Author Archives: Michal Necasek

First ROM Shadowing

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

A wunderBAR Story

Or, what should be broken became solid, and what should be solid became broken. While searching for something completely different, I came across a fascinating story of the “wunderBAR”. Since the story is very short, I’ll quote it here in … Continue reading

Posted in Computing History, Corrections | 21 Comments

Hash Tables FTW

Over the last few weeks I did a bit of tinkering on a hobby software project. The code was written by two other people during the last several years and I finally managed to find some time to fix a … Continue reading

Posted in Development | 2 Comments

Windows 3.x VDDVGA

While working on my Windows 3.x display driver, I ran into a vexing problem. In Windows 3.1 running in Enhanced 386 mode, I could start a DOS session and switch it to a window. But an attempt to set a … Continue reading

Posted in 386, Development, Documentation, Graphics, PC history, Windows | 59 Comments

Learn Something Old Every Day, Part VII: 8087 Intricacies

The other day I investigated a report that a C runtime library modification causes programs to hang on a classic IBM 5150 PC with no math coprocessor. The runtime originally contained two separate routines, one to detect the presence of … Continue reading

Posted in 8086/8088, Development, IBM, Intel, PC history, x87 | 10 Comments

A House of Cards

As one step in the development of the Windows 3.x/2.x display driver, I needed to replace a BIOS INT 10h call to set the video mode with a “native” mode set code going directly to the (virtual) hardware registers. One … Continue reading

Posted in Bugs, Development, Microsoft, Windows | 49 Comments

Win16 Retro Development

Several months ago I had a go at producing a high resolution 256-color driver for Windows 3.1. The effort was successful but is not yet complete. Along the way I re-learned many things I had forgotten, and learned several new … Continue reading

Posted in Debugging, Development, Microsoft, Windows | 25 Comments

Undefined Isn’t Unpredictable

The other day I discovered that 32-bit FreeBSD 11.2 has strange trouble running in an emulated environment. Utilities like ping or top would just hang when trying to print floating-point numbers through printf(). The dtoa() library routine was getting stuck … Continue reading

Posted in AMD, Development, Documentation, Intel | 22 Comments

Does (E)IP Wrap Around in 16-bit Segments?

The 8086/8088 is a 16-bit processor and offsets within a 64K segment always wrap around. If a one-byte instruction at offset FFFFh is executed on an 8086, execution will continue at offset 0. This is simply a consequence of the … Continue reading

Posted in 386, 8086/8088, Intel, x86 | 9 Comments

PC Disk Sector Sizes and Booting

Everyone knows that the IBM PC established 512-byte sectors on floppies and hard disks as the standard, which survived for several decades until the advent of “native” 4K-sector drives. Of course what “everyone knows” is not necessarily the whole story. … Continue reading

Posted in BIOS, DOS, IBM, PC history, Storage | 15 Comments