Category Archives: x86

Learn Something Old Every Day, Part XVIII: How Does FPU Detection Work?

Posted on March 28, 2026 by Michal Necasek

This post ended up being much longer than originally intended because halfway into writing it, I found that 286 and later CPUs don’t behave the way I had assumed they would… While investigating a bug related to a program using … Continue reading →

Posted in 286, 8086/8088, LSOED, x86, x87 | 39 Comments

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

Posted on October 29, 2022 by Michal Necasek

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

KEYBCS2

Posted on February 18, 2022 by Michal Necasek

After writing about the likely origins of IBM code page 852, I thought I should revisit the homegrown Czech alternative solution, the Kamenický brothers encoding and their keyboard driver. Its existence is well documented, and the so-called (somewhat misnamed) KEYBCS2 … Continue reading →

Posted in DOS, I18N, IBM, x86 | 34 Comments

How Old Is OMF?

Posted on July 9, 2020 by Michal Necasek

The Object Module Format (OMF), used by most DOS development tools, and eventually displaced by COFF/ELF in the 32-bit world, is quite old. It is a somewhat strange format because of its age, and it is quite complex, both because … Continue reading →

Posted in Development, Intel, Microsoft, PC history, x86 | 8 Comments

Better Late Than Never

Posted on May 30, 2016 by Michal Necasek

Better late than never, although in this instance, it’s really really late—about thirty years late. In the world of computing, that is eternity. The talk is about the new CR4.UMIP control bit documented in the latest (revision 58) Intel SDM, … Continue reading →

Posted in 286, Intel, x86 | 13 Comments

Curious Instructions

Posted on December 13, 2014 by Michal Necasek

Years ago, Geoff Chappell (the author of DOS Internals, among other things) published an article about mysterious instructions that Microsoft’s LINK knows but Intel’s documentation is silent about. The fourteen listed instructions were: LOADALL, CFLSH, WRECR, RDECR, SVDC, RSDC, SVLDT, … Continue reading →

Posted in Documentation, Intel, x86 | 11 Comments

Undocumented 8086 Opcodes

Posted on December 21, 2013 by Michal Necasek

A minor mystery recently surfaced while analyzing DOS boot sectors. DOS uses several criteria when deciding whether a boot sector contains a valid BPB, and one of the criteria is (oddly enough) checking whether the first two bytes of the … Continue reading →

Posted in 8086/8088, Documentation, Intel, x86 | 62 Comments

LOADALL Strikes Again

Posted on October 18, 2013 by Michal Necasek

A minor mystery recently popped up while running IBM’s OS/2 1.1 (1988), the first OS/2 version with the Presentation Manager GUI. While Microsoft’s and IBM’s releases of OS/2 were fully compatible from application perspective, there were differences in the drivers … Continue reading →

Posted in IBM, Intel, Microsoft, OS/2, x86 | 13 Comments

BSD Buglets

Posted on June 15, 2013 by Michal Necasek

Last week I ran into two wholly unrelated problems while researching the history of BSD-derived Unix systems on PCs. Both are classics in their category and merit a closer look. Y2K Strikes Again The first issue is a very typical … Continue reading →

Posted in BSD, UNIX, x86 | 11 Comments

If you ENTER, you might not LEAVE

Posted on June 7, 2013 by Michal Necasek

I’ve recently spent some time debugging curious hangs/aborts in two more or less exotic operating systems, Plan 9 and QNX 4.25. Both turned to be caused by the same innocuous-looking BIOS change, even though the circumstances were somewhat different and … Continue reading →

Posted in Intel, x86 | 9 Comments