Author Archives: Michal Necasek

DOS Memory, Managers & Extenders, Part I

Posted on June 17, 2011 by Michal Necasek

To understand why the maddeningly complex world of DOS memory managers and extenders came to be, it’s necessary to understand the evolution of the PC platform. Even though memory managers and DOS extenders reached their peak on 32-bit 386 and … Continue reading →

Posted in DOS, PC architecture, PC history, x86 | 27 Comments

The IHC Damage

Posted on June 14, 2011 by Michal Necasek

Trying to sort through a heap of floppy images recently, I found many of them to contain a suspicious ‘IHC’ signature in their boot sector in the location where an OEM identifier (such as ‘IBM  3.3’ or ‘MSDOS5.0’ would normally … Continue reading →

Posted in DOS, Windows, Windows 95 | 5 Comments

Interrupt 68h and EMM386

Posted on June 8, 2011 by Michal Necasek

While working with a modified BIOS image in a virtual machine, I ran into mysterious hangs when trying to load an old version of EMM386.SYS from MS-DOS 4.01. A newer version of EMM386 did not exhibit the problem. A quick … Continue reading →

Posted in BIOS, DOS, Virtualization | Leave a comment

Researching the history of PC computing

Posted on June 2, 2011 by Michal Necasek

Finding out what happened in the world of PC computing 20 or 25 years ago isn’t always easy. The 1980s were over before the Internet took off, which means that most of the information was never publicly available in electronic … Continue reading →

Posted in PC history | 2 Comments

OS/2 2.0 Comdex Fall ’91

Posted on May 30, 2011 by Michal Necasek

In October 1991, IBM brought an OS/2 2.0 pre-release (build 6.167) to the Fall Comdex in Las Vegas. This was a semi-official release of 32-bit OS/2, effectively a beta version, although it wasn’t called a beta. At the same Comdex, … Continue reading →

Posted in NT, OS/2 | 16 Comments

The floppy controller evolution

Posted on May 26, 2011 by Michal Necasek

The floppy subsystem in PCs hadn’t mutated over time quite as much as, say, the hard disk subsystem, but prior to its extinction in the early 21st century, the floppy disk controller (FDC) did evolve noticeably. In the original IBM … Continue reading →

Posted in PC architecture | 14 Comments

Xenix 286 in a VM

Posted on May 17, 2011 by Michal Necasek

The fixes which were included in VirtualBox 4.0.8 happen to help not only OS/2 1.x but also Xenix. The 386 versions of Xenix 2.3.x (not necessarily older versions!) should install in a VM without trouble, but the 286 versions are … Continue reading →

Posted in VirtualBox, Xenix | 10 Comments

Installing OS/2 1.x in a VirtualBox VM

Posted on May 17, 2011 by Michal Necasek

Installing 16-bit OS/2 in a virtual machine ranges between “tricky” and “impossible”, depending on the version of OS/2 and virtualization software used. In VirtualBox 4.0.8, things have moved further away from “impossible” and closer towards merely “tricky”. Version 4.0.8 fixed problems with floppy … Continue reading →

Posted in OS/2, VirtualBox | 29 Comments

The Fixed Disk Parameter Table

Posted on May 11, 2011 by Michal Necasek

The Fixed Disk Parameter Table, or FDPT, is a structure primarily used by the BIOS in IBM compatible computers, but is also of critical importance to some (especially older) operating systems which do not use the BIOS. The FDPT was … Continue reading →

Posted in BIOS, PC architecture, Virtualization | 3 Comments

Geometry Problems

Posted on May 11, 2011 by Michal Necasek

When introducing hard disk support in the PC/XT back in early 1983, IBM made a very unfortunate design decision: the information about drive geometry was exposed in the BIOS, and even worse, in the boot sector stored on the disk. SCSI … Continue reading →

Posted in BIOS, PC architecture, Virtualization | 2 Comments