Commodore Plus/4

From Academic Kids

The Commodore Plus/4 was a home computer released by Commodore International in 1984. The "plus 4" name refers to the four ROM-resident applications (word processor, spreadsheet, database, and graphing); it was billed as "the productivity computer with software built-in". It had some success in Eastern Europe, but was less popular in Western Europe. A total flop in the United States, it was derided as the "minus 60".



In the early 1980s, Commodore found itself engaged in a price war in the home computer market. The hot-selling VIC-20 was a design accident resulting from MOS Technology designing a videogame chip it couldn't sell, and companies like Texas Instruments and Timex Corporation undercutting the price of Commodore's PET line. The Commodore 64, the first 64-KB computer to sell for under US$600, was another salvo in the price war but it was far more expensive to make than the VIC-20 because it used discrete chips for video, sound, and I/O. Commodore president Jack Tramiel wanted a new computer line that would use fewer chips and at the same time address some of the user complaints about the VIC and C64.

Commodore's third salvo — which, as it turned out, was fired just as most of Commodore's competition was leaving the home computer market — was the C116, C16, and Plus/4. All three computers used a MOS 7501 CPU (6502 compatible but ~75% faster) and a MOS 7360 "TED" all-in-one video, sound, and I/O chip. The Plus/4's design is thus philosophically closer to that of the VIC-20 than of the C64.

The Plus/4 was the flagship computer of the line. The Plus/4 had 64 KB of memory while the C16 and 116 had 16 KB. The Plus/4 had built-in software, whereas the others did not. The Plus/4 and C16 had full-travel keyboards; the 116 used a rubber chiclet keyboard like less-expensive Timex-Sinclair computers and the original IBM PCjr. The C116 was only sold in Europe. All of the machines were distinguished by their dark gray cases and light gray keys.

The Plus/4 was introduced in June 1984 and priced at US$299. It was discontinued in 1985. It is not completely clear whether Commodore's intent was to eventually totally replace the C64 with the Plus/4, or whether they wanted to attempt to expand the home computer market and sell the Plus/4 to users who were more interested in serious applications than gaming. However, the Plus/4 succeeded at neither and quickly disappeared.

Plus/4 strengths

The TED offered 121-color (15 colors 8 luminance levels + black) video, which was revolutionary for its time, and 320200 video resolution, which was standard for computers intended to be capable of connecting to a television. The Plus/4's memory layout gave it a larger amount of user-accessible memory than the C64, and its BASIC programming language was vastly improved, adding sound and graphics commands as well as looping commands that improved program structure. Commodore released a high-speed floppy disk drive for the Plus/4, the Commodore 1551, which offered much better performance than the C64/1541 combination because it used a parallel interface rather than a serial bus. (The Plus/4 did not have the parallel interface built-in; it was provided by a plug-in cartridge supplied with the drive).

Unlike the C64, the Plus/4 had a built-in MOS Technology 6551 UART chip (the C64 emulated the 6551 in software). This allowed the Plus/4 to use high-speed modems without additional hardware or software tricks (the C64 required specially written software to operate at 2400 bit/s). However, since most people only could afford 300- or 1200-bit/s modems in 1984, and Commodore never released a 2400-bit/s modem, this feature went largely unnoticed. The Plus/4 keyboard had a separately placed "diamond" of four cursor keys, presumably more intuitive in use than the VIC's and C64's two shifted cursor keys. Also, for serious programmers, the Plus/4 featured a ROM-resident machine code monitor, which rekindled a tradition from the first Commodore computers, the PET/CBM series.

While the C64 had the advertised 64 KB of RAM installed, only about 38 KB was available for BASIC programs. The Plus/4's BASIC V3.5 made 59KB available, aided by its memory map that placed I/O at the top of memory ($FD00). In addition, the Plus/4's CPU was about 75% faster than the C64's.

Plus/4 weaknesses

The Plus/4 had three shortcomings, which proved fatal: unlike the C64's VIC II, the TED had no sprite capability, which strongly limited its video game graphics capabilities. Also, its tone generator was much closer to the VIC in quality than to the C64's SID, which, again, made the Plus/4 less attractive to game developers. Finally, the lack of these capabilities made C64 software compatibility impossible. Commodore may not have believed this to be a problem, as the successful C64 was incompatible with most VIC-20 software — but the C64 had developed a large software library by 1984, and while the C64 was a significant upgrade to the VIC-20 in almost every way, the Plus/4 was not.

Peripheral compatibility with the C64 was inconsistent. The Plus/4's serial, user, and video ports were compatible with the C64, but the Datasette port was changed, rendering previous units incompatible without third-party adapters that only became available later. This also posed a problem for the many third-party C64 printer interfaces that allowed one to connect a standard Centronics parallel printer to the Commodore serial port. Since most of these interfaces connected to the Datasette port to get +5 volts for power, they were incompatible with the Plus/4 unless the user modified the interface and risked voiding the warranty. For a computer intended to be used for productivity applications, this was a heavy weakness. Additionally, with the Plus/4, Commodore abandoned the Atari-style joystick ports used on the C64, replacing them with a proprietary mini-DIN port that was said to be less prone to emit RF interference. While this may have been seen as an advantage by the Federal Communications Commission and other regulatory agencies, end users did not share this view.

This made upgrading to the Plus/4 from the VIC-20 or C64 more expensive, since the user in many cases would have to buy new peripherals in addition to the new computer. It also made the Plus/4 less attractive to new buyers, since VIC and C64 peripherals were more plentiful and less expensive than their Plus/4 counterparts. The street price for a complete C64 system was lower than that of a comparable system based on the Plus/4.

The Plus/4, unlike the C64 and most other computers of its time (with the notable exception of the Coleco Adam), was equipped with ROM-resident application software (developed for Commodore by TriMicro). Unfortunately, the application suite, featuring a word processor, spreadsheet, database, and graphing, was quite rudimentary.

Most of the developers of the Plus/4 also worked on the Commodore 128 project, which was much more successful.


  • CPU: MOS 7501, 1.76 MHz
  • RAM: 64 KB, of which nearly 60 KB available to BASIC user
  • ROM: 64 KB including Commodore BASIC 3.5, machine code monitor; TRI-Micro's "3 Plus 1" (word processor, spreadsheet, database, graphing)
  • Text mode: 4025 characters (PETSCII)
  • Graphics modes: 320200 max. resolution with 121 colors
  • I/O ports:


The Plus/4 was originally named the Commodore 264 during prototype stage, and extant units bearing the 264 nameplate (some with and some without the integrated software) have been reported by hardware collectors.

An unreleased Plus/4 family prototype, the Commodore V364, included voice synthesis hardware and software. [1] (

External links

de:Commodore Plus4

hu:Commodore Plus/4


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