Category Archives: DOS

WordStar needs address wraparound?

Posted on September 24, 2011 by Michal Necasek

The CP/M compatible interface in DOS was initially documented, later forgotten, and then re-discovered every once in a while. In 1989, John Switzer described parts of the CALL 5 system call interface mechanism in a slightly hysterical article as a “back … Continue reading →

Posted in DOS, PC architecture, WordStar | 11 Comments

Who needs the address wraparound, anyway?

Posted on September 13, 2011 by Michal Necasek

The infamous A20 gate is well known and documented in hair-raising detail. What’s much less well documented is the real purpose of the A20 gate, that is, who actually needs the 8086 address wrap-around to be emulated in the first … Continue reading →

Posted in DOS, PC architecture | 4 Comments

IBM DOS 5.0 QBASIC hangs on non-IBM systems

Posted on September 11, 2011 by Michal Necasek

Much like its predecessors, IBM DOS 5.0 has no trouble running on clone systems, as long as they’re sufficiently compatible. That includes virtual machines. However, the QBASIC.EXE (and consequently EDIT.COM) program dated May 9, 1991 consistently hangs on any non-IBM … Continue reading →

Posted in BIOS, DOS, PC history | 20 Comments

Hang with early DOS boot sector

Posted on September 3, 2011 by Michal Necasek

While installing various versions of DOS for the DOS history series of articles, I was faced with a mysterious problem: Some versions of DOS would hang right away when booting from fixed disk, but not from floppy. I already knew … Continue reading →

Posted in DOS | 32 Comments

Happy 30th Birthday!

Posted on August 13, 2011 by Michal Necasek

In honor of the thirtieth anniversary of the IBM Personal Computer, the OS/2 museum started a new series of articles, this time dealing with the history of DOS—the predecessor of OS/2 and the first OS developed by Microsoft in cooperation … Continue reading →

Posted in DOS, PC history | Leave a comment

The Danger of Knowing Too Much

Posted on August 3, 2011 by Michal Necasek

A few days ago I had to look a little closer at Microsoft’s KEYB.COM because it was misbehaving in a virtualized environment. As a reminder for those readers who perhaps forgot, KEYB.COM was the DOS keyboard “driver” with support for … Continue reading →

Posted in DOS, PC architecture | Leave a comment

DOS boot sector, BPB, and the media descriptor byte

Posted on July 25, 2011 by Michal Necasek

All DOS programmers know that DOS storage media (floppies, fixed disks, even RAM drives) have a BPB (BIOS Parameter Block) which describes the basic layout of the storage medium. All DOS programmers also know that every DOS disk contains a … Continue reading →

Posted in DOS | 1 Comment

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