Inside the IBM PC 5150: The first-ever IBM PC

As the forebear of all PCs turns 35, we take a look inside this pioneering machine.

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Credit: Benj Edwards
The first IBM PC

35 years ago (today), IBM launched the most influential commercial computer system of all time, the IBM PC 5150. Over the past three and a half decades, architectural descendants of this single machine have taken over the desktop, workstation, server, and even game console markets. And despite inroads from ARM-based smartphones, its digital descendants are still relied upon for just about all the heavy lifting in the computer industry.

On the anniversary of such a monumentally important computer, I thought it would be instructive to take a deeper look into the machine that started it all. How? By taking apart one of these bad boys on my trusty workbench, of course. And that’s exactly what you’ll see in the slides ahead.

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Credit: Benj Edwards
IBM's CGA display

Before we take apart the IBM PC, I’d like to point out a few peripheral components that come together to form a complete IBM PC system. The most obvious component is the monitor. Many monitors were available for the IBM PC in the years following its release, including an IBM brand monochrome unit and the IBM CGA monitor you see here (on which I'm playing the shareware classic ZZT). This monitor required the optional purchase of a CGA video card. In its standard 320x200 graphics mode, CGA could only display four colors at a time from a palette of 16. In this case, ZZT uses text-mode graphics, so it can display any of the 16 colors at once.

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Credit: Benj Edwards
The first IBM PC keyboard

The first IBM PC keyboard, seen here, borrowed heavily from the industrial-strength IBM System/23 Datamaster computer which preceded it. The keyboard is hefty (six pounds!), loud, and clicky, and its layout was slightly unusual at the time of its launch. (It wouldn’t be until the Model M keyboard in 1984 that the standard 101-key layout we all know and love today would be finalized.)

Despite its awkward layout, this first PC keyboard won high praise from critics for its precision and durability. You could knock someone stone cold unconscious with it and it would still work. In the 1980s, PCWorld editors used to test computers that way.

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Credit: Benj Edwards
Cracking the case

The only things separating us from the inside of an IBM PC were five precision flathead IBM screws, which came out easily. The heavy gauge sheet metal case slides off with no trouble, exposing the machine’s tender insides.

You can’t see it too clearly in this photo, but I always like to point out that the IBM PC first shipped with a cassette drive port (right next to the keyboard port), which allowed users without floppy drives to save their IBM BASIC programs to an audio cassette tape. In 1981, floppy drives were an expensive option, so IBM covered every possible market segment with custom build options—from bare-bones to decked-out.

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Credit: Benj Edwards
Everything was bigger back then

Look at all that circuitry crammed into this metal case. It’s huge, heavy, and throws off a lot of heat. Today, we have incredibly more complex systems with tiny, integrated surface-mount components on double-sided circuit boards. Back then, electronic components were still relatively big and discrete (although incredibly miniature compared to that which came before). Such is progress. Let’s take a closer look at what’s inside.

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Credit: Benj Edwards
The ISA slots

The longevity and versatility of the IBM PC came from its user-expandable nature, a concept which IBM borrowed heavily from the Apple II and S-100 based computer systems that preceded it. With five ISA (Industry Standard Architecture) slots on the motherboard, users could plug in just about any combination of peripheral expansion cards (each its own circuit board with its own capabilities) to customize the machine however they wanted. Users could install different interface boards for serial ports, parallel ports, removable drives, and more; new graphics cards; audio cards; and even an early memory expansion board, which we’ll see ahead.

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Credit: Benj Edwards
Peripheral cards

These were the cards installed in my particular IBM PC, a system which I bought second-hand from its original owner many years ago. From top left, clockwise, we see a combination parallel and serial port card (it’s tiny and of a later era); an expansion memory board, which brought the IBM PC to 640 KB of RAM; a CGA video card; and a floppy disk controller card. Of these, only the floppy controller is an original IBM part, which further illustrates how third-party-extensible the PC was.

In the first PC, no ports other than keyboard and cassette were integrated, so unless you were going to stick with IBM’s BASIC language (which was included in ROM), you needed to use at least a few ISA cards to create a practical system.

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Credit: Benj Edwards
Removing the disk drives

At launch, the cheapest IBM PC came with no floppy drives and 16KB of RAM. For an extra fee, users could buy a single-sided, full-height 5.25-inch floppy disk drive option that stored 180KB of data per disk. Not long after that, IBM introduced a double-sided, double-density drive that stored 360KB per disk, which is what my unit came with. (Five years ago while playing with the IBM PC for PCWorld, I added a second DS/DD disk drive (half-height) to make using the machine easier.)

Here you can see the full-height floppy drive, which weighs almost four pounds, removed from the chassis. Also notable is the IBM PC’s speaker (seen here on the table attached to its rusty orange plastic mount), which was the first-ever sound output for PCs. It could only produce clicks, buzzes, and beeps.

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Credit: Benj Edwards
Removing the motherboard

Compared to modern PCs, it is mercifully easy to remove the IBM PC’s motherboard, which is held in place with two screws and four plastic stand-offs. After taking out the screws (and unplugging everything, of course), the motherboard slides right out from the side. All that’s left in the chassis now is the hulking power supply, seen here with it’s spider-like appendages.

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Credit: Benj Edwards
The motherboard up close

Electronically, the motherboard is the heart and the soul of the IBM PC. There’s lots going on. In the lower-right corner we see RAM chips that bring this machine up to 256KB of RAM (a further 512K comes from the ISA card we saw earlier). In the lower-left corner we see the five 8-bit ISA expansion slots that hold those peripheral cards. And the CPU is in the upper left, which we’ll take a closer look at in a moment. All the other chips on the motherboard don’t really do anything useful (kidding!).

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Credit: Benj Edwards
The brains of the IBM PC

The IBM PC shipped with a 16-bit 4.77MHz Intel 8088 CPU, a version of the Intel 8086 with an 8-bit bus. IBM chose the 8088 because its 8-bit bus meant that IBM could utilize relatively inexpensive 8-bit support chips, drastically lowering the cost of the system. As an added bonus, the team of engineers behind the PC also had experience designing with those 8-bit chips from the previous Datamaster project.

When I bought this IBM PC, it came with a surprise: a Zilog V20 CPU that served as a drop-in replacement for the machine’s native 4.77MHz Intel 8088. The V20 provided a slight speed boost with some applications. Years ago, I replaced the V20 with a Fujitsu-manufactured 8088 (seen here near the center of the photo) for authenticity’s sake. Today it hums along just as good as new.

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Credit: Benj Edwards
How far we've come

As I remove the last component from the IBM PC—the system’s four-pound, 150-watt power supply—let’s take a moment to reflect (pun intended) on how far we’ve come since 1981. My Apple iPhone 6 Plus sitting beside it on the table weighs about six ounces and is vastly more capable than an IBM PC, all in a tiny, portable package that fits in a pocket.

It’s incredible progress. But now that I think of it, will the iPhone work 35 years from now? Most likely not. The fact that my IBM PC still boots and runs like new (with minimal repairs over the years), is a testament to its tank-like build quality. It seems poetically fitting that the IBM PC’s functional longevity is just as robust as its cultural one has been over these past three and a half decades. Happy birthday, IBM PC.