Category Archives: PC architecture

Who Knew What When

Posted on June 28, 2024 by Michal Necasek

When Microsoft released the unique early beta build of Multitasking DOS 4, I quickly found out that it does not run in VirtualBox: This was a bit of a surprise, because the more-or-less released versions of Multitasking DOS 4 from … Continue reading →

Posted in 286, DOS, IBM, Microsoft, PC architecture, PC history, VirtualBox | 30 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 | 22 Comments

Failing to Fail

Posted on June 16, 2023 by Michal Necasek

The other day I was going over various versions of the venerable DOS/16M DOS extender from Rational Systems (later Tenberry Software). The DOS/16M development kit comes with a utility called PMINFO.EXE which is meant to give the user some idea … Continue reading →

Posted in Bugs, Intel, PC architecture, x87 | 13 Comments

First ROM Shadowing

Posted on April 21, 2023 by Michal Necasek

The other day I was asked an interesting question: What was the first BIOS with support for ROM shadowing? In the 1990s, ROM shadowing was common, at first as a pure performance enhancement and later as a functional requirement; newer … Continue reading →

Posted in C&T, Compaq, IBM, PC architecture, PC history | 55 Comments

Xenix 2.2 vs. VGA

Posted on April 15, 2022 by Michal Necasek

The other day I started wondering why certain old versions of 286 and 386 XENIX look a bit weird in emulation: The characters are cut off, because XENIX sets up an EGA text mode with 8×14 character matrix but uses … Continue reading →

Posted in 286, 386, PC architecture, PC history, VGA, Xenix | 14 Comments

Learn Something Old Every Day, Part IV: Ctrl+Scroll Lock is Ctrl+Break

Posted on February 26, 2022 by Michal Necasek

The other day I tried running NSNIPES, a multiplayer networked game that came with old versions of NetWare. The game worked fine, but I couldn’t get out of it. Esc did nothing, any “usual” combinations like Alt+X, Alt+Q or similar … Continue reading →

Posted in NetWare, PC architecture, PC history | 3 Comments

8237A DMA Page Fun

Posted on March 28, 2021 by Michal Necasek

The other day I was trying to fill a couple of gaps in my understanding of the Intel 8237A DMA controller documentation. I wrote a small testcase that performed a dummy transfer and modified the base address and count registers … Continue reading →

Posted in Intel, PC architecture, PC history | 5 Comments

386 Cache Coherency

Posted on April 8, 2020 by Michal Necasek

I’ve been slowly chewing my way through U.S. Patent 5,724,549, titled Cache Coherency without Bus Arbitration Signals, initially filed by Cyrix Corporation in 1992 and published in 1998 (when it was utterly irrelevant, but such is the life of patents). … Continue reading →

Posted in 386, Cyrix, PC architecture, PC history | 18 Comments

A Brief Visit to Disk Geometry Hell

Posted on February 20, 2020 by Michal Necasek

Several weeks ago I thought I’d install NetWare 3.12 in a virtual machine using the BusLogic SCSI controller emulation. While configuring a 1.5 GB virtual drive, I thought I should be safe and not run into any trouble with a … Continue reading →

Posted in BusLogic, IBM, NetWare, PC architecture, Storage | 32 Comments

More Fun with ISA DMA

Posted on January 13, 2020 by Michal Necasek

A reader comment on a previous post on ISA DMA pointed out that UMBPCI (or rather the DMACHK utility distributed with it) does something unusual with regard to ISA DMA. There was a suspicion of somehow accomplishing the mythical memory-to-memory … Continue reading →

Posted in PC architecture, PC hardware, Software Hacks | 8 Comments