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 system. Why is that?

Analysis shows that the key is the presence (or absence) of ROM BASIC, which was built into the ROM of all IBM PC systems (including PS/2) from the very first PCs until approximately 1991, sometime after IBM DOS 5.0 was released. But why does QBASIC care when it’s supposed to replace the ROM BASIC anyway? 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 that DOS 4.x is very sensitive to BIOS stack usage; if a BIOS needs more than 100 bytes or so of stack to process a disk read request, it will fail to boot DOS 4.x from fixed disk, even though the same DOS 4.x can access the same disk just fine when booted from floppy.

However, the hangs I was observing were happening with DOS 2.x and 3.x, and those do not have such tight stack usage requirements. I quickly realized that the problem is caused by a bug in the DOS boot sector: the boot sector code tries to optimize the loading of IBMBIO.COM and attempts to read a whole disk track at a time. That sounds like a good idea, but it’s not. 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 with IBM. The first two installments deal with the prehistory of DOS and with the DOS 1.x releases. As usual, VirtualBox was quite helpful for supplying screenshots of vintage software.

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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 international keyboard layouts. The MS-DOS version of KEYB.COM was incorrectly detecting the keyboard type (thought an 84-key variant was attached, when it was a 101/102-key of course), but IBM’s KEYB.COM from PC DOS 7 had no problem, and the FreeDOS version likewise worked fine. What I found in KEYB.COM was a little surprising.

The authors of KEYB.COM clearly had a lot of  inside knowledge about PC hardware. It’s quite likely that IBM had a hand in writing the utility, but Microsoft probably maintained it and made sure it worked (more or less) on non-IBM systems. Continue reading →

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IBM PC/AT 8042 Keyboard Controller Commands

Posted on August 3, 2011 by Michal Necasek

Anyone trying to use the classic 8042-style keyboard controller (KBC) found in the IBM PC/AT and nearly all later PCs typically runs into a problem with lack of accurate documentation. The 8042 (or 8742, or any number of compatible parts built into later Super I/O chips) is actually quite well documented. The catch is that the 8042 is a programmable micro-controller with its own control software in (usually) ROM. Until recently, no one outside a few companies (IBM, AMI, Phoenix) knew exactly what the control software did.

IBM documented a number of commands the host can send to the KBC. It should be understood that all those commands are a pure software construct, with nothing about the 8042 hardware dictating that the commands need to follow any specific format, function, or that they even need to be there at all. Therefore understanding the 8042 ROM code is the only way towards understanding exactly what the commands are and what they do, with the caveat that different controllers may and do have somewhat different code in their ROM. Continue reading →

Posted in PC architecture | 22 Comments

Detecting floppy drive presence and type

Posted on July 28, 2011 by Michal Necasek

Detecting floppy drives is surprisingly tricky and poorly understood. The difficulty lies mainly in the interface between the floppy disk controller (FDC) and the actual drive(s). The interface isn’t particularly complex, but it isn’t straightforward, and it does not follow the intended design of the original NEC µPD765A FDC.

Many if not most operating systems do not even attempt to detect floppy drives and use the value configured by the user in the non-volatile CMOS memory. Even the excellent Undocumented PC (van Gilluwe) cops out and uses the BIOS to detect floppy drives. Yet a simple and reliable method for drive presence detection has been published in the mid-1980s, in the classic IBM PC/AT Technical Reference. Continue reading →

Posted in BIOS, PC architecture | 1 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 boot sector with a 0x55, 0xAA signature in the last two bytes and the BPB stored at offset 3 and starting with an 8-byte OEM identifier. Sadly, it’s not true. There are DOS disks with no signature in the boot sector and no BPB stored in the boot sector. Even when there is a BPB, it’s not necessarily correct.

The basic problem with BPBs is that they were only introduced in DOS 2.0; previous versions of DOS did not use the concept at all. It’s easy to see why: DOS 1.0 only supported a single storage medium format (a single-sided 5¼” floppy with 40 tracks and 8 sector per track, 160KB total capacity). There was no real need to distinguish between various formats, although there was a way to do so through the media descriptor byte in the FAT. Continue reading →

Posted in DOS | 1 Comment

Windows/386 2.01

Posted on June 21, 2011 by Michal Necasek

The OS/2 museum recently obtained a boxed copy of the original Microsoft Windows/386. That is, version 2.01 of Windows/386 from September 1987.

Windows/386 was Microsoft’s first product utilizing the 386 hardware in the DOS world. Microsoft was involved in the port of XENIX to the 386, but that was an entirely different product with a tiny market share. Continue reading →

Posted in PC history, Windows | 48 Comments

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 later systems, the foundation for their existence was laid with the original IBM PC.

To recap, the IBM PC, released in 1981, was built around the Intel 8088 chip. The PC was designed as a short-lived stopgap product and was meant to be simple and cheap, rather than extensible and future-proof. Of course it was exactly the low price and relative simplicity that made the PC into a major force, taking everyone–including IBM–by surprise.

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 be. I first suspected a virus, but it turned out to be something worse — a testament to the ubiquity of Windows 9x and the idiocy of its developers.

The ‘IHC’ signature is most likely the first three letters of ‘CHICAGO’ (codename of Windows 95) in reverse order. Windows 9x by default rewrites the OEM identifier in the boot sector of every floppy disk that it accesses, even if the access is something as simple as listing directory contents. Continue reading →

Posted in DOS, Windows, Windows 95 | 5 Comments