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 door” into DOS and called it a security risk, despite the fact that it was a compatibility interface very deliberately maintained in every version of DOS. However, the article correctly pointed out that the CALL 5 interface bypassed INT 21h hooks. In theory, that could have been used for nefarious purposes; then again, worrying about that on the DOS platform was like being gravely concerned that cold wind could get in through a small crack in the window in a house without a roof. DOS simply wasn’t a secure foundation and patching a tiny hole couldn’t fix the fact that any program running on DOS effectively owned the entire system. 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 place.

There is precious little information on that topic. But as it turns out, all versions of DOS, including the OS/2 DOS box and NTVDM in Windows NT, implement a CP/M-like system call interface which cannot work without address wrap-around being emulated one way or another. In other words, any DOS application may in fact need the wrap-around, without necessarily being even aware of the fact. The key is the so-called ‘CALL 5’ compatibility interface, invoked by a near call to the offset 5 of the PSP (Program Segment Prefix). This interface was intended for COM programs, but will work as long the CS register points to the PSP regardless of the program type. So what’s at offset 5 of the PSP? 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 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.

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 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 →

Posted in DOS, PC architecture | Leave a comment

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