When you watch documentaries about early computer innovations, particularly the late 70s, early 1980s, most of the documentaries tend to focus on Apple and Microsoft, and maybe IBM as the big innovators. But, I think often companies like Commodore, and Atari, and Tandy don’t get nearly enough credit for the role that they played.  Let’s take a look at the Commodore PET!

Most of my readers are familiar with the Commodore 64, one of the best selling computers of all time. Well renowned for its great graphics and sound, but Commodore history didn’t start with this machine. So, let’s go back a little bit to the late 1970s and figure out where it all started!

Commodore History Part 1 Video

The Processor that Started it All

It all began in 1974 when Chuck Peddle and a group of engineers started up a chip fabrication company called MOS Technology. Most of these guys had worked at Motorola on the 6800 processor, and so they set out to develop a compatible CPU known as the 6501 that could simply be substituted for the much more expensive Motorola CPU. As you might imagine, Motorola sued and to make a long story short, the 6502 was born, which was pretty much the same chip but changed just enough that it was no longer completely compatible with the 6800.

The 6502 Processor

Since the chip was no longer compatible with the 6800, customers would need some way to test the chip. In 1976 Chuck Peddle also designed the KIM-1 development computer. This was a single board computer that used the 6502, and could be programmed in machine language from the keypad on the top. However, later it was possible to connect a dumb-terminal display and actually run BASIC. Programs could be saved to a cassette tape. The computer proved to be popular with hobbyists as well as engineers.

The 6502 would go on to be a huge success and eventually found its way into the Apple II series, the Atari 2600, the Nintendo Entertainment System, the entire line of Atari 8-Bit computers, the BBC Micro, and of course the entire line of Commodore 8-Bit machines. But back to 1976 for the moment. MOS Technologies was bought up by Commodore Business Machines, who at this point was primarily in the calculator business. Chuck Peddle managed to convince Commodore boss Jack Tramiel that calculators were a dead-end business and that they needed to produce a computer to compete with the upcoming Apple II.

The MOS Technology KIM-1

In 1977 the Commodore PET 2001 was born, using much of the same design as the KIM-1. Much like the Apple II, the PET was all inclusive, having an integrated monitor, keyboard, and cassette tape storage device. 1977 was a big year for the personal computer revolution. With the market introduction of the big 3, the Apple II, Commodore PET, and TRS-80 computer, it was the first time that a regular person could buy an affordable computer without having to assemble it themselves.

The cool thing about the PET is that it was actually designed by the same guy who designed the 6502 processor. If you look at the prices of the big 3, you’ll see the PET was competitively priced. Although the Apple II did have superior hardware, which we’ll get into shortly, the PET did have the advantage that it came with a monitor and tape drive, where the Apple II required those as separate purchases.

A Closer Look at the PET

Let’s take a closer look at the design of the Commodore PET. The first thing I want to draw your attention to is the keyboard. It’s insane, and it will drive you insane if you actually try to type on it. One thing that isn’t communicated well by video and pictures is just how small this keyboard is. The main part, excluding the number pad measures just 6in. by 2.75in. Just to put that into perspective, my iPhone 6 will essentially cover the entire keyboard. The Apple mini keyboard is actually huge by comparison.

the pet keyboard

The keyboard size isn’t the only problem. They layout is crazy. While the keys are technically in a QWERTY arrangement, normally the rows are offset creating diagonal lines. Not so on the PET. They are squared up. The weirdness doesn’t end there. The space bar is tiny! Normally you would expect numbers across the top row of keys, but there aren’t any. Instead you have just symbols. If you want to type a number, you have to use the number pad.

PET keyboard is insane

What is even more infuriating is the symbols. For example, if I want to type a Dollar sign or a number sign, I would instinctively press shift before the key. However, when I do that I wind up with a totally different character instead of the one I wanted. Then there’s the cursor keys. Notice that there are only 2 of them. One key cursors down, and the other key cursors right. If you want to reverse that, you have to hold down the shift key. So by using the combination of the cursor keys and shift you can cursor anywhere on the screen.  The little back arrow? You might think that’s a backspace key. But, it’s not. It actually prints that character on the screen, and so when you make a mistake, and believe me you will, you’re going to go to push this key and it’s not going to fix your mistake and you’re going to go even more insane than you were before. The actual delete key is all the way over at the other side of the number pad.

