Deeper Into ATA History

Posted on October 14, 2021 by Michal Necasek

While looking for something completely unrelated (namely the Rock Ridge extensions to ISO 9660), I came across a cache of old X3T9 committee documents from 1990. In retrospect I’m a little surprised that I hadn’t found these earlier, since the archive appears to have been published on one of Walnut Creek CD-ROMs circa 1994, but I’m not sure how long it’s been online.

What’s interesting is that the Walnut Creek archive appears to overlap with the X3T9.2 archive that has available for a long time, but contains numerous documents that the X3T9.2 archive does not. Notably there’s a directory with CAM Committee documents. While the CAM Committee’s primary objective was to define a Common Access Method (CAM) for software accessing SCSI devices, an effort that ultimately went nowhere, the CAM Committee also started a rather more successful side project, the AT Attachment (ATA) standard.

The archive is far from complete, but it does include one complete ATA draft, revision 2.1 from June 11, 1990. That’s one revision older than the oldest ATA draft I was aware of until now, which is revision 2.2 from August 1990. The rev 2.1 draft is provided as a ZIP archive containing WordStar files, which is excellent for seeing exactly how the draft was edited (and the WordStar files include a couple of editorial comments that do not show up in the printed version), but the downside is that getting from WordStar to PDF was not entirely trivial. In the end I was able to produce a PDF of ATA Rev 2.1 in 2-up format that’s quite similar to the scanned documents in the X3T9.2 archive.

Even better, the Walnut Creek archive includes what appears to be the very first and quite incomplete ATA standard draft from March 30, 1989. Said draft also provides a hint why a SCSI oriented committee started ATA in the first place: The early ATA drafts also included a specification of EATA (Extended AT Attachment), a SCSI pass through mode of ATA devices (completely separate from and much older than ATAPI).

Sadly the initial draft—which is so old that it’s called DAD, for Disk ATBus Definition, rather than ATA—does not include the EATA sections. In the next oldest currently available draft (revision 2.1), EATA had been already removed again. ATA revisions 1.x appear to have included the SCSI pass through functionality defined by EATA.

EATA was the brainchild of DPT (Distributed Processing Technology), one of the larger SCSI HBA vendors. An overview of EATA can be found here. I don’t believe anyone besides DPT implemented EATA, but the idea behind it was quite interesting.

CHM’s oral history of Dal Allan describes how EATA was created by DPT and desired by Quantum, but WD successfully fought to remove it from the standard for cost reasons, only to implement the same idea (SCSI pass through over ATA) as ATAPI a couple of years later.

The first ATA draft from March 1989 notably already defines the IDENTIFY DRIVE command as well as READ/WRITE MULTIPLE, but there is no sign of DMA support yet. The DASP signal for letting drive 0 detect drive 1 was also already defined, although the details were refined many times since then.

Finding the very first ATA draft is something I doubted would ever happen. Now I wonder if the revision 1.x ATA drafts might eventually turn up, too.

The list of early ATA drafts on this site has now been appropriately updated.

Posted in IDE, PC history, Standards | 17 Comments

Weird Tales

Posted on October 1, 2021 by Michal Necasek

While researching various aspects of the history of computing, sometimes I come across what can best be described as “weird tales” — unsourced claims that sound interesting but are either provably wrong or there’s no evidence to support them. In the 1980s and 1990s, Bill Gates was a frequent purveyor of such weird tales, often designed to burnish the image of Microsoft.

Listening to Bill Gates, one might for example get the impression that he personally designed the IBM PC, even though Microsoft’s involvement with the actual hardware design was at best peripheral. At the same time, there’s often enough some kernel of truth in the implausible stories.

Some time ago, one of such weird tales caught my attention, the story of Microsoft putting the Wang word processing character set into the IBM PC. Here’s one version of the story:

They told us for this keyboard, they said Lexington had to use a certain layout. We wanted these function keys to be up here, that the Lexington layout forced them to be over here. We ended up with this funny big key over here because of Lexington. But here we’d put in our favorite fully-extended character set. Came up with some new ideas there. Put some special things in here that we thought we’d be able to use. In fact, we put in the Wang word processing character set because we weren’t sure whether to do a Wang-clone word processor, or start with our own approach at that time.

