The IBM PC, 41 Years Ago

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.

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.

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.

The Datamaster Legacy

The IBM PC hardware was very strongly influenced by the design team’s prior experience with the Datamaster. That may seem odd given that the Datamaster was announced in July 1981, just weeks before the PC. But in reality, the Datamaster development started in 1978 and the hardware was finished in Summer 1980. The product release was significantly delayed by difficulties with implementing IBM’s chosen BASIC variant.

The Datamaster used an 8-bit Intel 8085 CPU, 8259 interrupt controller, 8237 DMA controller, and 8253 programmable timer. It also utilized an expansion bus remarkably similar to the PC’s.

CPU Choice

The Datamaster team learned that the 64K address space of an 8-bit CPU wasn’t quite big enough for the tasks IBM had in mind for it. The Datamaster used paging to expand the addressing capabilities, but that complicated things significantly.

With that in mind, IBM wanted a CPU with a much larger address space for the PC. The Intel 8086 or 8088 was a logical choice; the slightly better performance of an 8086 was not considered worth the increased complexity and cost. The 8088 provided a good compromise between supporting 16-bit software with a huge (for the time) 1 MB address space while utilizing cheaper and familiar 8-bit infrastructure.

There was already an existing 8086 BASIC (Microsoft’s) and other tools, and porting software from the 8085 was not difficult. As an added bonus, IBM already had Intel MDS development systems that supported both 8085 and 8086 development.

With the above in mind, it’s easy to answer questions such as “why did IBM not use the Motorola 68000”. The CPU was barely available in 1980, there was no BASIC for it yet and no software, and IBM had no experience with it. Choosing the 68000 would have delayed the PC release well beyond what IBM was willing to accept; that alone was a sufficient reason to not pick the 68000.

Internal Layout

The initial design called for a power supply taking up the left side of the PC’s enclosure, and adapter cards competing for space with the internal floppy drives. The actual design moved the power supply behind the floppy drives, leaving more room for long adapter cards.

IBM took advantage of the extra space and the adapter cards released with the IBM PC were on the larger side.

Expansion Bus

The Datamaster didn’t only influence the choice of the PC’s CPU and support chips. It also strongly influenced the PC’s 62-pin I/O expansion bus, later known as the ISA bus. How significant was the Datamaster influence? The following two diagrams should answer that:

The above is a diagram from an IBM Datamaster service manual dated December 1980. Below is a diagram of the expansion connector from the IBM PC Technical Reference dated August 1981.

The diagram is mirrored (A pins on the left vs. A pins on the right), but clearly extremely similar. What used to be page select bits 0-3 neatly turned into address bits 16-19. Interrupt and DMA levels that had specific purpose on the Datamaster are generic on the PC. There are real differences, e.g. pin B20 used to be DMA request 0 on the Datamaster but turned into a system clock signal on the PC. Since the PC dedicated DMA channel 0 to DRAM refresh, DMA request 0 no longer made sense. Pin B04, unused on the Datamaster, turned into interrupt request 2.

This similarity meant that existing adapter cards for the Datamaster could be used in the IBM PC with very minimal changes, if any. This no doubt greatly sped up the initial stage of development because there was no need to design a brand new floppy controller or serial adapter, for instance.

Memory

The first IBM PC system board supported 16 to 64 KB RAM, with 64 KB being about the maximum of what a “personal computer” of the era would support. Also available were 32/64 KB cards which plugged into the PC’s expansion slots. With three such cards (a practical maximum; two of the five expansion slots would be taken by a display adapter and a floppy controller), the PC could be expanded to 256 KB RAM.

The prototype board shown below has Mostek MK4232 memory chips (32K×1), with room for 18 chips total. That allows installing up to 64 KB RAM (two banks of 32 KB) with parity.

Using parity was not at all typical for low-end computers at the time, but IBM felt that the added ability to detect errors was worth the small additional expense.

