IBM released the IBM 5150 Technical Reference manual in August, 1981, and included in it the fully commented source code listing for the BIOS. I find this odd for two reasons:

  1. IBM must have realized that creating a legal "clone" PC would be simplified by having this source code.
  2. Other manufacturers (Commodore, Apple, for example) fully documented their systems for programmers without including firmware source code.

Since cloning the PC ultimately undermined IBM's personal computer business, this seems like a case of incompetence in handing your competitor something that makes it easier for them, while providing no benefits for IBM. Is there an alternate or documented explanation for IBM doing this, besides simple incompetence?

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    Given the long and convoluted legal history of IBM mainframes and the status of non-IBM peripherals, it was probably a reflexive move on the part of IBM...
    – Jon Custer
    Commented Aug 13, 2019 at 13:50
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    Back then, it was really not unusual to publish proprietary firmware code as documentation, just like publishing full hardware schematics - some are detailed enough, even allow you to reproduce the entire system - but doesn't mean it's legal. They are mostly used for developing 3rd party software/hardware, repairs, or modding. Another classic example is HP45 pocket calculator, the complete ROM Listing is published in US patent 4001569 (pmonta.com/calculators/us-patent-4001569.pdf, see page 47). Reading the code really helps to understand scientific calculating techniques at that time. Commented Aug 13, 2019 at 16:10
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    @比尔盖子 That should be an answer, I would answer that if not for your comment. I myself studied a lot the ROMs of ZX Spectrum, XT and AT to be able to write better software. Commented Aug 14, 2019 at 9:05
  • Pretty sure their incompetent part came with their deal with Microsoft.
    – LarsTech
    Commented Aug 15, 2019 at 22:18
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    The source was under copyright. This is the same mechanism that protects GNU licensed software today. Commented Mar 10, 2020 at 12:00

8 Answers 8


In the late '70s and very early 80s it was not unusual to make BIOS source code available. Apple did indeed do so; the full source listing starts at page 76 of the Apple II Reference Manual. Atari did the same in their Operating System Source Listing section of their Atari 400/800 Technical Reference Notes.¹ For CP/M machines, having the BIOS source was near essential if you wanted to add new hardware to the system that could be used by CP/M. So IBM's publication of their BIOS source code in Appendix A of the Technical Reference Manual was not unusual.

Nor does having BIOS source available simplify making a clone; in fact it may make it more complex. To avoid copyright issues, your clone's BIOS must not copy anything from the published source. This means that even if you independently come up with a similar solution for similar reasons, you may have to rewrite your solution not to be so similar to what the published source does because you could be accused of copying it from the original source. Having source code with comments increases the scope of work that can't be copied, as compared to just object code.

Nor was it incompetent not to foresee, before the release of the first IBM PC, that clones would eat IBM's lunch. Nobody at that point even could be sure that a single standard system for microcomputers would ever become a thing; it certainly hadn't happened up to that point. Even if someone had had such amazing precognitive abilities that they could have seen this, I'm not sure that IBM ended up making less from their fraction of a huge PC market than they would have made from the totality of a much smaller IBM PC market that they owned exclusively. The whole point of clones was that they were significantly cheaper, and if the market couldn't move to cheaper by cloning PCs, it would have moved to cheaper by using less-compatible hardware and moving the compatibility burden to software developers.

¹Of the early PC manufacturers, Commodore appears to be one of the very few that did not distribute their BIOS (which they called "KERNAL") source, though it later leaked out. Perhaps this was because it was somewhat more substantial than other early BIOS code, being a bit closer to an actual operating system.

