk:
80105 MARGIN #1,132 \ QUOTE #1 \ X=0'&&&&&- VAX Basic User Manual (1990)
- VAX Basic Reference Manual (1990)
- Let's Build a Compiler, by Jack Crenshaw (1989)
- A bunch of HP 9000 Basic references
- BASIC 4.0 for the HP 9000 series 200/300 computers (Archived)
- Small BASIC Interpreters
There's code. It does stuff. But it does not interpret Basic (yet).
Cause the name would not make sense.