From the beginning, the 1 MB address space was carved into several regions. At the top, 128 KB was reserved for system firmware, 128 KB for other memory, and 128 KB for display memory. That left 640 KB available for system RAM, but that was purely theoretical—the original PC supported only up to 256 KB RAM. Even the PC/AT only supported 256 or 512 KB on the system board.

The PC’s memory map was quite reasonable and the infamous 640 KB memory limit didn’t come into play until the late 1980s, well after the expected design life of the PC. At that point, the problem wasn’t hardware (286 and 386 processors) but rather software (DOS) holding things back.

Storage

As the drawings show, the initial PC design anticipated external 8″ drives for the PC. That ended up not happening. But the rest of the PC storage subsystem turned out more or less exactly as the initial design.

While the Datamaster used 8″ drives, the PC used 5¼″ drives. The media are much more convenient to work with (if you’ve ever seen 8″ floppies, they’re huge), and the PC form factor would not have been possible with 8″ drives.

The Datamaster used the NEC μPD765 floppy controller. It did the job, IBM engineers knew how to use it, and saw no reason to pick a different controller for the PC.

The PC used floppies with 512-byte sector size, something that later became an ubiquitous default, so much so that any other size was extremely exotic; yet in 1980/81, typical sector sizes were 128 bytes, 256 bytes, or even 1,024 bytes. Once again, the Datamaster also used 512-byte sectors, except it used 128-byte sectors on the first track of a floppy, as was then common. The PC fortunately simplified things and used 512-byte sectors throughout.

The PC could probably have maximized the floppy storage capacity by using 1,024-byte sectors, but that was perhaps not even considered. There are interesting tradeoffs in that each sector requires a certain amount management overhead and additional slack on the floppy, but at the same time each file tends to waste some unused space in its last sector. 512-byte sectors strike a good compromise between keeping the disk sector overhead low and not wasting too much allocation space per file.

The PC storage subsystem was not required to be compatible with existing systems and set its own standards.

Display Hardware

A closer look at the PC design drawings shows that one area where the final PC noticeably differed from the initial design is display hardware. Not coincidentally, that’s also where the PC significantly differed from the Datamaster.

The Datamaster display was text only and used the Intel 8275 CRT controller (CRTC). To be able to display graphics, the PC needed a different CRTC; the Motorola 6845 was chosen. The PC also needed to support TVs as well as dedicated monitors, and the characteristics of NTSC television determined what IBM’s CGA would do.

The initial PC design notably called for 80×24 text resolution, the same resolution the Datamaster used, and also the standard resolution of terminals. The PC instead ended up using the 80×25 resolution, which is extremely common today but was far from typical in 1981.

The initial design also called for a 280×192 graphics resolution (same as Apple II). The released PC (CGA) instead used 320×200 graphics. The most likely answer as to why is ‘because that was possible within the constraints of NTSC TV and 16 KB display RAM’.

The CGA had 16 KB RAM, or 16,384 bytes. A 4-color (2 bits per pixel) 320×200 resolution or a 2-color 640×200 resolution uses exactly 16,000 bytes. There would still be enough memory for a few more lines of graphics, but not for another line of text using 8×8 character cells (about the smallest cell size that’s still legible).

The text-only MDA similarly had 4 KB (4,096 bytes) of RAM. With 160 bytes required to store a line of text (80 characters each with an attribute byte), there was room for 25 lines of text—utilizing 4,000 bytes of memory—but not more.

Although the 320×200 resolution appears to have been new with the IBM PC, the Commodore 64 (introduced a few months later) used it as well. On the PC, the supported text and graphics resolutions were about the maximum possible given the display memory size and attached display hardware.

Keyboard

The PC’s physical keyboard layout was identical to the Datamaster (Model F keyboard), but the key labels were different and the keyboard was not built into the system unit. While the Datamaster used a parallel connection to the keyboard within the system housing, the PC used a serial link through a long coiled cable.

