Type Home computer
Release date 1980 (VIC-1001) / 1981
Discontinued 1985
Operating system Commodore BASIC 2.0
CPU MOS Technology 6502 @ 1.108404 MHz (PAL) [1] @ 1.02 MHz (NTSC)
Memory 20 KB total, 5 KB for BASIC Programming
Graphics VIC 176 x 184 3-bpp
Sound 3x square, 1x noise, mono.[2]
Predecessor Commodore PET
Successor Commodore 64

The VIC-20 (Germany: VC-20;[3] Japan: VIC-1001) is an 8-bit home computer which was sold by Commodore Business Machines. The VIC-20 was announced in 1980,[4] roughly three years after Commodore's first personal computer, the PET. The VIC-20 was the first computer of any description to sell one million units.[5]


Origin, marketing

The VIC-20 was intended to be more economical than the PET computer. It was equipped with only 20 kB of RAM (of this, only 5 KB were available to the BASIC programmer) and used the same MOS 6502 CPU as the PET. The VIC-20's video chip, the MOS Technology VIC, was a general-purpose color video chip designed by Al Charpentier in 1977 and intended for use in inexpensive display terminals and game consoles, but Commodore could not find a market for the chip. As the Apple II gained momentum with the advent of VisiCalc in 1979, Jack Tramiel wanted a product that would compete in the same segment, to be presented at the January 1980 CES. For this reason Chuck Peddle and Bill Seiler started to design a computer named TOI (The Other Intellect).

The TOI computer failed to materialize, mostly due to the fact that it required an 80-column character display which in turn required the MOS Technology 6564 chip. However, the chip could not be used in the TOI since it required very expensive static RAM to operate fast enough. In the meantime, freshman engineer Robert Yannes at MOS Technology (then a part of Commodore) had designed a computer in his home dubbed the MicroPET and finished a prototype with some help from Al Charpentier and Charles Winterble. With the TOI unfinished, when Jack Tramiel was confronted with the MicroPET prototype, he immediately said he wanted it to be finished and ordered it to be mass-produced following a limited demonstration at the CES.

The prototype produced by Yannes had very few of the features required for a real computer, so Robert Russell at Commodore headquarters had to coordinate and finish large parts of the design under the codename Vixen. The parts contributed by Russell included a port of the operating system (kernel and BASIC interpreter) taken from John Feagans design for the Commodore PET, a character set with the characteristic PETSCII, an Atari 2600-compatible joystick interface, and a ROM cartridge port. The serial IEEE 488-derivative interface (which could use far cheaper cabling than a real IEEE-488 as was used on the PET)[6] was designed by Glen Stark. Some features, like the memory add-in board, were designed by Bill Seiler. At the time, Commodore had an oversupply of 1 kbit×4 SRAM chips, so Tramiel decided that these should be used in the new computer. The end result was arguably closer to the PET or TOI computers than to Yannes' prototype, albeit with a 22-column VIC chip instead of the custom chips designed for the more ambitious computers.

In April 1980 at a meeting of general managers outside London, Jack Tramiel declared that he wanted a low-cost color computer. When most of the GMs argued against it, he said: "The Japanese are coming, so we will become the Japanese." This was in keeping with Tramiel's philosophy which was to make "computers for the masses, not the classes". The concept was championed at the meeting by Michael Tomczyk, newly hired marketing strategist and assistant to the president, Tony Tokai, General Manager of Commodore-Japan, and Kit Spencer, the UK's top marketing executive. Then, the project was given to Commodore Japan; an engineering team led by Yash Terakura created the VIC-1001 for the Japanese market. The VIC-20 was marketed in Japan as VIC-1001 before VIC-20 was introduced to the US.

When they returned to California from that meeting, Tomczyk wrote a 30-page memo detailing recommendations for the new computer, and presented it to Tramiel. Recommendations included programmable function keys, full-size typewriter-style keys, and built-in RS-232. Tomczyk insisted on "user-friendliness" as the prime directive for the new computer, and proposed a retail price of US$299.95. He recruited a marketing team and a small group of computer enthusiasts, and worked closely with colleagues in the UK and Japan to create colorful packaging, user manuals, and the first wave of software programs (mostly games and home applications).

Scott Adams was contracted to provide a series of text adventure games. With help from a Commodore engineer who came to Longwood, Florida to assist in the effort, five of Adams's Adventure International game series were ported to the VIC. They got around the limited memory of VIC-20 by having the 16 kB games reside in a ROM cartridge instead of being loaded into main memory via cassette as they were on the TRS-80 and other machines. The first production run of the five cartridges generated over $1,500,000 in sales for Commodore.

