Commodore PET and CBM

The Commodore PET 2001 was one of the first three personal computers that came to market in 1977, launching the home computing era along with Tandy/Radio Shack and Apple. Commodore’s entry was unique for having an all-in-one design.

Before the PET

Much like Texas Instruments, Hewlett-Packard, Radio Shack, and Motorola, Commodore got its start long before personal computers arrived. It was founded in Toronto, Ontario, Canada in 1954 as the Commodore Portable Typewriter Company by Polish immigrant Jack Tramiel. In 1962, it was listed on the New York Stock Exchange as Commodore International.

When Japanese imports forced most North American typewriter manufacturers out of business in the late 1950s, Commodore moved to adding machines. Then in the late 1960s, Japanese adding machines hit the North American market, once again threatening Commodore’s existence.

Tramiel went to Japan to figure out how to compete and came back with the idea of producing electronic calculators instead of mechanical adding machines. Once again, Commodore had a successful product on its hands – until Texas Instruments (TI) flooded the market in 1975 with calculators that sold for less than Commodore was paying TI for parts (TI was one of Commodore’s important suppliers).

Tramiel decided to head in another direction, took out a $3 million loan, and acquired chip manufacturer MOS Technology, Inc. to assure a steady supply of chips for his gear. Part of the agreement was that Chuck Peddle, MOS Tech’s chip designer, would join Commodore.

KIM-1 single-board computer

Commodore’s First Computer

Before home computers, there were hobbyist computers. The KIM-1 was one of the first, developed as a way for MOS Technology to demonstrate its 6502 CPU. The KIM-1 was the world’s first single-board computer when it was released in 1976. It had 1 KB of memory, a 6-digit LED display, a cassette interface, and 15 input/output ports. It sold for $245 and only required a power supply and some sort of enclosure for the 9″ x 10″ board.

The KIM-1 became the basis for many different projects and laid the foundation for Commodore’s first personal computer, which was released the following year.

The Commodore PET

Commodore PET 2001Peddle convinced Tramiel that calculators were a dead end, so Commodore should turn its attention to the fledgling personal computer market (often called home computers back then). Peddle designed a machine with a metal case, a keyboard, a built-in monitor, and a built-in cassette tape drive for loading and saving software and data files. This was the Commodore PET 2001, which was displayed at the June 1977 Consumer Electronics Show and came to market in October 1977.

The PET was named in part for the pet rock fad, which lasted about six months during 1975. Silly as it sounds, 1.5 million pet rocks were sold during that period for $4 each, making their creator a millionaire. Officially, P.E.T. stood for Personal Electronic Transactor.

Apple II with 9-inch composite monitor and two floppy drivesThere were only two other home computers at that time: The Apple II, built around the same MOS 6502 CPU, had arrived in June 1977, replacing the Apple 1, which had been more of a do-it-yourself project. The Apple II sold for $1,298 with 4 KB of memory and could be used with a composite monitor or with a color or black-and-white TV (with an RF modulator) and cassette tape recorder, which many homes already had.

Radio Shack TRS-80 Model IThe other was the TRS-80 Model I, on sale at Radio Shack stores across the US since August 1977. It was based on the Zilog Z-80 CPU, and the 4 KB version retailed at $599, which included a screen and tape recorder.

The PET sold for $495 with the same 4 KB as its competitors, making it the value champion. Commodore could only produce 30 units per day at that time, and orders came in so quickly that Tramiel decided to raise the price to $595. Next Commodore started pushing the 8 KB version at $795, since the 4 KB model left only 3 KB available for the user. Commodore sold 500 PETs in 1977.

A Better BASIC

Commodore acquired a BASIC license from Microsoft, allowing it to install Commodore BASIC on as many machines as it could produce for a single, one-time fee. Over the years, it was installed on tens of millions of Commodore computers. Microsoft BASIC included floating point operations, a Commodore advantage; the BASIC used in the TRS-80 and Apple II at that time only worked with integers.

The Datasette

The Datasette tape drive reads and writes data to a cassette tape at 1500 bits per second, but the computer reads and writes data twice to verify integrity as well as using a parity bit. Unlike the cassette tape units used with other personal computers, the Datasette is a digital device, not analog.