Bill Gates, Smithsonian Video History Interview

There is another variant of the story, similar but not the same:

Yeah, we were also fascinated by dedicated word processors from Wang, because we believed that general-purpose machines could do that just as well. That’s why, when it came time to design the keyboard for the IBM PC, we put the funny Wang character set into the machine–you know, smiley faces and boxes and triangles and stuff. We were thinking we’d like to do a clone of Wang word-processing software someday. Most personal computers back then didn’t even have upper- and lower-case characters.

Bill Gates, Fortune Magazine interview, October 2, 1995

First a side note about “Lexington”. No one appears to know of an IBM keyboard codenamed “Lexington”. On the other hand, it is well known that the original IBM PC keyboard layout came straight from System/23 Datamaster. It is also well known that IBM’s keyboards were manufactured in Lexington, Kentucky at the time (later Lexmark). Bill Gates most likely mixed things up, perhaps due to incomplete information he had.

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Posted in IBM, Microsoft, PC history, PC press | 22 Comments

DOS 2.11 From Scratch

Posted on September 21, 2021 by Michal Necasek

Warning: Long post!

After having good luck with rebuilding core PC DOS 1.1 from source code, I thought I’d do the same with the DOS 2.11 source code released by the CHM. What follows is largely a collection of notes that I wrote down while banging the released source code into shape. That turned out to be a lot harder than building DOS 1.1 for two reasons.

One is that the released DOS 2.11 source code is a lot more extensive and includes source code for numerous utilities (CHKDSK, DEBUG, EDLIN, SYS, etc.). The other, bigger reason is that the CHM unfortunately created a bit of a mess when releasing the code and sorting out the pieces was not trivial.

Microsoft style DOS 2.11 boot

The CHM placed all DOS 2.x related files in just two directories, ‘v20object’ and ‘v20source’. It is now clear that the files came from at least three distinct sources:

  • MS-DOS 2.00 OEM distribution disks
  • MS-DOS 2.11 source code of unknown provenance
  • Miscellaneous debris such as WordStar 3.20 overlay files

Fortunately for me, Jeff Parsons has done a lot of legwork reconstructing the DOS 2.0 OEM distribution disks. These disks were clearly an early version of what Microsoft later called OAK (OEM Adaptation Kit). The disks contain generic DOS 2.0 binaries, with the notable exception of IO.SYS which had to be supplied by the OEM. There is example “skeletal” IO.SYS source, together with source for PRINT.COM that OEMs might modify, and example OEM source module for FORMAT.COM which OEMs had to write.

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Posted in Development, DOS, Microsoft, PC history | 43 Comments

PC DOS 1.1 From Scratch

Posted on September 10, 2021 by Michal Necasek

A number of years ago, the Computer History Museum together with Microsoft released the source code for MS-DOS 1.25 (very close to PC DOS 1.1) and MS-DOS 2.11. I never did anything with it beyond glancing at the code, in no small part because the release was rather poorly organized.

PC DOS 1.1 rebuilding itself

Now I finally decided to look at the code for DOS 1.1 and see how far I could get with it. For both DOS 1.1 and 2.0, there are ‘object’ and ‘source’ directories. The ‘object’ directory for DOS 1.1 simply contains a copy of PC DOS 1.1, which is not particularly revealing or useful on its own (and strictly speaking I’m not even sure why the CHM thought it could publish those files).

The ‘source’ directory is much more interesting and contains the following files:

05/09/1983 09:59 AM   63,781 ASM.ASM
05/17/1983 06:19 PM   67,064 COMMAND.ASM
07/02/1982 11:33 AM    3,625 HEX2BIN.ASM
08/03/1982 12:29 AM   36,882 IO.ASM
05/17/1983 06:15 PM  114,253 MSDOS.ASM
05/17/1983 06:20 PM      649 STDDOS.ASM
07/01/1982 11:54 PM   16,223 TRANS.ASM

This turns out to be an interesting mix, and an included 2013 e-mail from Tim Paterson explains its origin: Those files are the source code for what SCP (Seattle Computer Products) shipped to its customers. ASM, HEX2BIN, and TRANS were SCP’s development tools used for initial DOS development. MSDOS.ASM and COMMAND.ASM are source code for core DOS components. IO.ASM is source code for SCP’s IO.SYS (i.e. IBMBIO.COM equivalent).

