Category Archives: BIOS

PC Disk Sector Sizes and Booting

Posted on October 26, 2022 by Michal Necasek

Everyone knows that the IBM PC established 512-byte sectors on floppies and hard disks as the standard, which survived for several decades until the advent of “native” 4K-sector drives. Of course what “everyone knows” is not necessarily the whole story. … Continue reading →

Posted in BIOS, DOS, IBM, PC history, Storage | 15 Comments

Reconstructing the EGA BIOS

Posted on March 19, 2022 by Michal Necasek

A few weeks ago I had a sudden need to understand certain finer points of the operation of EGA/VGA BIOS. I found common reference materials to be inadequate—they tend to do a good job of documenting the data structures the … Continue reading →

Posted in BIOS, Development, Documentation, Graphics, IBM, PC history | 32 Comments

The IBM PC, 41 Years Ago

Posted on August 10, 2021 by Michal Necasek

No, the OS/2 Museum does not have either a time machine or difficulty doing basic math. As of this writing, it is August 2021 and the IBM PC was announced in August 1981, 40 years ago. But in August 1980, … Continue reading →

Posted in 8086/8088, BIOS, DOS, IBM, Intel, PC hardware, PC history | 60 Comments

Reading From Void

Posted on February 21, 2019 by Michal Necasek

Recently I came across the following question: What happens when software reads the registers of a non-existent IDE controller? That is, what happens when software reads for example ports in the 1F0h-1F7h/3F6h range (primary IDE channel) when there is no … Continue reading →

Posted in BIOS, IDE, PC hardware | 10 Comments

FasterModeSwitch: Is It Really?

Posted on October 31, 2018 by Michal Necasek

Short answer: Yes. Before launching into the long answer, let’s recap what it even is. FasterModeSwitch is a SYSTEM.INI setting in Windows 3.1 which applies only to Standard (286) mode and can therefore be found in the [standard] section of … Continue reading →

Posted in 286, BIOS, Microsoft, Windows | 10 Comments

Resetting Disks Is Hard

Posted on February 26, 2017 by Michal Necasek

After digging through BIOS listings and disassembling the Adaptec 1542C BIOS as well as the PC DOS 2000 boot sector, it’s clear why the floppy-less SCSI boot does not work on my test system. It’s because disk reset done by the boot … Continue reading →

Posted in BIOS, Bugs, SCSI | 22 Comments

Booting Is Hard

Posted on February 23, 2017 by Michal Necasek

So I had this brilliant idea of using SCSI drives with old 286/386/486 boards which have old BIOSes that can’t handle IDE drives bigger than 500-ish megabytes. The SCSI HBA is the first one I happened to grab, an Adaptec 1542C … Continue reading →

Posted in 386, BIOS, CompactFlash, DOS, SCSI | 21 Comments

Windows NT 3.1 (and OS/2) Memory Detection

Posted on January 15, 2016 by Michal Necasek

It is common knowledge that Windows NT 3.1 only recognizes up to 64 MB RAM, unlike NT 3.5 and later versions. This statement can be found in various places, including this blog. The limitation was empirically determined by installing NT … Continue reading →

Posted in BIOS, NT | 39 Comments

The IBM PC BIOS and Intel ISIS-II

Posted on May 30, 2014 by Michal Necasek

An interesting question recently popped up: How exactly did IBM build the ROM BIOS for the IBM PC? Knowing what tools were used should make it possible to use the ROM listing published in the IBM PC Technical Reference and … Continue reading →

Posted in BIOS, IBM, Intel, PC history | 44 Comments

How to please WDCTRL

Posted on February 2, 2012 by Michal Necasek

As any user of 16-bit Windows knows, Microsoft Windows 3.1x in 386 Enhanced mode supported a coveted feature called 32-bit disk access (sometimes also called FastDisk). The “32-bit” designation was slightly misleading as there was no 32-bit data path to … Continue reading →

Posted in BIOS, Windows | 3 Comments