To be fair, when this computer came out in 1977, most of the customers had never even used a personal computer before or a computer of any kind and so they didn’t have any pre-conceptions for what a keyboard layout should be – like we do today. It probably wasn’t quite as weird for them as it would be for us.

Opening the Commodore PET

Let’s take a look inside the PET, it opens like the cab of a semi truck, and even gives you a little kick stand to hold it open. Looking at these 16 RAM chips, you might think the PET came with a lot of RAM. But, you’d be wrong. The original PET only came with 4K of RAM. These are 1K by 4-bit static RAM chips. Being that cost was such a consideration for this computer, you might be wondering why they didn’t use the cheaper dynamic RAM, or DRAM? Well, static ram was and still is today much more expensive than dynamic RAM. However, DRAM has one drawback, it requires that it is refreshed every so often, which requires additional circuitry to handle that. When you’re dealing with only 4K, it actually ended up being cheaper just to use static RAM.

opening the PET

OK. So 4K is a ridiculously small amount or RAM, but it’s worse than that because the operating system actually needs at least 1K of that, leaving about 3K left over for the user. So how much is 3K of RAM? Well, the screen on the pet is 40-characters by 25-lines, meaning you need 1,000 bytes of RAM, or almost an entire kilobyte just to store one screen full of text. Essentially you had enough RAM for about 3 screens of text! To be fair though, the Apple II and TRS-80 only had 4K when they came out as well.

PET RAM chips

This particular PET has been upgraded a little daughter board. It’s an aftermarket 32K RAM expansion module, and that’s why it shows 31K available to BASIC on the boot screen. Let’s take a closer look at this cassette drive. This was actually just an off-the-shelf cassette recorder that Commodore bought and slightly modified. You can see the whole unit is actually mounted, in a rather clunky way in my opinion. The cassette drive was really the only storage device available for the PET at first. And with 4K of RAM, this wasn’t much of a problem.

The PET Disk Drives

It wasn’t until 1979 that Commodore came out with a matching disk drive. Since the PET was never really designed to use a disk drive, they decided to use the IEEE-488 parallel port as a means to connect the disk drive.  Unlike the Apple II, the Commodore PET has no card slots inside so there’s nowhere to add a floppy disk controller card. So, what they had to do was essentially design an entire computer inside the floppy drive unit, which would handle controlling the floppy drives as well as an entire disk operating system.

Indeed if you take a look inside the disk drive, you’ll see it’s quite sophisticated, having it’s own 6502 processor, RAM, ROM, and I/O controllers. The PET didn’t really interact with information on the disks directly, rather it would send commands to the disk drive, such as telling it to fetch a file, and then the disk drive would take care of all of the work of finding the right data on the disk. In fact, it could even copy files or even entire disks from one drive to the other all by itself, just with a single command.

The PET was popular with schools and found its way into many computer labs. And while the disk drive was expensive, one of these floppy drive units could actually be connected to multiple PETs at the same time, thus saving space and money. In fact, you can see this arrangement being used in this photo from a computer lab where each table has 8 PETs connected to a single floppy drive and printer.

chained PETs to disk drives

The Commodore PET’s Display

Let’s talk about the screen on the PET. The original model here is black and white. A lot of people assume it’s green, but that actually wasn’t until later models. The original one was black and white. In fact, there’s not even any grayscale. It’s just literally two colors, black and white. The screen was controlled by a clone of the Motorola 6845 CRT controller, which was also used in the IBM CGA card, among other computers. However, there was no circuitry here for color.

Commodore PET Display

There are also no graphics modes. I mean, literally none. There is no way to put graphics on this machine at all. And what’s worse is that the character set is in ROM and it cannot be moved, so there’s no way to modify what the characters look like. So, you are pretty much stuck with putting characters on the screen and only the characters that came built into ROM. And that’s it.