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    Even today, for most microprocessors, particularly in the ARM implementation world, the processor manufacturer publishes extensive example code or HDK including bootstrap/bootloaders, baremetal IO drivers, sample applications, that inevitably proliferate if the platform takes off, this is done in order to encourage adoption. In 30 years someone might ask, why was android bootloader public? Because otherwise nothing gets done.
    – crasic
    Commented Aug 14, 2019 at 2:06
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    @crasic Perhaps true for microcontrollers, but in the higher-performance ARM SoC's, most documentation is available only with strict non-disclosure agreements, and most of the firmware is only a binary blob.
    – jpa
    Commented Aug 14, 2019 at 7:03
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    There were clones for IBM mainframes,the Amdahl mainframes, but in that case, Amdahl didn't end up dominating the market, so perhaps IBM was thinking a similar thing would happen with PCs. However, early PC's used a collection of off the shelf components, so cloning would be easier. The big change in clone dominance occurred with the 386, with IBM pushing for its micro-channel based PS/2 systems, while a group of clone makers developed a cheaper EISA standard.
    – rcgldr
    Commented Aug 15, 2019 at 4:18
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    continuing - It was during this era that the top 20 PC companies accounted for only about 50% of the market, with the remainder being small operations building 386 AT clones with over the shelf components. This also coincided with Apple raising prices for Mac across the board in late 1989, followed by the 386 clones and Windows 3.0/3.1, reducing the Macs share of market from 25% down to about 5%.
    – rcgldr
    Commented Aug 15, 2019 at 4:19
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    "it was not unusual to make BIOS source code available" => Not only that, but also IBM was used to shipping source code to customers for their mainframe and midrange products. It was only in February 1983 that IBM made their infamous "Object Code Only" announcement, that they were going to stop shipping the source code to their mainframe software products. So, releasing the source code to the IBM PC BIOS in 1981 was rather consistent with IBM's broader culture at the time, even if that culture would soon drastically change. Commented Aug 18, 2022 at 0:01

When other manufacturers attempted to copy the BIOS from the source listings, IBM sued them for copyright violation and won. Besides, even without the listings, anyone would have been able to dump and disassemble the BIOS. Publishing the source code made it harder to argue that the engineers hadn’t seen or used it.

What took IBM by surprise was the strategy of a company called Phoenix Technologies in 1984. They had one team of engineers look at the BIOS source and write a complete specification of what each part of it did, and a second team, who had never seen IBM’s copyrighted code, re-implement the BIOS from the spec in a “clean room.” This stood up in court and became the missing piece that allowed other companies to make 100% IBM-compatible PCs.

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    Should be noted that this approach would have worked even without source code. IBM stumbled with the PC in the early days (remember the PC Jr.?) and users had a voracious appetite for compatible hardware. That meant the clones could hit scales that IBM simply wasn't able to handle, at least not with ramp up time, and the clean room work on the BIOS was justified commercially. Even without source code this equation would not have changed. Commented Aug 14, 2019 at 7:13
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    Having the source code surely saved Phoenix at least a little effort reverse-engineering the software.
    – Davislor
    Commented Aug 14, 2019 at 7:22
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    A little effort, yes, but in those days your source tended to be assembler anyway. It's not even in the same order of magnitude of effort as, for instance, trying to reverse engineer a complex C++ program from just the binary. Commented Aug 14, 2019 at 20:53
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    @madscientist159 One of Ralf Brown’s books talks about obtaining an OEM MS-DOS developer’s package from Microsoft that included the .OBJ files. He wrote that it was just as good as having the source code: it gave him everything but the comments. Which were probably out-of-date and misleading anyway.
    – Davislor
    Commented Aug 14, 2019 at 22:22
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    @madscientist159 reverse engineering hand-crafted assembler can often be harder than doing it for compiled code (I've done plenty of both). Native assembler code is often far less modular, with the semantic intent behind chunks of code being harder to reason about than when you get lots of readily identifiable small functions.
    – Alnitak
    Commented Aug 15, 2019 at 11:16

Just because they released the source code didn't mean that copyright no longer applied. They didn't "open source it".

Having access to the source was as effective documentation on interoperating with the machine as anything was.

Back in the day, we had a stack of microfiche (I'd guess 100+ pages of fiche film) with (apparently) the source code to DECs VMS on it. It came with the system. Pages of assembly printouts.

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    They did open source it, but they didn't license it as FOSS. Open source does not imply copyleft.
    – forest
    Commented Aug 14, 2019 at 5:55
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    @forest No, they published the source. To be open source, the licence must give the distributee the right to use, modify and redistribute the software. If IBM were suing other companies for using their source code, it was not open source.
    – JeremyP
    Commented Aug 14, 2019 at 12:33
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    @forest Nope. If you publish the source code but people cannot reuse it, it is not open source. en.wikipedia.org/wiki/The_Open_Source_Definition
    – JeremyP
    Commented Aug 14, 2019 at 22:21
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    @forest Even your link denies your definition of open source although it criticises the term because people (you included) wrongly think it means "you can look at the source code".
    – JeremyP
    Commented Aug 14, 2019 at 22:25
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    @forest the OSI definition is what almost everyone means when they use the term. It can essentially be assumed that when people use the term that they are referring to the OSI one rather than the not well known FSF version.
    – Qwertie
    Commented Aug 15, 2019 at 3:12

Harry Potter and the Half-Blood Prince is another work whose source code is entirely available for public view, yet it's definitely under copyright and you will get in big trouble for commercially copying it.