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Posted in Development, DOS, Microsoft, PC history | 28 Comments

Learn Something Old Every Day, Part III

Posted on September 8, 2021 by Michal Necasek

As part of a hobby project, I set out to reconstruct assembly source code that should be built with an old version of MASM and exactly match an existing old binary. In the process I learned how old MASM versions worked, and why programmers hated MASM. Note that “old versions” in this context means MASM 5.x and older, i.e. older than MASM 6.0.

The way old MASM works is relatively straightforward but its documentation often explains it very poorly or not at all. MASM is a two-pass assembler, and that indirectly explains almost everything about its quirks. This is different from more modern N-pass assemblers which automatically run multiple passes to resolve ambiguities.

The core of the problem is that MASM tries to be clever, but it’s not nearly clever enough. It is very questionable whether MASM’s cleverness is a solution or a problem; other assemblers are stricter, relying on programmers to resolve ambiguities. This perhaps puts slightly more of a burden on the programmer but results in more readable, consistent source code.

Most ambiguities result from the fact that like most assemblers, MASM does not require symbols to be declared before they’re referenced. In the first pass, MASM generates “provisional” code, making guesses about what unknown symbols are. At the end of the first pass, all symbols are known (if they’re not, the assembly will fail).

In the second pass, MASM applies what it learned in the first pass and generates the final object code. If the guesses made in the first pass turn out to be incompatible with the second pass, MASM will report the dreaded “phase error”. More about that later.

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Posted in Assembler, Development, Microsoft, PC history | 11 Comments

PC-86-DOS

Posted on September 3, 2021 by Michal Necasek

A number of years ago, an 8″ disk containing Seattle Computer Products (SCP) 86-DOS 1.0 was successfully imaged. The newest files on the disk are dated April 30, 1981, making the disk the oldest complete release of what was soon to be known as PC DOS and MS-DOS, about a month older than a pre-release of PC DOS from early June 1981.

While it is possible to run the 8″ disk image with 86-DOS version 1.00 under an emulator, it of course doesn’t run on a PC or any PC emulator/virtualizer. That’s a shame because most of the utilities included with SCP’s 86-DOS run under DOS just fine. In theory, it should be possible to provide a PC compatible “BIOS” component (IBMBIO.COM or IO.SYS equivalent) and run the rest of the system more or less unmodified on a PC.

In practice, it can in fact be done. Behold PC-86-DOS 1.00, running from this disk image:

SCP’s 86-DOS 1.00 on a PC

Actually achieving this wasn’t complicated but there was one non-trivial snag.

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Posted in Development, DOS, IBM, PC history | 18 Comments

The Dual-Drive IDE Hell

Posted on August 25, 2021 by Michal Necasek

I have perhaps inaccurate but very strong memories from my PC-building days (in the early to mid-1990s) that one of the most failure-prone and frustrating endeavors was trying to get two IDE drives working together on a single cable as a master/slave pair, or as Device 0/1 in the language of the ATA Standard.

It’s not that it was fundamentally impossible… but it was insanely random and unpredictable. No one could tell in advance whether any given pair of IDE drives would work. Mixing drive vendors seemed to increase the likelihood of failure, but using two drives from the same vendor was by no means a guarantee of success.

Various jumpers used for configuring IDE hard disks

There were two major reasons for why this was the case:

  • The whole thing started out as a giant hack, pretending that two separate IDE drives are really two drives attached to a single AT style controller.
  • The vast majority of IDE users only ever had one drive, thus not putting enough pressure on IDE vendors to get their act together.