The original 83-key PC keyboard now seems alien, since the enhanced 101/102-key layout took over in the late 1980s.

Note that the Ctrl-Alt-Del sequence (deliberately) required two hands on the original PC keyboard, since Ctrl and Alt were only on the left side of the keyboard and Del was on the right.

Technical Reference

A key ingredient of the PC’s success was the Technical Reference. While the Tech Ref wasn’t exactly a tutorial and didn’t have a whole lot of explanatory text, it included complete schematics of the IBM PC, as well as a full listing of the PC BIOS, quite thoroughly commented. Any halfway competent engineer could locate datasheets of the components IBM used and see exactly how they were connected in the PC. Or build a clone of the PC.

The IBM Technical Reference was, again, a direct result of the tight development schedule. There was no time to engage a team of technical writers and develop documentation in the style then typical for IBM products. Publishing existing schematics and BIOS source code was, on the other hand, very quick and easy—and much appreciated by engineers developing hardware and software for the IBM PC.

ASCII Island in a Sea of EBCDIC

In the 1980s, IBM systems (including the Datamaster) almost exclusively used the EBCDIC character set, different from ASCII. The PC, as the design drawings show, was meant to use ASCII from the very beginning. Perhaps surprisingly, this did not cause any significant problems during development.

The PC development team was deliberately separated from the rest of IBM. Microsoft’s BASIC used ASCII, DOS used ASCII. The Intel MDS machines also used ASCII, and that was where the PC BIOS was developed.

The ASCII/EBCDIC divide only caused a minor inconvenience for the PC development team when the BIOS listings (ASCII) were transferred to IBM’s mainframe systems before publication in the Technical Reference.

BIOS Interrupts

The Datamaster used interrupts as firmware entry points and this approach was continued on the IBM PC. While direct calls to known addresses were typical for similar machines at the time, the Datamaster could not easily use that approach because of paging.

While the software interrupt approach may seem unnatural, it turned out to be extremely flexible because it is easy to “hook” existing interrupts, add new functionality, and “chain” to the existing interrupt handler; the same concept as inheritance in object-oriented programming. IBM notably used this approach with the PC/XT hard disk controller (adding BIOS INT 13h for hard disks in a new ROM and chaining to the old BIOS service for floppy access) and the EGA (using add-on ROM to drive the EGA but falling back to the system BIOS for CGA/MDA support).

As it happens, DOS independently chose the same software interrupt approach to provide system services, with the same resulting flexibility and extensibility (and sometimes chaos).

PC Software

IBM’s original plan was to ship the PC with ROM BASIC and the CP/M operating system, both standard for personal computers at the time. Microsoft was the preeminent supplier of BASIC to OEMs and already had 8086 BASIC, so all IBM had to do was negotiate a contract for BASIC and implement the OEM interface required by Microsoft.

The PC shipped with a 32 KB ROM BASIC which was called ‘Cassette BASIC’, alluding to the fact that the BASIC ROM could only use an attached cassette tape for storage. As part of DOS, IBM also shipped Disk BASIC and Advanced BASIC (BASIC.COM and BASICA.COM) which both utilized the ROM BASIC and among other things provided additional “device drivers” allowing the ROM BASIC to use floppy disks for storage.

On the operating system side, things did not go so smoothly. There are many legends and conflicting stories, and not so many facts.

The biggest unanswered question is why the IBM PC did not ship with CP/M. There are many rumors about how Gary Kildall, the CP/M inventor and Digital Research (DRI) boss refused to meet with IBM executives and went flying instead, or how he flew in too late for a meeting. Which doesn’t really make any sense at all, since meetings can be rescheduled.

Other rumors say that Gary Kildall was not involved at all since it was his then-wife, Dorothy McEwen, who was in charge of negotiating OEM contracts. Another version of the story goes that she wouldn’t meet with IBM execs because she was already in a different meeting with HP and then went on vacation. Again, I do not find this credible. Meetings can be rescheduled.