It would take over 2 minutes to load an 8 KB program from tape, something developers addressed starting in 1980. The PET Rabbit for 16 KB and 32 KB PET and CBM (Commodore Business Machines) computers used routines that made saving and loading data 4x as fast. At $30, it was a lot less expensive than a disk drive.

The Display

The original PET has a 9″ display showing 25 lines of text with up to 40 characters per line (this uses just 1000 Bytes of memory). When Commodore introduced the PET 4000 series, it displayed the same amount of text, but on a 12″ screen, and the CBM 8000 family provided 80 columns of text, twice as much as earlier Commodores (also using twice as much system memory, 2000 Bytes), on the same 12″ display.

The Keyboard and Character Set

Commodore PET 2001 keyboard

One clever feature of the PET and later Commodore computers was the PETSCII character set, also known as CBM ASCII. In addition to the standard characters found on a keyboard, Commodore computers had a whole range of graphical symbols that could be accessed from the keyboard – these characters were printed on the keycaps for easy access.

Commodore PET 2001-N with a real keyboardThe biggest complain about the original PET was its keyboard, which was laid out in a grid (above) and not like a typewriter keyboard. Within a year Commodore introduced the PET 2001-N (right), which removed the tape drive and added a standard keyboard along with a numeric keypad (something the TRS-80 had but the Apple II did not).

In 1978, Commodore expanded into Europe, where it sold its computers for twice as much as in the States – and it found ready buyers. European models were called CBM models, since Philips had a trademark on the PET name. The first models were the CBM 3008 (8 KB), 3016 (16 KB), and 3032 (32 KB).

Floppy Drives

In 1979, Commodore introduced its first floppy drive, the CBM 2040 dual drive with DOS 1.0 built into it. Like later Commodore floppy drives, it is an intelligent device with its own CPUs (two of them) and RAM that connected to the PET’s IEEE-422 parallel port. Each drive can store 170 KB of data on a single-sided 5-1/4″ floppy. Single drive mechanisms came later.

Because Commodore DOS was built into the floppy drive, you had to buy a new drive to get the latest version of DOS. The 2040 had DOS 1.0, the 4040 used 2.0 and later 2.1, the 8050 included DOS 2.5 and support for 500 KB high-density disks, and the 8250 had DOS 2.7 and could store 1 MB on a double-sided 5.25″ high-density floppy.

Improving PET

By 1980, the PET had moved from its original 9″ display to a 12″ screen, and along the way the metal case had been replaced by a plastic one. The European versions of the updated PET were the called the CBM 4016 and 4032.

Commodore also introduced the 8000 series, which featured 80 characters per row instead of 40 and was geared more toward business use.


In 1981, Commodore designed a new computer for programmers. It was designed in conjunction with the Computer Systems Group at the University of Waterloo in Waterloo, Ontario, Canada. Founded in 1957, the University of Waterloo was known for its strong engineering and mathematics programs, especially computers. The university developed many software tools for programmers starting in the 1960s and licensed them to meet growing demand from other organizations.

To help with demand from computer science students, the university already had a number of Commodore computers on campus to take pressure off the mainframe, which supported only 30 simultaneous users.

The SuperPET 9000, also known as a Micro-Mainframe, shared most of its design features with the CBM 8000 family, but it added a second system board with a 6809 CPU and 64 KB of system memory. It also had a built-in RS-232 display, so it could be connected to an IBM mainframe. This allowed programmers to work from home and then transfer their programs to other computers.

The SP 9000 was initially build in Canada by Commodore’s Canadian division and included Waterloo Structured BASIC, APL, Fortran, Pascal, and COBOL programming languages. It was possible for CBM 8032 owners to upgrade their machines, turning them into SuperPETs.

The SuperPET went on sale to the public in September 1981, barely a month after the original IBM PC had first shipped, and it never caught on in the broader personal computing market. In September 1982, Commodore announced that it sold only 7,000 SP 9000s – less than the number of VIC-20 computers it could build in one day.

Although the SP 9000 remained in production, it was largely ignored, and Commodore finally ended production in 1985.

Further Reading

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