Commodore Plus/4 - Plus/4 Strengths

Plus/4 Strengths

The TED offered 121-color (15 colors × 8 luminance levels + black) video, a palette matched only by Atari Computers at the time, and 320×200 video resolution, which was standard for computers intended to be capable of connecting to a television. 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, 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 1551 plugged into the cartridge port.

The TED chip had identical resolutions and video modes to the VIC-II (bitmap or character graphics which could be high-resolution or multicolor), but lacked hardware sprites. It also had 128 colors (in practice 121 since several of them were identical gray shades) against the VIC-II's fixed 16 color palette in addition to blinking text. Its sound was a weak 2-voice square wave generator.

Unlike the C64 which emulated the 6551 chip in software, the Plus/4 had a built-in MOS Technology 6551 UART chip that could perform up to 19200 bit/s. 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). But most people could only afford 300- or 1200-bit/s modems in 1984—and Commodore never released a 2400-bit/s modem—so this feature went largely unnoticed. 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 C-64. Also, for serious programmers, the Plus/4 featured a ROM-resident machine code monitor, which rekindled a tradition from the first Commodore computers.

The Plus/4 has one of the earliest examples of 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. Bowen's name will be in reverse-field and Ryan's will be blinking.

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 59 kB 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 MOS-6502-based computers have to use, at the top of memory ($FD00), while in the C64 they had been located at the much lower address $D000. On the C64, a program that did not need BASIC was able to swap out the ROMs and the I/O registers manually and thus gain access to the full 64 kB, 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.

While the Plus/4's CPU could run about 75% faster than the C64's, the computer was still designed with a shared memory architecture, in which screen data resided in main memory. This means that the video chip has to access the memory while it is displaying the picture, in effect slowing down the CPU to less than half its full speed in this screen area. Only during those periods when the video chip is displaying the screen border or putting out the horizontal and vertical retrace signals is the CPU able to run at full speed. This means that, on average, the Plus/4's CPU runs only slightly faster than that of the C64.

"The PLUS/4 was derived from an existent commercial program “TRILOGY” published by Pacific Tri – Micro. Under license to CBM a total of 600,000 units were built. It was warmly received and critically acclaimed in Europe." "It was translated into 3 languages English, French and German. Commodore dropped support of the PLUS/4 when Jack Tramiel left to run Atari Corp. Pacific Tri Micro continued to support PLUS/4 users until 1988. PLUS/4 users were a loyal group and remember fondly, their running, of their small business with the built-in programs. In those days it took about 20 minutes to load “TRILOGY” from tape, but with the PLUS/4 it was reduced to several seconds." David W. Johnson, developer of the PLUS/4 software