Category Archives: Undocumented

Mystery CPUID Bit

Posted on April 17, 2026 by Michal Necasek

Yesterday I had the opportunity to test a recently acquired Athlon 1200 CPU (Thunderbird core, ceramic PGA package). I dreaded the first boot-up attempt because I have had rather bad experience with slightly newer Palomino and Thoroughbred OPGA processors—a surprisingly … Continue reading →

Posted in AMD, K7, PC hardware, Undocumented | 6 Comments

Learn Something Old Every Day, Part XIII: InDOS Is Not Enough

Posted on February 12, 2025 by Michal Necasek

The other day I spent a while trying to understand the purpose of a rather strange looking piece of code inside Borland’s THELP.COM utility shipped with Turbo Pascal 6.0 (THELP.COM was misbehaving under emulated DOS). The THELP utility performs the … Continue reading →

Posted in DOS, PC history, Undocumented | 14 Comments

Learn Something Old Every Day, Part XI: DOS Directory Searches are Bizarre

Posted on May 7, 2024 by Michal Necasek

A while ago I started playing with EMU2, a piece of software which calls itself “A simple text-mode x86 + DOS emulator”. It is indeed relatively simple, only emulating an 8086 (or maybe 80186, with little bits of 80286 here … Continue reading →

Posted in Development, DOS, Undocumented | 53 Comments

The Secret History of ATAPI

Posted on November 24, 2021 by Michal Necasek

The other day I asked myself a seemingly trivial question: What was the first ATAPI CD-ROM drive and when was it available? Given that ATAPI was a major technology which instantly obsoleted all proprietary CD-ROM interfaces and made SCSI much … Continue reading →

Posted in CD-ROM, PC history, Standards, Undocumented | 61 Comments

Disabling Quick Edit Mode

Posted on December 18, 2020 by Michal Necasek

Last week I decided to finally solve a minor annoyance that’s been pestering me since I switched my main development machine from Windows 7 to Windows 10 over a year ago. As it is with these things, a certain threshold … Continue reading →

Posted in NT, Undocumented, Watcom | 22 Comments

The Answer To 0x49: Fujitsu FMR

Posted on December 11, 2020 by Michal Necasek

This is a guest post by A. N. Other. The following was originally intended as a comment to “Not MSX Either“, the 4th installment in the hunt for the mysterious 0x69 FAT VBR-start byte which was allowed in DOS. Due … Continue reading →

Posted in DOS, NT, PC hardware, PC history, Undocumented | 7 Comments

Seagate Cheetah Date of Manufacture

Posted on May 12, 2020 by Michal Necasek

Lately I found myself in the possession of several Seagate Cheetah 15K.7 SAS drives. These represent the pinnacle of hard disk engineering; with 15,000 RPM, the drives deliver up to around 200 MB/s sustained throughput (both read and write!) and … Continue reading →

Posted in Seagate, Storage, Undocumented | Leave a comment

More About That Strange Pentium 4

Posted on October 26, 2019 by Michal Necasek

A few years ago I wrote about a strange NetBurst processor with SL7HY S-spec that landed at the OS/2 Museum. After renewed reader interest I pulled it out of the closet and tested the processor again. A collection of miscellaneous … Continue reading →

Posted in Intel, PC hardware, PC history, Pentium 4, Undocumented | 34 Comments

A Brief History of Unreal Mode

Posted on June 15, 2018 by Michal Necasek

After a run-in with a particularly crazy manifestation of unreal mode (Flat Assembler, or fasm), I decided to dig deeper into the history of this undocumented yet very widely used feature of 32-bit x86 processors. For the purposes of this … Continue reading →

Posted in 386, Corrections, Microsoft, PC history, Undocumented | 47 Comments

ICEBP Finally Documented

Posted on May 25, 2018 by Michal Necasek

After more than 30 years, Intel finally documented the INT1 instruction, also known as ICEBP (opcode F1h), in the latest (May 2018, -067) edition of the SDM. This was probably forced by security concerns, because from a security standpoint, having … Continue reading →

Posted in 386, Documentation, Intel, Undocumented | 14 Comments