However, there are 256 characters using a special character set called PETASCII or later just shortened to PETSCII. The character set contains a lot of custom symbols and can be used together to create rudimentary graphics. There are also a set of 16 block character patterns that can be used to create rudimentary pixels. In fact,


I wrote a program a while back called PETDraw, which actually runs on the PET. It allows you to pick characters and place them wherever you want on the screen. Of course, you don’t actually need a special program to do this because the different characters are on the keyboard itself, all you have to do is use the SHIFT key to type them. However, my program makes it easier, especially if you’re using an emulator with a regular computer keyboard. Plus it has a special pixel draw mode that automatically figures out which of the 16 block characters to use and allows you to freely draw in a matrix of 80 x 50 pixels. This program is available for free on my website here. The PET actually does have two character sets. I’ve been showing you the default, but if you type POKE 59468,14 then it will switch to an upper/lower case set, which looks better for typing, but has fewer graphics characters.

Sound on the PET (or lack of)

The PET also had no sound.. I mean, like literally none. There is no speaker and there’s no audio output anywhere. However, early PET users figured out that they could use two wires on the user-port, which connected to a handshake register on one of the I/O controllers and it could essentially operate as a one-voice square wave generator. Soon after, many software programs started using this method as the default method for sound production on this computer. Interestingly enough, the same concept will actually work on any of the Commodore 8-Bit machines user port. However it was generally not needed because all of these machines already have a sound synthesizer built in.

PET user port sound

The Redesigned PET

Eventually Commodore redesigned the PET. This 4016 model has a very similar look, but you’ll notice the screen is quite a bit larger, and now uses a green phosphor display instead of the black and white of the original model. The keyboard is redesigned so that it’s easier to type on, however most of the weird key arrangement I mentioned earlier still remains. The cassette drive is gone. At this point you’d have to use an external cassette drive or a floppy drive. The case opens in a similar manner, but the stand is in a different place. Also the case is now plastic, instead of the metal found on the original PET. Also, they have included a speaker inside the computer so it can produce sound without needing to connect anything to the user port.

It’s obvious that the Commodore PET was designed in the United States for US customers. Everything in the design is inches based. This means you may need to break out the fraction to decimal conversion chart when talking to your buddies across the pond about them!

Notice this is a 4016, which basically means it has a 40 column display with 16 kilobytes of RAM. Although this one has been upgraded to 32K, which seemed to be the standard on later PETs. But they also had several different models in this numbering scheme, with the high end models having 80 columns display and as much as 96K of RAM. There were also some other weird variations including one called a SuperPET. The PET would later evolve into Commodore’s CBM line of business computers which kept a similar form factor, but much more ergonomic with a detachable keyboard. I’ve never actually seen any of the CBM line in person as I think they were mostly sold in Europe. However, the 40-column PETs seem to have the most software available for them. And, generally speaking, software designed for the 40-columns won’t run on the 80-columns and vice-versa.

Commodore PET 4016

BASIC on the PET

Next I want to talk about a cool feature of Commodore BASIC. Using the screen editor, if I press the reverse key, it will change the text to reverse. I can press shift and reverse to disable that feature. Pressing the clear/home key by itself, it will return the cursor to the top of the screen. If I hold down shift and press that key, it will clear the screen.

If I use these commands in a BASIC program. Once I type quotation marks, when I press the clear key, it just leaves a little symbol. Then, I can type some text and maybe use the reverse key. It leaves another little symbol. I can even use the cursor keys. When I go to execute this program, it will actually remember all of the screen commands, which is cool because it makes formatting text in your BASIC programs very simple!


Speaking of BASIC, the PET used Microsoft BASIC right from the beginning. And while the Apple II and TRS-80 did eventually offer Microsoft BASIC, they both used proprietary forms of BASIC during the first year or two on the market. And being that Microsoft BASIC was only designed in 1975, just 2 years before this machine came on the market, you could pretty easily make the case that this was probably the first mass-market computer that actually shipped with Microsoft BASIC.