Making it public and releasing it from copyright are two separate things.

In order to clone the IBM BIOS, they had to write totally new software that did the same things, but prove in court that the writers of this software did not lift or copy any part of the original.

This process is covered in detail in the first few episodes of Halt and Catch Fire.


When IBM published this source code, it was to make it easier for other companies to make peripherals.

They wanted there to be a lot of cards to could be slotted into the PC and just work. Having all these cards available would increase the marked for the PC itself.

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    ... then came that micro channel nightmare Commented Aug 16, 2019 at 18:23

I can assure you it was not altruism! In fact, it made it darn hard to copy legally (see Eagle Computer) and get away with it. Phoenix had to prove that none of the people who wrote the code EVER read the IBM published code. And I can tell you that nearly all of us in that era had. So while it seems simple, it was a great way to freeze the competition. They had to wait until there were enough assembler programmers on the 808x series who resisted temptation. Copyright law specifies about 60% commonality I think. So it was harder than it looked.

Eagle did rewrite theirs under duress, and while it was a darn good copy, the rest of the industry was already moving on to Phoenix. IBM pulled a subtle, well executed strategy. Sadly they blew it in the MCA debacle, but hey, all of the companies at that time made a few of those!


You are working from the rather popular assumption that hogging a brook will be preferable to tapping a river. The comparatively open nature of the IBM PC internals created a market that would not have existed otherwise, and IBM's profits from that market exceeded their expectations.

Now make no mistake: this was not particularly unprecedented. CP/M had example BIOS code available, my own NASCOM II came with a complete assembly listing of its system ROM, and I have the same for an Atari 400 here in an official Atari binder. It was quite customary for home computers to have schematics and system ROM listings readily available, and the market was sort of split here. In comparison, it was a bit unusual that the typical Microsoft ROM Basic that was also included with a number of home computers never came with the assembly listings or even API documentations and was a black box.

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    One of my prized possessions when I was, you know, quite a bit younger, was a book that was a commented disassembly listing of Microsoft BASIC for the TRS-80. As I recall, it was missing the opcode for each line, so they wouldn't break copyright, but that was easy to fill in by hand. I learned so much from that book. I thought it was this one: classiccmp.org/cpmarchives/trs80/mirrors/pilot.ucdavis.edu/…, but this is a little different than the one I had
    – Mohair
    Commented Aug 14, 2019 at 22:06
  • "popular" assumption? How on earth does one "hog" a brook (or even tap a river? wouldn't your knuckles get wet?) ??
    – Mawg
    Commented Aug 15, 2019 at 15:17
  • @Mawg you hog a brook by diverting all of its water to irrigate your field. Commented Aug 16, 2019 at 16:42
  • One lives & learns. T o think that I had to read a retro computing forum to learn that :-)
    – Mawg
    Commented Aug 18, 2019 at 8:33

I find the position of this decision as incompetence rather interesting. This is presuming that every business is only, ever, out to make money and/or monopolize the market. I realise this is common place in a capitalist system, but don't like to assume that it is prevalent EVERYWHERE. Perhaps they just provided the source code to be nice, or to advance personal computing faster than they as a company could do themselves.

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    Altruism is a poor survival strategy for individuals. It is a great survival strategy for groups of individuals. Take for example the teams of searchers who go looking for lost children. Many times searchers are lost during these kinds of rescue operations. This is a poor survival strategy for the people doing the searching (yet is is wired into our brains (well, most of us)). However, it is great for the tribe because the children carry the tribe forward through time, increasing the tribes survival. Commented Aug 14, 2019 at 13:27
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    Not "to be nice" - that is not the IBM way. Not "to advance personal computing faster" - the original IBM PC was not a leading edge product. Pure and simple capitalism - provide the information necessary for 3rd parties to write software and build hardware compatible with the IBM PC and IBM sells more PCs. That was a big change from the mainframe era and it worked. Commented Aug 14, 2019 at 14:28
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    I agree with manassehkatz that was IBM's reasoning. After all, a little PC could never, every compete with their mainline Mainframe and Mini product lines!!!
    – kmarsh
    Commented Aug 14, 2019 at 19:48
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    @JonathanFite It's not necessarily altruism for altruism's sake. A rising tide lifts all boats.
    – forest
    Commented Aug 15, 2019 at 3:15
  • Altruism requires an altruistic motive, otherwise it's not altruism. And looking back on it and justifying the decision based on effects does not mean that's why they did it. But user66001's answer seemed more about the nature of altruism than this specific decision by IBM. Commented Aug 15, 2019 at 12:32

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