Talk:TREK7

Divide by zero bug

The article mentions an "obstinate ... 'divide by zero' problem that has been plaguing the code for decades hindering any successful game". Curiousity is piqued - what was the bug, what were the circumstances that provoked it, and what were the results when it was? (I'm assuming a fatal total failure, for the latter?) It sounds like the code most have been subtracting one quantity from another, and using the difference to divide something - without checking to see if the difference was 0? Jnc (talk) 22:06, 25 March 2023 (CET)

That's what happened:

%SYSTEM-F-FLTDIV_F, arithmetic fault, floating divide by zero at PC=0001060C, PS L=03C00024
%TRACE-F-TRACEBACK, symbolic stack dump follows
 
module name     routine name     line   relative PC  absolute PC

                HORTA              21      0000012C     0001060C
                ATACK              59      00000280     0001A600
                FOR$MAIN          321      00000D3D     0001013D

and the FORTRAN line causing it was:

SINA=FLOAT(ITAR-ISTR)/X

As a first step I put in the line:

IF(X.EQ.0)X=0.1

just before the division takes place to stop division by zero, but that was just a stupid workaround and not the real solution.

About a month later I found the causing bug in subroutine ATACK: IF(MIN.EQ.21)GO TO 106 instead of IF(MIN.EQ.21)GO TO 108.

I found it by chance and luck - not by systematic approach ...

So a simple misread jump target caused so much trouble.

There is a lot of "spaghetti code" in the TREK7 sources, which makes understanding and debugging an arduous job! Vaxorcist (talk) 09:20, 26 March 2023 (CEST)

Comments

The mention of scarce comments reminded me about a favourite MIT story about an obscure comment - perhaps the only comment in a program! So, instruction (I don't think it was a line number - this predates BASIC) 1827 had the comment "LVB RIP". What? Eventually (this was long before the Internet, too :-) the penny dropped - Ludwig van Beethoven died in 1827! Or maybe it was 'JSB RIP' on instruction 1750 - that actually sounds more likely, now that I think about it - old-time hackers were more fans of Bach than Beethoven! I tried to find this story online, but couldn't. I think I read it in an old copy of the Hacker's Dictionary. Jnc (talk) 22:06, 25 March 2023 (CET)

I wish someone would counter-check my comments on the TREK7 sources and add (at least some of) the ones still missing. Vaxorcist (talk) 09:20, 26 March 2023 (CEST)

A search for "JSB RIP" (augmented with "PDP") turned up two hits. It's from "History of TOPS - Life in the fast AC's": https://www.inwap.com/pdp10/tops-history.txt Larsbrinkhoff (talk) 08:55, 5 April 2023 (CEST)