I believe the real story is much more prosaic and straightforward: DRI did not have a product to sell. CP/M-86 simply didn’t exist in 1980, or really 1981 for that matter.

IBM was not going to wait and it was time for Plan B. Microsoft, already contracted to provide language tools for the IBM PC (assembler, Pascal, FORTRAN, etc.) needed an OS, and knew where to find one.

The DOS story is reasonably well documented. Back in 1979, Seattle Computer Products (SCP) started building 8086-based systems and needed an operating system. CP/M for the 8086 wasn’t available, and it was unclear when it might be. Tim Paterson stepped in and threw together QDOS, Quick and Dirty OS, soon renamed to 86-DOS: A bare-bones CP/M workalike that was good enough to manage files on a floppy and launch programs. The major advantage of 86-DOS was that it enabled relatively simple porting of existing 8085 CP/M applications to the 8086, largely accomplished through machine translation.

Microsoft bought 86-DOS for cheap, licensed it to IBM, and for much of the 1980s and the early 1990s, thus acquired a license to print money.

The IBM PC, Maker of Empires

It is fairly obvious that the IBM PC laid the foundations for two business empires, neither of them IBM.

Thanks to the IBM PC, DOS became the standard PC software and Microsoft was happy to license it to any OEM. For many years, Microsoft raked in cash from licensing DOS without needing to put much effort into improving the product. Digital Research briefly threatened Microsoft’s cash cow (and it’s clear that Bill Gates was very worried), but Microsoft managed to replace DOS with Windows before DRI could really eat into Microsoft’s bottom line.

Intel, by all appearances, stumbled into their x86 empire entirely by accident. The 8086 was considered a stopgap product. The 80286 was seen as a minor update and the 80386 started out as a sort of dead-end project, before turning into a matter of major strategic importance for Intel after the iAPX 432 abysmally failed. The PC effectively forced Intel to go the x86 route.

The importance of being in the right place at the right time cannot be possibly overstated.

Summary

The IBM PC development cycle was very short, only one year from the start of the design phase to a finished and announced product. The design was jumpstarted by heavily leaning onto the development team’s experience with the System/23 Datamaster. The core PC architecture was more Datamaster than not, with the notable exception of a CPU upgrade (Intel 8088 instead of 8085). The PC’s I/O subsystem, on the other hand, only had some (storage, communications) or barely any (display) relation to the Datamaster, setting new standards.

The tight schedule determined almost everything about the design of the PC, from the hardware (significant reuse of Datamaster design) to the software (using existing 3rd party software, no waiting for CP/M-86). The IBM PC was the right product at the right time, and its success and durability outstripped anyone’s wildest expectations.

Explainer

The design drawings use shorthand that may require explanation. Here’s an attempt to decode some of the acronyms and IBMese.

• CD: Card (feature card or I/O card)
• CH: Channel
• DEC: Decoding or decoder
• DRV: Drive
• 1LPC: 1 Line Per Channel—how many wires fit in the 0.1 inch space between pins on the components; higher LPC implies more expensive to manufacture
• MPU: MicroProcessor Unit aka CPU
• PCK: Parity Check
• Planar: System board
• ROS: Read-Only Storage aka ROM
• RQ/GT: Request/Grant

Sources

• The Creation of the IBM PC, David J. Bradley, BYTE, September 1990
• A Personal History of the IBM PC, David Bradley, IEEE Computer, August 2011
• Whence Came the IBM PC, Jon Titus, EDN, September 15, 2001
• Recollections of Gary Kildall, an interview with Gordon Eubanks by Clive Akass
• IBM PC Technical Reference, IBM, publication no. 6025008, August 1981
• IBM 5322 Computer Service Manual, IBM, publication SY34-0171-0, December 1980
• Personal correspondence with Dr. David J. Bradley