In the early PET models, if you typed in WAIT 6502,1 it would show an Easter egg from Microsoft. In fact, if you change the second argument in the command to a different number, it will display the Microsoft logo that many times. I’ve actually looked at the code and it is clear that Microsoft essentially encrypted their name into this so that it could not be easily found. Apparently Commodore was furious about this, mostly because space was very limited on this machine and those Easter egg routines took up precious space on a machine where every byte counts.

Gaming and Demos on the Commodore PET

Despite the limited graphics and sound capabilities, there are no shortage of games for the PET. Most of the games were designed for the 40-column version of the PET and there seems to be a lot of incompatibilities between PETs with different keyboard types as well. Millipede is surprisingly playable and enjoyable. As is Space Invaders. Some of these graphics are so cleverly designed using the built in character set, that you could almost be tricked into thinking these were bit-mapped graphics. My friend Robin Harbron ported his RPG style game Minimae over to the PET as a fun project. There’s also a pretty good version of Tetris available and it even uses digitized sounds, but it requires a digital-to-analog converter connected to the user port similar to the Covox system that was available for the IBM platform.

The Commodore PET has also made it into the demo scene a few times. Most notably this demo by ORB, which exploits a new found way to display graphics on the PET. What they’re doing here is using some critical timing to reset the video chip every scan line so that it only displays the top line of every character. So by carefully using characters who’s top row of pixels more or less match up with what they need, they can almost do pixel-perfect bitmaps. This demo also exploits the use of a SID chip if you happen to have one installed in your PET. Of course, Commodore never put SID chips in the PET, but it is possible to wire one in yourself if you want. Not that there’s a lot of software that will support it, though.

PET demo scene ORB

Can you Damage the PET via Software?

So, one mantra I’ve always had when talking with inexperienced computer users who are afraid to try things on their computer. I’ll always tell them “Look, there’s nothing you can do. Nothing you can type on this keyboard or click with your mouse that can possibly damage the computer. The worst thing you could do is erase or corrupt some data, and that can always be restored.” However, it turns out in the case of the Commodore PET that that advice is not always true! It was discovered that on certain models if you typed the command POKE 59458,62 it would throw the CRT display out of sync, and after a short time could actually damage some of the circuits and burn out the display. This was referred to as the KILLER POKE. I think I’ll go ahead and test the killer poke right here on this machine. After all, this isn’t mine, it actually belongs to The Obsolete Geek. Surely he won’t mind!

Steve Gray decided to modify his PET so that it could display color on an external monitor… and eventually integrated the color CRT inside of the PET. This required extensive modifications to the logic board and Kernal ROMs to make this possible, but it is certainly a neat project.

The Commodore PET in Movies and TV

The Commodore PET has also been featured in countless movies and TV shows. Just to show a few of my favorite, you can see one in Star Trek II, the wrath of Khan. It’s apparently part of Captain Kirk’s antique collection. You can also clearly see one in many episodes of the IT crowd. Apparently Steven Hawking used one as shown in the movie The Theory of Everything. At the end of Terminator 3, when they arrive at the fallout shelter, you can clearly see a Commodore PET behind John Connor. And in the movie Wayne’s World, the local TV station has a Commodore PET in the office.

Collecting Commodore PETs: Should You?

The PET makes a great collector’s item. And, if you have room for one in your house, they make a great thing to put on display. It’s definitely a good conversation starter when people come over to visit. But truth be told they’re really not that great of computers to actually use. I know a lot of vintage computer enthusiasts. Of those, I don’t know anybody who really wants to spend hours on end sitting in front of a PET. I mean, they’re pretty cool to look at and they have a really cool place in history, but they’re not really all that much fun to actually use!

More in the Commodore History Series

In the next few articles and episodes I’m going to be covering the Commodore VIC-20, the Commodore 64, the Plus/4, the Commodore 128, etc. ne of the things that you’re going to see about those designs is that they were not revolutionary, rather they were evolutionary from the PET. You’re going to see a lot of technology carried over to those machines. And many of the characteristics of these machines can be traced back to their origins at the PET or perhaps even further back all the way to the KIM-1!