After designing and building an XT IBM compatible PC design in several modern ATX form factor systems, I became increasingly interested in the complete history of the development of Personal Computers by IBM.

I have read a lot of historical information about the creation of Personal Computers which has been quite fascinating and a real eye opener about how the IBM compatible Personal Computer, PC/XT and PC/AT concepts were actually created, and why these became such historically important and meaningful designs.

By creating my own version of the IBM PC/AT model 5170 Computer in a completely new ATX mainboard design, it has been my intention to revive and understand in much more depth this most essential “next level” historic milestone of computer history.

Growing up in the 1980s and 1990s, as a student at the technical school MTS Graafschap College in Doetinchem taking their Electronics classes, I saved for a long time to be able to buy my first real “IBM-Compatible” PC. I got interested in PCs thanks to my school days at the old MTS building which doesn’t exist anymore, where they had (a lot of Tulip branded) PCs in their CAD classrooms which provided various CAD software for the students to get acquainted and familiar with. Seeing the much more advanced usefulness of these PC computers as a tool to create electronics designs did inspire me a lot that getting a PC at home would be the best thing to upgrade to from my old Commodore 64 setup which I had previously used for my school work. I realized that owning an actual PC would help me a lot in gaining a more professional working environment to do all sorts of work, including to develop my own new electronics designs.

After going through several computer stores in my city, I remember finally ending up in an IBM store, just going there out of curiosity, thinking that the computers sold there must have been much too expensive to be able to afford an actual PC made by the famous PC creator IBM themselves, however I was very pleasantly surprised by IBM because at that exact time, they actually had a very affordable model for sale in this IBM store, which was the IBM PS/1 model 2121. This very compact and appealing PC model surprisingly came in a very complete setup with all the software and complete printed manuals for everything, and including a small VGA monitor, all of this within the budget that I had saved up. When I found this PC, this inspired a lot of courage to make the decision on the spot to buy this IBM instead of getting a “clone” PC. Even though this PC was meant as the lower budget model of IBM PC, and being of a less powerful edition than most other PCs sold at the time, it turned out to be a very versatile and useful computer when I explored the possibilities of this PC. I was very happy with the purchase and taught myself a lot about PCs from working with this PC at home than ever before, and developed a much deeper appreciation and fascination for IBM as a manufacturer from experiencing this PC as a user and doing a variety of modifications to the system after buying it.

After working in the field of IT and PCs for years, I revisited the days of early PC computing by first recreating the XT design, which I had never seen anywhere before in the 80s and 90s when I was a student because these systems were not affordable at the time. This XT redesign I have built was able to run a lot of software that I did use in the 1990s but soon I explored the technical limits of what the 8-bit IBM PC/XT was able to provide in its most expanded form. It was time to have a detailed look at the next design, the 16 bit PC/AT.

Historical background of the IBM PC and subsequent PC/AT model 5170

As I understand it, IBM had the surprising intention to get involved with “microcomputing”, in other words in the field of home computing, and to compete with other manufacturers such as Commodore and Apple with a line of microcomputers of their own. It was understood within IBM that this would be impossible to do in a short time span without deciding to develop this new project in a very unconventional and untypical process for IBM. In order to cut out a lot of corporate bureaucracy, IBM decided to form a separate division within their Entry Level Systems department in Boca Raton, Florida. In this division they employed some unusually brilliant people who operated in a undesirable way according to IBMs main corporate structural methods. This made these people to be shifted to “unimportant” positions, in “less important” departments. Little did IBM know that these people would develop something so radical and extremely functional that these systems would take over a large portion of the entire business of IBM in the near future…

The first person to lead IBMs new home computer development project was Bill Lowe. However soon Bill was promoted out of this department and needed to find his successor to take on the project and make it into a reality. The important decision was made by Bill to recommend Don Estridge to take over the project in order to make it a success. Don was the best man for this position and really demonstrated many amazing and unique leadership talents and had his own very insightful vision for IBM to develop a home computer which made him the best person for fulfilling this very historic role within IBM. Don was a brilliant man of many talents which included one very crucial talent, the ability to explain his intentions and plans for the project to IBM CEO Frank Cary.

The relationship which Don Estridge developed with Frank Cary, who shared all his visions for the project, was an essential key part of computing history, and exactly this is the reason why IBM, with the help of Don Estridge, was able to provide our world with the most useful computer tool of all, the IBM compatible PC. Don was strongly inspired by using home computers himself, he owned an Apple II computer which he used at home. When starting the new project, he took the Apple with him to the office in order to help convince people that creating a microcomputer would be a great idea, and show off some of the capabilities of such a type of system.

