The Plus/4 is a home computer released by Commodore International in 1984. It is part of the Commodore 264 series, which also includes the Commodore 16 and Commodore 116 models. The Plus/4 was marketed as "the productivity computer with software built in", with a four-application ROM-resident office suite: word processor, spreadsheet, database, and graphing software.
Internally, the Plus/4 shares the same basic architecture as the Commodore 16 and 116, allowing it to use software and peripherals designed for these models. It is incompatible with the Commodore 64's extensive software library and some of its hardware. The Plus/4 was intended to expand the home computer market by targeting users interested in serious applications rather than gaming.
The Plus/4 has a compact plastic casing with a full keyboard, although the cursor and function keys are made of rubber. Its BASIC version 3.5 is software incompatible with the Commodore 64. Despite its potential, the Plus/4 was criticized for its lack of compatibility with the C64 and its deficiencies in sound and graphics compared to its predecessor.
Commodore produced approximately 827,000 units worldwide, with a significant number sold in Germany. The Plus/4 was discontinued in 1985, but systems remained available from liquidators for several years afterward.
History
In the early 1980s, Commodore found itself engaged in a price war in the home computer market. Companies like Texas Instruments and Timex Corporation were releasing computers that undercut the price of Commodore's PET line. Commodore's MOS Technology division had designed a video chip but could not find any third-party buyers. The VIC-20 resulted from the confluence of these events and it was introduced in 1980 at a list price of $299.95. Later, spurred by the competition, Commodore was able to reduce the VIC's street price to $99, and it became the first computer (of any kind) to sell over 1 million units. The Commodore 64, the first 64 KB computer to sell for under in the US, 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. Still, the C64 went on to become a best-seller and was selling for $199 at the time of the Plus/4's introduction. Even while C64 sales were rising, 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.
Rumors spread in late 1983 of a new computer in 1984 called the "Commodore 444" or "Ted", with built-in word processing and spreadsheet software, and that it would be one of four new computers that would replace the VIC-20 and C64, which the company would discontinue. The company'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 264, which became the Plus/4. The original concept allowed customers to select the application software they wanted, with dealers installing the corresponding ROM chip. Commodore abandoned this approach by the summer of 1984. There were also prototypes of a 232, basically a version of the Plus/4 without the software ROMs, and a V364, which had a numeric keypad and built-in voice synthesis. The latter two models never made it to production. All these computers used a 6502 compatible MOS 7501 or 8501 that was clocked approximately 75% faster than the CPUs used in the VIC-20 and C64, and a MOS Technology 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 that of the C64.
The Plus/4 was introduced in June 1984 and priced at . The Plus/4 was the flagship computer of the line, featuring of RAM while the C16 and C116 had . The Plus/4 had built-in software, whereas the others did not. All of the machines were distinguished by their dark gray cases and light gray keys. This was a reversal of the color scheme on the C64 and VIC, which used lighter cases and darker-colored keys.
Commodore's intent with the Plus/4 was not to replace the C64, but to expand the home computer market and sell the Plus/4 to users who were more interested in serious applications than in gaming. By 1984, however, in the US, most of these customers were beginning to switch to the new, low-cost IBM PC compatibles such as the Leading Edge Model D and Tandy 1000 series.
It was discontinued in 1985. Although, like the Commodore B128, Plus/4 systems remained available from liquidators for years after its discontinuation, the Plus/4 disappeared from Commodore's major markets by 1988.
Hardware
Graphics
The TED offered 121-color (15 colors × 8 luminance levels + black) video, a palette matched only by the Atari 8-bit computers and the Enterprise at the time, and 320×200 video resolution, similar to many computers intended to be capable of connecting to a television.
The TED chip had identical resolutions and video modes to the C64's VIC-II (bitmap or character graphics which could be high-resolution or multicolor), but lacked hardware sprites. Its sound capability was a two-voice square wave generator.
The first eight colors of the TED's palette are the same as the VIC-II, but colors 8-15 are different. It allowed each color except 0 (black) to be set to one of eight possible luminance levels, thus making 121 total colors possible.
The power-on default configuration places the screen memory at and the color memory at . Color memory is integrated into the TED and there is no separate color RAM like on the VIC-20 and C64. Bits 0-3 of each byte in color RAM hold the color value and 4-6 hold the luminance. Bit 7 is a flag that sets blinking text. Bitmap mode is similar to the C64, however in hi-res mode the color RAM is used to supply the luminance values for each block (bits 0-3 the luminance for color 0 and 4-7 the luminance for color 1) while it is not used at all in C64 hi-res graphics. In multicolor bitmap mode, the same setup is used, but while on the C64, color RAM holds the values for color 3, on the Plus/4 it instead holds the luminance values for colors 1–2. Color 3 is instead global and obtained from the register at .
