Author Archives: Michal Necasek

Tarbell to Cromemco

Posted on January 23, 2024 by Michal Necasek

While playing around with old versions of 86-DOS, I came across a disk image of 86-DOS 1.14. I ran the older 86-DOS versions in the SIMH simulator which can emulate the Cromemco disk controller supported by 86-DOS. Unfortunately the 86-DOS … Continue reading →

Posted in Development, DOS, PC history | 16 Comments

Misconceptions on Top of Misconceptions

Posted on January 18, 2024 by Michal Necasek

While researching the precise meaning of the Ctrl-Z (26 decimal, hex 1Ah, ASCII SUB) character in DOS, I was somewhat taken aback by this article which purports to correct a common misconception. The article is, for the most part, entirely … Continue reading →

Posted in DOS | 21 Comments

86-DOS Revisited

Posted on January 15, 2024 by Michal Necasek

At the end of December 2023, several disk images of very old versions of Seattle Computer Products 86-DOS unexpectedly turned up. This includes previously unseen releases of 86-DOS version 0.11 and 0.34 (going by the version number in the 86-DOS … Continue reading →

Posted in DOS, PC history | 24 Comments

From the Annals of Preprocessor Hackery

Posted on October 22, 2023 by Michal Necasek

Over the last few days I’ve been slowly attacking the source code for 386MAX, trying to build the entire product. One of the many problems I ran into turned out to be quite interesting. There are several (16-bit) Windows components … Continue reading →

Posted in 386MAX, C, Development, Microsoft | 25 Comments

VDS: Borne out of Necessity

Posted on October 5, 2023 by Michal Necasek

In July 1990, Microsoft released a specification for Virtual DMA Services, or VDS. This happened soon after the release of Windows 3.0, one of the first (though not the first) providers of VDS. The VDS specification was designed for writers … Continue reading →

Posted in 386, Documentation, Microsoft, PC history, Windows | 10 Comments

386MAX and EISA DMA

Posted on September 28, 2023 by Michal Necasek

A few weeks back I was reminded that the source code to the 386MAX (later Qualitas MAX) memory manager was released in 2022 on github. Back in the 1990s I used primarily EMM386 and QEMM, but I have some experience … Continue reading →

Posted in 386MAX, DOS, EISA, Source code | 11 Comments

Learn Something Old Every Day, Part IX: AHA-154xB and ASPI4DOS.SYS

Posted on August 31, 2023 by Michal Necasek

The other day I had a pressing “need” to examine the behavior of Adaptec 154x and compatible SCSI HBAs and their DOS drivers. I found the hard way that the AHA-154xB does not work with Adaptec’s last DOS drivers from … Continue reading →

Posted in Adaptec, Bugs, BusLogic, SCSI | 6 Comments

Learn Something Old Every Day, Part VIII: RTFM

Posted on August 22, 2023 by Michal Necasek

In my quest to understand the intricacies of x87 behavior and especially floating-point exceptions, I pulled out my trusty old Alaris Cougar board. The system board had a 100 MHz Intel OverDrive 486 DX4 plugged in and worked quite well. … Continue reading →

Posted in 486, Documentation, Intel, PC hardware, x87 | 9 Comments

This Code Smells of Desperation

Posted on August 5, 2023 by Michal Necasek

A few weeks ago I had the questionable pleasure of diving into the math exception handler of WIN87EM.DLL, the Windows 3.1 math emulator and FPU support library. Actually WIN87EM.DLL appears to have been first shipped with Windows 3.0, and the … Continue reading →

Posted in Bugs, Microsoft, PC architecture, x87 | 19 Comments

Retro-Porting to NT 3.1

Posted on July 26, 2023 by Michal Necasek

In another useless project, I decided to find out why even trivial programs created with the Open Watcom compiler refuse to run on Windows NT 3.1. Attempting to start an executable failed with foo.exe is not a valid Windows NT … Continue reading →

Posted in Development, Microsoft, NT, Watcom | 12 Comments