Eventually, by the late 1990s, things did work reasonably well. The key ingredient was cable select and (barely) special cables which allowed all drives to be configured identically and work as either master or slave depending on their cable position. This was not exactly a revolutionary concept; PC floppy drives and ST506 interface hard disks used the same idea, using a twist in the cable, since the early 1980s.

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Posted in IDE, PC hardware, PC history | 20 Comments

Drive Price Mystery

Posted on August 17, 2021 by Michal Necasek

While digging into the history of DoubleSpace and DriveSpace, I came across a handy article about the rocky relationship between Stac Electronics and Microsoft. Only I was distracted by the bold claim (certainly claimed in very bold letters) that a 40 MB drive cost $1,200 in 1989. Now, I don’t have a good sense of what cost how much in the US of A in 1989, but that just did not sound right.

Sure enough, a PC Magazine article about mail-order hard disks published in the June 1989 issue showed that in 1989, a 40 MB hard disk that cost more than $500 was an outlier, and finding one for $399 was not difficult. It also seemed odd to pick a Western Digital hard disk as representative of 1989 drive prices, because although WD was a major force in disk controllers, and did sell its own hard disks after the acquisition of Tandon’s drive division, the clear market leader at the time was Seagate. Other common mass-market drive makers were MiniScribe, Priam, or Micropolis… but not Western Digital.

So where did the outrageous $1,200 price came from? For a moment I thought perhaps that was adjusting 1989 dollars for inflation, but even in 2021, $500 (not a great price for a 40 MB drive) in 1989 dollars would be the equivalent of $1,100, not $1,200, and the article is several years old. So it’s probably something else…

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Posted in PC history, Storage | 16 Comments

How Not to Erase Data

Posted on August 16, 2021 by Michal Necasek

From past blog posts it is fairly obvious that the OS/2 Museum occasionally purchases used hard disks. Most of the time, the disks are either completely erased (overwritten with zeros) or don’t have anything very interesting on them.

But sometimes they do. Especially old SCSI disks tend to have old data on them, simply because very few people have the necessary equipment to use them anymore. But recently I encountered a case that appears to be an epic failure to delete data.

The hard disk is a Maxtor 25128AT, a 2.5″ IDE drive with 128 MB capacity that started its life as an OEM drive in an IBM ThinkPad. It was originally set up with MS-DOS 6.20 and Windows 3.11 for Workgroups.

A 128 MB hard disk formerly found in a ThinkPad laptop

At some point in the past, someone attempted to wipe the disk by high-level formatting it, i.e. putting in place a fresh FAT file system. That normally does quite a lot of damage by destroying the root directory and FAT tables. If the file system was fragmented, recovering the files can be quite difficult and very labor intensive. Yet on this particular drive, reformatting the drive resulted in no appreciable data loss and recovering all files was fairly easy. How is that possible?

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Posted in DOS, PC hardware, Storage | 17 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.

IBM Personal Computer (IBM promotional photo)

But in August 1980, one year earlier, IBM started putting together the design of the IBM PC. If that (one year) sounds like an awfully short development cycle for a product like the PC, that’s because it was, especially for a company like IBM where the typical product development cycle was closer to five years at the time. The tight schedule determined the PC design: No custom or not yet available chips, no major software development, favor proven and familiar technologies.

With these constraints in mind, the design of the PC was sketched out on a stack of papers on August 10, 1980. Some of those drawings—not previously published—are now presented here; see explainer at the end of this article for a glossary.

IBM PC Design, August 1980

A closer look at the drawings reveals that even though there were quite a few changes between August 1980 and August 1981, the core was all there from the beginning: A box with two floppy drives, five I/O expansion slots, and a detachable keyboard; an Intel 8088 CPU with optional 8087 FPU, 8259A interrupt controller, 8237-5 DMA controller; RAM on motherboard, with an option to install additional RAM expansion cards; a display adapter with separate monitor or TV.

Choosing these particular chips was no coincidence, and there was one very clear reason for it: The IBM System/23 Datamaster.

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Posted in 8086/8088, BIOS, DOS, IBM, Intel, PC hardware, PC history | 67 Comments