The Plus/4's TED has several advantages over C64's VIC+SID. All TED registers that are available can be read and written. TED may realize the blinking cursor and the characters in the reverse-video mode. It may display 256 characters in the text mode. It may use graphics split by raster interrupt and show pictures at 320x288 resolution. This, with interlaced mode, makes it possible to show 320x496 images. In addition, the TED has 16 address lines, thus it can "see" the entire memory space of the computer unlike the VIC-II. The video buffers may thus be placed anywhere in memory and there are no mirrors of the character ROM to get in the way like on the C64.
Input and output
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 1551 plugged into the cartridge port.
The Plus/4 had a built-in MOS Technology 6551 UART chip that could perform up to , unlike the C64, which emulated the 6551 chip 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 ), at a time when 300- or 1200-bit/s modems were more common and Commodore never released a 2400-bit/s modem so this feature went largely unnoticed. The Plus/4's serial port is the standard Commodore user port used since the PET, featuring TTL voltage which is incompatible with RS-232. This requires a voltage converter to use modems or other serial devices from non-Commodore vendors.
The Plus/4 keyboard had a separately placed directional "diamond" of four cursor keys, presumably more intuitive to use than the VIC's and C64's two shifted cursor keys.
A reset button was added on the right side of the system, a feature lacking on the C64.
The Plus/4 also revived the built-in machine language monitor from the PET days, a feature missing on the VIC-20 and C64.
Specifications
CPU: MOS Technology 7501/8501, approximately when the raster beam is on the visible screen and (PAL) / (NTSC) the rest of the time RAM: , of which nearly (60671 bytes) is available to BASIC users. There are known RAM expansions with and 1 MB ROM: including Commodore BASIC 3.5, a machine code monitor, and TRI-Micro's "3 Plus 1" (word processor, spreadsheet, database, graphing). It is possible to add up to more ROM with cartridges. Graphics: MOS Technology TED (TED 7360) Text mode: 40×25 characters (PETSCII). There are three text modes: standard, extended color, multicolor Graphics modes: 160x200 (multicolor, 2bpp, no sprites) / 320×200 (hi-resolution, 1bpp) at 121 colors (all can be visible at the same time) I/O ports: Tape connector (for Commodore 1531 Datassette with ; incompatible with C64, although adapters allowing the machine to use earlier and aftermarket datasettes were common.) ROM cartridge slot (incompatible with C64) Two 8-pin mini-DIN game controller ports (incompatible with C64) Commodore serial bus (compatible with C64) User port (for modems and nonstandard devices, incompatible with C64) Composite video connector including S-Video and mono audio signal (compatible with C64) RF modulator to TV antenna connector (compatible with C64) The power connector is compatible with C64 power supplies in some Plus/4s and takes the same 9V AC and 5V DC voltages, but uses a non-standard "Square DIN" plug, like the C128, in most Plus/4s.
Software
BASIC
The Plus/4's memory map, which used bank switching far more extensively than the C64, gave it a 56% larger amount of user-accessible memory than the C64 for programming in BASIC, and its BASIC programming language was vastly improved, adding sound and graphics commands as well as looping commands that improved program structure.
The Plus/4 does not have the Restore key on the VIC and C64, but a similar function may be achieved by holding down Run/Stop and pressing the reset button. This will reboot the computer into the machine language monitor, but any BASIC programs in memory will be left intact.
The Plus/4 contains an Easter egg. Entering the command SYS DEC("CDAB") or SYS 52651 puts up the names of three programmers and a hardware designer: Fred Bowen, John Cooper, Terry Ryan and Bil Herd, with Bowen's name at the top in reverse-field and Ryan's at the bottom blinking on and off.
While the C64 had the advertised of RAM installed, only about was available for BASIC programs. The Plus/4's BASIC V3.5 made available, aided by its memory map that swapped the ROMs in and out of memory as needed, and that placed the memory mapped I/O registers, which all 6502-based computers have to use, at the top of memory (), while in the C64 they had been located at the much lower address . On the C64, a program was able to swap out the ROMs and the I/O registers manually and thus gain access to the full , but this was not compatible with BASIC on that machine; on the Plus/4, on the other hand, most of the ROM area was automatically switched out when not needed, rendering the RAM existing at the same addresses accessible for BASIC programs.
The BASIC program area on the Plus/4 begins at , but the BASIC ROM starts at so the automatic switching of the OS ROMs is not initiated unless a BASIC program grows big enough to reach , or 28K in size. Since RAM on the C16 never exceeds the line, banking does not occur on that machine. As on the C64, writing to the ROM areas will alter the RAM underneath.