So, with a renewed interest in OS/2 betas, I’d been getting stuff into the direction of doing some full screen video. I’d copied and pasted stuff before and gotten QuakeWorld running, and I was looking forward to this challenge. The whole thing hinges on the VIO calls in OS/2 like VioScrLock, VioGetPhysBuf, VioScrUnLock etc etc. I found a nifty sample program Q59837 which shows how to map into the MDA card’s text RAM and clear it.

It’s a 16bit program, but first I got it to run on EMX with just a few minor changes, like removing far pointers. Great. But I wanted to build it with my cl386 experiments and that went off the edge. First there are some very slick macros, and Microsoft C just can’t deal with them. Fine I’ll use GCC. Then I had to get emximpl working so I could build an import library for VIO calls. I exported the assembly from GCC, and mangled it enough to where I could link it with the old Microsoft linker, and things were looking good! I could clear the video buffer on OS/2 2.00 GA.

Now why was it working? What is a THUNK? Well it turns out in the early OS/2 2.0 development, they were going to cut loose all the funky text mode video, keyboard & mouse support and go all in on the graphical Presentation Manager.

Instead, they were going to leave that old stuff in the past, and 16bit only for keeping some backwards compatibility. And the only way a 32bit program can use those old 16bit API’s for video/keyboard/mouse (etc) is to call from 32bit mode into 16bit mode, then copy that data out of 16bit mode into 32bit mode. This round trip is called thunking, and well this sets up where it all goes wrong.

Then I tried one of the earlier PM looking betas 6.605, and quickly it crashed!

Well this was weird. Obviously, I wanted to display help

This ended up being a long winded way of saying that there is missing calls from DOSCALL1.DLL. Looking through all the EMX thunking code, I came to the low level assembly, that actually implemented the thunking.

EXTRN   DosFlatToSel:PROC
EXTRN   DosSelToFlat:PROC

After looking at the doscalls import library, sure enough they just don’t exist. I did the most unspeakable thing and looked at the online help for guidance:

So it turns out that in the early beta phase, there was no support for any of the 16bit IO from 32bit mode. There was no thunking at all. You were actually expected to use Presentation Manager.

YUCK

For anyone crazy enough to care, I uploaded this onto github Q59837-mono

It did work on the GA however so I guess I’m still on track there.