Initially, the first IBM PC models which came out were very limited in specifications, however it was Don Estridge himself who had much more clear visions and ideas about how important the new IBM PCs were going to be in the world, far beyond what Bill Lowe imagined initially. Subsequently, the development of computer technology mechanisms within the IBM PC designs was much more advanced than what you normally would find in home computers. The complexity and advanced level of functionality and reliability distinguished the IBM PC design greatly from everything else which was on offer at the time in such entry price ranges. This became much more apparent to me after studying all the schematics in much detail, and after constructing the first models of recreated designs of my own. This made me experience and relive up close why PCs are so advanced and how they became such extremely useful machines, much more so than anyone in IBMs management could ever have anticipated. And to me it was really disappointing to find out how much under appreciated and unacknowledged Don Estridge has been in his time. A typical example of how true brilliance and epic work done by such a genius and talented man is quickly overlooked and overrun by the steamroller effect of commercial successes which takes no time to stop, notice and acknowledge how and why the success was created in the first place. Don Estridge was the essential key historical person in creating the technology which in fact formed a whole industry of technology which started a computer revolution with a powerful momentum of its own, which continued to run for decades and has never stopped since.

The creation and further development of the IBM PC and PC/XT were extremely important, however equally important was the development of the next level and even more powerful advanced PC design, the IBM PC/AT model 5170 computer. This computer was so essential because it provided not only a multiple of processing speed compared to the PC/XT, it did much more, this 16 bit “AT” design presented the new and improved Intel 80286 processor.

The design of the IBM PC/AT was so advanced that it managed to provide complete backward compatibility with all 8-bit PC hardware and software. Basically Don Estridge managed to develop a much faster generation of PC which inside at its core included the complete functionality of the 8 bit PC/XT system. So no previous investments of users would be wasted at all and could all be re-used within a PC/AT system. At the time this provided a level of backward compatibility which was unseen in this type of computers. The PC/AT additionally provided so much added computing power that it became a serious business machine. After the PC/AT model 5170 came out, for a long time IBM dominated the market because no competitor at that time was able to match the price and performance level of the 5170. The PC/AT model 5170 provided humanity with not only the best practical computer application of the Intel 80286 processor of its time, it also played a crucial part in history to completely solidify the importance of the IBM compatible industry PC standard by showing us what was possible with 16 bit PC/AT technology.

The way that the PC/AT system standard was designed and created makes it extremely suitable to even be able to provide multitasking and multi user abilities, especially if using unix. The 5170 powered a revolution in computing and actually was starting to even become a serious competitor for the much more expensive mini computers of the same time period. Traces of this epic 5170 design and technology became an integral part of PC technology for decades into the future, a profound effect!

There are many people who have criticized the 286 CPU, however after working with such a CPU in my AT system, I have come to appreciate the technology. The 286 had some shortcomings, for example not being able to switch in and out of protected mode without using a CPU reset procedure while the rest of the system waits for the CPU to return. In DOS we can’t access the extended memory unless using difficult and inefficient programming tricks. However the fact still remains that the 286 is a fast 16 bit CPU. I have been able to clock it at 20Mhz so far which provides a really fast and responsive system!

The IBM PC/AT model 5170 in more depth

The IBM 5170 provides many important core components and functions. It provides extremely flexible hardware and software interrupt capabilities in order to call subroutines to automatically handle a variety of processes, DMA capability both onboard and on expansion cards containing their own DMA solution, a system timer enabling the system to perform many crucial functions at repeated exactly synchronized intervals, a 16 bit 80286 CPU which is able to address 16 Megabytes of memory, of which at least 15MB is potentially available as usable RAM memory space. More simple computer functions such as the keyboard interface, printer port, serial port, interrupt control are all done by much more simple 8 bit devices in the AT core system. This is possible thanks to the brilliantly designed and very advanced 8 to 16 bit conversion technology which is a core part of the IBM 5170 PC/AT system. This technology does much more than to provide compatibility, it enables much more simple 8 bit I/O to be used for simpler tasks, enabling to interface these 8 bit devices with the powerful 80286 CPU. This greatly simplified the system and made it much more compact. The AT is often discribed as a 16 bit system, however it is much more than that. It actually combines all the advantages of 8 bit computers with those of 16 bit computers by integrating these two technologies into a single system, a stroke of genius by Don Estridge!

Please continue reading on the project design page where I am describing the whole project in detail.

You also can look at my GitHub page of the project where I have shared the project as open source.

Further reading is available on My project thread at the VCF forums.

Kind regards,

Rodney