While the PET was sold through authorized dealers, the VIC-20 primarily sold at retail—especially discount and toy stores, where it could compete more directly with game consoles. It was the first computer to be sold in K-Mart. Commodore took out advertisements featuring actor William Shatner (of Star Trek fame) as its spokesman, asking: "Why buy just a video game?" Television personality Henry Morgan (best known as a panelist on the TV game show I've Got a Secret) became the iconic voice on a series of clever Commodore product ads.

The VIC-20 had 5 kB of RAM (netted down to 3.5 kB on startup, exactly 3583 bytes), which is roughly equivalent to the words and spaces on one sheet of typing paper, meeting a design goal of the machine. The computer was expandable up to 40 kB with an add-on memory cartridge (a maximum of 27.5 kB was usable for BASIC). Although the VIC-20 was criticized in print as being underpowered, the strategy worked.

In 1981, Tomczyk contracted with an outside engineering group to develop a direct-connect modem-on-a-cartridge (the VICModem), which at US$99 became the first modem priced under US$100. The VICModem was also the first modem to sell over 1 million units. VICModem was packaged with US$197.50 worth of free telecomputing services from The Source, CompuServe and Dow Jones. Tomczyk also created an entity called the Commodore Information Network to enable users to exchange information and take some of the pressure off of Customer Support inquiries, which were straining Commodore's lean organization. In 1982, this network accounted for the largest traffic on CompuServe.


In 1982 the VIC-20 was the best-selling computer of the year, with 800,000 machines sold. One million had been sold by the end of the year and at one point, 9000 units a day were being produced. That summer, Commodore unveiled the Commodore 64, a more advanced machine with 64 kB of RAM and considerably improved sound and graphics capabilities. Sales were slow at first due to reliability problems and lack of software, but by the middle of 1983, the latter had turned into a flood and VIC-20 sales abruptly plunged. It was quietly discontinued in January 1985.[7]


Because of its small memory and low-resolution display compared to some other computers of the time, the VIC-20 was primarily used for educational software and games. However, productivity applications such as home finance programs, spreadsheets, and communication terminal programs were also made for the machine. Its high accessibility to the general public meant that quite a few software developers-to-be cut their teeth on the VIC-20, being introduced to BASIC programming, and in some cases going further to learn assembly or machine language. A young Linus Torvalds, the eventual creator of Linux, was given a VIC-20 as his first computer. Another notable software developer who began his computing career with a VIC-20 was the OpenBSD creator Theo de Raadt.

Several computer magazines sold on newsstands, such as Compute! and CBM-produced publications such as Commodore Power Play, offered programming tips and type-in programs for the VIC-20. Many VIC users learned to program by entering, studying, running, and modifying these type-ins.

The ease of programming the VIC and availability of an inexpensive modem combined to give the VIC a sizable library of public domain and freeware software, although much smaller than that of the C64. This software was distributed via online services such as CompuServe, BBSs, as well as offline by mail order and by user groups.

The VIC's low cost led to it being used by the Fort Pierce, Florida Utilities Authority to measure the input and output of two of their generators.[8]

As for commercial software offerings, an estimated 300 titles were available on cartridge, and another 500+ titles were available on tape. By comparison, the Atari 2600—the most popular of the video game consoles at the time—had a library of about 900 titles near the end of its production life (although many titles were extremely similar). Most cartridge games were ready to play as soon as the VIC-20 was turned on, as opposed to games on tape which required a time-consuming loading process. Titles on cartridge included Gorf, Cosmic Cruncher, Sargon II Chess, and many others. A handful of disk applications were released for the VIC-20.

Technical specifications

BYTE in 1983 published a series of technical articles on the VIC-20:

Basic features

The VIC-20 had proprietary connectors for program/expansion cartridges and a tape drive (PET-standard Datassette). It came with 5 kB RAM, but 1.5 kB of this was used by the system for various things, like the video display (which had a rather unusual 22×23 char/line screen layout), and other dynamic aspects of the ROM-resident BASIC interpreter and KERNAL (a low-level operating system). Thus, only 3583 bytes of BASIC program memory for code and variables was actually available to the user of an unexpanded machine.

The computer also had a single DE-9 game controller port, compatible with the digital joysticks and paddles used with Atari 2600 videogame consoles[9] (the use of a standard port ensured ample supply of Atari-manufactured and other third-party joysticks; Commodore itself offered an Atari-protocol joystick under the Commodore brand); a serial bus (a serial version of the PET's IEEE-488 bus) for daisy chaining disk drives and printers; a TTL-level "user port" with both RS-232 and Centronics signals (most frequently used as RS-232, for connecting a modem[10]).

