Author Archives: Michal Necasek

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

The PC floppy subsystem

Posted on April 5, 2011 by Michal Necasek

The PC floppy subsystem, ubiquitous and indispensable until the early 21st century, suffered the typical fate of many “legacy” subsystems: The initial design was adequate, but did not adapt to newer and more complex hardware. With the original IBM PC, … Continue reading →

Posted in PC architecture | Leave a comment

HIMEM.SYS, unreal mode, and LOADALL

Posted on March 18, 2011 by Michal Necasek

The previous post talked about real mode on 286+ processors which behaves more like a slightly modified variant of protected mode rather than the old 8088/8086 processors. Real mode with non-compatible selector bases or limits is usually called unreal mode … Continue reading →

Posted in DOS, x86 | 8 Comments

Will the real Real Mode please stand up?

Posted on March 11, 2011 by Michal Necasek

Every programmer familiar with the x86 architecture understands the difference between real and protected address mode of the processor. It is well known that the real mode is compatible with Intel’s old 16-bit 8088/8086 CPUs, while protected mode was a … Continue reading →

Posted in x86 | 7 Comments

Display Drivers, NT and NeXTSTEP

Posted on January 21, 2011 by Michal Necasek

It is instructive to compare the OS/2 and 16-bit Windows display driver model with other operating systems. Why NT and NeXTSTEP? NT because it was Microsoft’s third take on a (mostly) PC operating system, and NeXTSTEP because it was an … Continue reading →

Posted in NeXTSTEP, NT | 2 Comments

Display Drivers, OS/2 and 16-bit Windows

Posted on January 19, 2011 by Michal Necasek

Not surprisingly, the display driver model of Windows 1.x/2.x/3.x and OS/2 1.x/2.x was quite similar. This was in sharp contrast to the drivers for just about every other device; while disks or network adapters already had existing drivers in DOS, … Continue reading →

Posted in OS/2, Windows | Leave a comment