Arithmetic Games Set 1: a Peek into One of the First-Ever IBM PC Games

For whatever reason, I'm a sucker for "firsts", and the earliest games on the IBM PC platform hold special interest for me, with an extra touch of morbid curiosity reserved for stuff like BASIC games (and for IBM's own offerings). Thus, I was quite intrigued when I recently came across something that answers all these criteria: a disk dump of Arithmetic Games Set 1, developed by Science Research Associates, Inc. and published by IBM.  

There was zero coverage of this particular game online, even at the usual suspect places, but (to quote a certain bunch of mean, green, caterpillar-like alien overlords) you will note that this oversight has been rectified: I added a MobyGames entry for it. This post won't cover AGS1's qualities as a game (or as a pair of games), so if you fancy some screenshots and a general description, head on over there.  

What got my attention first was the time-frame. Have a look at this (and keep in mind that the IBM PC was officially introduced on August 12, 1981):

If all the meaningful file dates are from '81, what's with the 1983 BASICA.COM, you might ask? The answer is in the two batch files (one for single-floppy and one for dual-floppy systems), intended to make the original disk bootable by copying the system and BASIC interpreter over from a DOS disk; evidently, they've been used. As for why someone might do something back-asswards like that, that's because the original disk is copy-protected... but more on that later.  

The BASIC files are themselves "protected" too - you know, the type that cannot be listed or edited. But since there are known ways of getting around that, we can look inside, and see some more interesting timestamps:

These date strings place the game right around the time of the PC's launch date, and the .BIN files (just some graphics data for the title screen) are even older. What do we make of this?

SRA was an educational subsidiary of IBM, having been acquired by Big Blue "several years ago" as of 1982 (see Infoworld, 23 Aug. 1982). A web search turned up this resume by Richard Cavagnol, who worked at IBM/SRA until Dec. 1980, and "managed the contract programming teams that developed [...] a set of arithmetic games for the first IBM PC under development" (the same person later worked at WICAT Systems, *also* mentioned in the BASIC comments seen above - probably the origin of these porgramming teams).  

So, yep - we're looking at the result of experimental development done on the PC when it was still a prototype. Which makes it one of the first PC games ever, perhaps beaten to the punch only by Microsoft Adventure (and/or by Bill Gates' infamous Donkey, if that one counts)!
Of course, this meant that I had to poke around a little and see what makes it tick. This is where things get a little strange.    
 

Under the Hood

COLOR.BAS, the first program that runs on launch, has some code to detect a CGA card (or, to use its imposing regnal title, a Color/Graphics Monitor Adapter). If it's satisfied that one is present, it does this odd little thing:

This routine revectors interrupt 1D - the pointer to the Video Parameter Table (normally in ROM), which holds the raw video register values used by the BIOS to set video modes. Here it's relocated to 0000:0490 (inside the read/write BIOS data area!), and this location is populated with a near-facsimile of the real table from ROM. The only difference is the hsync position for graphics and 40-column text modes, which is 0x2C instead of 0x2D (this changes the horizontal centering of the image by a single character width).  

Beats me what they were trying to achieve with that - changing a single value hardly seems to justify duplicating the entire table. The target location is problematic, too: it might have been unused in the original 5150 PC, but later BIOSes use e.g. 0000:0496 for keyboard flags. As a result, running the unmodified game on a later PC (or in DOSBox) makes the keyboard freak out: hit Ctrl or Alt at any point in the game, and you'll freeze the machine.  

Next up is LOGO.BAS, responsible for (you guessed it) the logo and title screen. But before it does any of that, it executes this machine language routine:

Here's that copy protection we were talking about. Disassembling it shows that it tries to verify a disk sector, expects it to fail, and drops out of BASIC if it doesn't. Easily bypassed, but once again, the routine is POKEd right into the BIOS data area (this time at 0x4D0, a reserved area AFAIK). I suppose that doing this offered some advantage on early PCs, which came with a maximum of 64KB of RAM on board; between the overhead of DOS, the Advanced BASIC interpreter, and the program itself, this area might have been the only place they could trust to remain free for machine language routines!

For a final quirk, you have the error handlers, and the messages they print:

I especially like the message in lines 65050-65070... but the real mystery is line 65175: it seems to pay extra-special attention to error code 241, urging you to call SRA and report it immediately(!). BASICA, however, does not define error codes greater than 70-ish (not according to available documentation, that is). It's possible to trigger user-defined errors, with a statement such as ERROR 241, but the game doesn't do this at any point in the code.  

So 241 isn't a predefined BASIC error, nor a user-defined error - what is it, then? Merely a forgotten leftover from development/testing? Or does this seemingly-innocuous number hide something altogether more sinister and undocumented? And most importantly, will Arithmetic Games Set 2 ever turn up?

Perhaps a future installment of this blog shall hold the answers one day. Until that happens...  

...Beware the fumblings of Stanley Stickyfingers!