Importantly, like most video game consoles and many computers at the time the VIC had a ROM cartridge port to allow for plug-in cartridges with games and other software as well as for adding memory to the machine. Port expander boxes were available from Commodore and other vendors to allow more than one cartridge to be attached at a time. Cartridge software ranged from 4 - 16 kB in size, although the latter was uncommon due to its cost and only larger software houses produced 16 kB cartridges.

The graphics capabilities of the VIC chip (6560/6561) were limited but flexible. At startup the screen showed 176×184 pixels, with a fixed-colour border to the edges of the screen; since an NTSC or PAL screen has a 4:3 width-to-height ratio, each VIC pixel was much wider than it was high. The screen normally showed 22 columns and 23 rows of 8-by-8-pixel characters; it was possible to increase these dimensions but the characters would soon run out the sides of the monitor. Like on the PET, 256 different characters could be displayed at a time, normally taken from one of the two character generators in ROM (one for upper-case letters and simple graphics, the other for mixed-case—non-English characters were not provided). Normally, the VIC-20 was operated in high-resolution mode whereby each character was 8×8 pixels in size and used one color. A lower-resolution multicolor mode could also be used with 4×8 characters and three colors each, but it was not used as often due to its extreme blockiness.

The VIC chip did not support a true bitmap mode, but programmers could define their own custom character set. It was possible to get a fully addressable screen, although slightly smaller than normal, by filling the screen with a sequence of different double-height characters, then turning on the pixels selectively inside the RAM-based character definitions. The Super Expander cartridge added BASIC commands supporting such a graphics mode using a resolution of 160×160 pixels. It was also possible to fill a larger area of the screen with addressable graphics using a more dynamic allocation scheme, if the contents were sparse or repetitive enough. This was used, for instance, by the game Omega Race. The VIC chip did not support sprites.

The VIC chip had readable scan-line counters but could not generate interrupts based on the scan position (as the VIC-II chip could). However, the two VIA timer chips could be tricked into generating interrupts at specific screen locations, by setting up the timers after a position has been established by repetitive reading of the scan-line counter, and letting them run the exact number of cycles that pass by during one full screen update. Thus it was possible, but difficult, to e.g. mix graphics with text above or below it, or to have two different background and border colors, or to use more than 200 characters for the pseudo-high-resolution mode. The VIC chip could also process a light pen signal (a light pen input was provided on the DE-9 joystick connector) but few of those ever appeared on the market.

The VIC chip had three rectangular-wave sound generators. Each had a range of three octaves, and the generators were located on the scale about an octave apart, giving a total range of about five octaves. In addition, there was a white noise generator. There was only one volume control, and the output was in mono.

The VIC chip output composite video; Commodore did not include an RF modulator inside the computer's case because of FCC regulations. It could either be attached to a dedicated monitor or a TV set using the external modulator included with the computer.

Memory expansion

The VIC-20's RAM was expandable through the cartridge port. RAM cartridges were available in several sizes: 3 kB (with or without an included BASIC extension ROM), 8 kB, 16 kB, 32 kB and 64 kB, the latter two only from third-party vendors. The internal memory map was dramatically reorganized with the addition of each size cartridge, leading to the situation that some programs would only work if the right amount of memory was present (to cater for this, the 32 kB cartridges had switches, and the 64 kB cartridges had software setups, allowing the RAM to be enabled in user-selected sections).

The most visible part of memory that was reorganized with differing expansion memory configurations was the video memory (with text and/or graphics display data). This was because the video chip could only use the built-in memory for its display data, and at the same time free memory had to remain contiguous for the BASIC interpreter to be able to use it. An unexpanded VIC had 1 kB of system memory, followed by a 3 kB "hole", then 4 kB of contiguous user memory up to address 8191. The 3 kB cartridge would fill the "hole", so on unexpanded and +3K VICs the video area was placed at the top of user memory (8 kB - 512 bytes). If an 8 kB or 16 kB cartridge was added instead, this memory appeared at addresses above 8 kB; the video memory was then placed at the start of user memory at 4 kB, just above the "hole", to provide the maximum amount of contiguous user memory.

The 32 kB cartridges allowed adding up to 24 kB to the BASIC user memory; together with the 3.5 kB built-in user memory, this gave a maximum of 27.5 kB for BASIC programs and variables. The extra 8 kB could usually be used in one of two ways, set by switches:

  1. Either it could be mapped into the address space reserved for ROM cartridges, which sat "behind" the I/O register space and thus was not contiguous with the rest of the RAM. This allowed running many cartridge-based games from disk or tape and was thus very useful for software pirates; especially if the RAM expansion allowed switching off writing to its memory after the game was loaded, so that the memory behaved exactly like ROM.
  2. Or, 3 kB of the 8 kB could be mapped into the same memory "hole" that the 3 kB cartridge used, letting 5 kB lie fallow. These 3 kB were contiguous with the rest of RAM, but couldn't be used to expand BASIC space to more than 27.5 kB, because the display data would have had to be moved to cartridge RAM, which was not possible.

Some 64 kB expansion cartridges allowed the user to copy ROM images to RAM. The more advanced versions even contained an 80-character video chip and a patched BASIC interpreter which gave access to 48 kB of the memory and to the 80-column video mode. As the latter type of cartridges, marketed primarily in Germany, were not released until late 1984—two years after the appearance of the more capable C64—they went by mostly unnoticed.

Address Size [kByte] Description Cartridge
0x0000 1.0 RAM with jump vectors etc.
0x0400 3.0 Expansion *
0x1000 4.0 RAM for BASIC and screen
0x2000 4.0 Expansion block 1 *
0x4000 8.0 Expansion block 2 *
0x6000 8.0 Expansion block 3 *
0x8000 4.0 ROM character bitmap
0x9000 1.0 I/O for VIC, 6522 VIA#1, 6522 VIA#2, block 0
0x9400 0.5 Used for color RAM when expansion RAM at block 1
0x9600 0.5 Color RAM (normally)
0x9800 1.0 I/O block 2 *
0x9C00 1.0 I/O block 3 *
0xA000 8.0 Decoded for expansion ROM *
0xC000 8.0 ROM Basic
0xE000 8.0 ROM Kernal


While noting the small screen size and RAM, BYTE stated that the VIC 20 was "unexcelled as low-cost, consumer-oriented computer. Even with some of its limitations ... it makes an impressive showing against ... the Apple II, the Radio Shack TRS-80, and the Atari 800". It praised the price ("Looking at a picture ... might cause you to think $600 would be a fair price ... But it does not cost $600—the VIC 20 retails for $299.95"), keyboard ("the equal of any personal-computer keyboard in both appearance and performance. This is a remarkable achievement, almost unbelievable considering the price of the entire unit"), graphics, documentation, and ease of software development with the KERNAL.[13]


The VIC-20 could be hooked into external electronic circuitry, using parts available from parts outlets like RadioShack and Maplin. Interfaces were designed to use the joystick port, the so-called "user port", or the memory expansion–cartridge port, which exposed various analog to digital, memory bus, and other internal I/O circuits to the experimenter. The BASIC language could then be used (using the PEEK and POKE commands) to perform data acquisition from temperature sensors, control robotic stepper motors, etc. The VIC-20 did not originally have a disk drive, with only a relatively high cost, but extremely reliable digital tape storage system (using audio cassette tapes); the VIC-1540 disk drive was released in 1981. Many experimenters built adapters that allowed any conventional audio cassette recorder to be used for program and data storage (since these were generally cheaper than Commodore's own Datasette recorder, though only as reliable as other manufacturers analog cassette storage solutions).

As on other Commodore 8-bit systems, certain system functions could be accessed by the SYS command. For example, even though the VIC had no hardware reset button, SYS 64802 would cause the computer to restart, much like Control-Alt-Delete on IBM compatible PCs or the Reset button on Apple's Macintosh computers. This is because memory location 64802 in the standard memory map was the entry point to the VIC's KERNAL reset routine.

The Commodore VIC-20 continues to have a loyal following today. Programmers continue to write demo, utility, and game programs for the machine (most often shared through the Denial community), and also through commercial retro-software developers such as Psytronik. Recent programs compiled in machine language tend to reveal features of the machine that were never utilized during its production years. A common goal of these programs (and the programmers writing them) is to "show off" how many complex program/graphic features (such as scrolling and pseudo-sprites) and/or intense/realistic gameplay that can be packed into the VIC-20's small amount of available RAM and resolution. Recent software releases such as Frogger '07 (2007 release) and Berzerk MMX (2010 release) have gameplay, graphics, and sound (including voice synthesis in Berzerk) that rival the original arcade machines. Even a port of Doom, a 1993 game popularized on much more powerful platforms, has become available for the VIC-20 in 2013. New hardware has also recently been released, including the Mega-Cart, which contains all known cartridge games and utilities.[14] Hundreds of unique programs are available on the Internet for use on the VIC-20.

Image gallery

See also



External links

  • Denial – the Commodore VIC-20 Community
  • OLD-COMPUTERS.COM online-museum VIC-20 page
  • VIC-20 Programmers reference guide and more
  • VIC-20 Gamer - Videos of games produced for the Vic 20
  • Rick Melick's Commodore VIC-20 Tribute Page
  • A 40/80 Character Expansion For The VIC-20 (November 1982)
  • 40 and 80 column boards

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.