Rainbow 100

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Rainbow 100
DEC Rainbow 100.jpg
DEC Rainbow 100 as a museum piece in Citilab
Type Personal computer
Release date 1982; 35 years ago (1982)
Operating system CP/M, MS-DOS, UCSD p-System, Concurrent CP/M, Venix
CPU Zilog Z80 @ 4.012 MHz and Intel 8088 @ 4.815 MHz
Memory 64 - 896 kB
Successor VAXmate
Rainbow 100 floor model and software packages

The Rainbow 100 was a microcomputer introduced by Digital Equipment Corporation (DEC) in 1982. This desktop unit had a monitor similar to the VT220 in a dual-CPU box with both 4 MHz Zilog Z80 and 4.81 MHz Intel 8088 CPUs.[1] The Rainbow 100 was a triple-use machine: VT100 mode (industry standard dumb terminal for interacting with DEC's own VAX), 8-bit CP/M mode (using the Z80), and 16-bit CP/M-86 or MS-DOS mode using the 8088.[1][2]


The Rainbow came in three models, the 100A, 100B and 100+. The "A" model was the first released, followed later by the "B" model. The most noticeable differences between the two models were the firmware and slight hardware changes. The systems were referred to with model numbers PC-100A and PC-100B respectively; later "B" models were also designated PC-100B2.


The "A" model was the first produced by Digital. The distinguishing characteristic of the "A" model from an end-user perspective was that the earlier firmware did not support booting from a hard disk. Other distinguishing hardware features included the three 2764 ROM chips holding the system firmware[3] and the case fan/power supply combinations.[4] Versions of the 100A shipped outside the USA included a user-changeable ROM chip in a special casing. The user changed the built-in ROM for this one to support their keyboard layout and language of the boot screen.


The "B" model followed the "A" model, and introduced a number of changes. The "B" model featured the ability to boot from a hard disk (referred to as the Winchester drive) via the boot menu due to updated firmware. The hardware changes included bigger firmware stored on two 27128 ROMs (16 kB)[3] and an improved case fan/power supply.[5] The firmware allowed selection of the boot screen language and keyboard layout, eliminating the need to switch ROM. The "B" model also allowed remapping of hardware interrupts to be more compatible with MS-DOS.

The "100+" model was actually a marketing designation signifying that the system shipped with a hard drive installed; the "100+" and "B" models were identical in all other respects. When a hard-disk option was installed on the Rainbow, the kit included the 100+ emblem for the computer's case.[6]



The Rainbow contained two separate data buses controlled by the Zilog Z80 and the Intel 8088 respectively.[7] The buses exchanged information via a shared 62 kB memory.[8]

When not executing 8-bit code, the Zilog Z80 was used for floppy disk access.[8] The 8088 bus was used for control of all other subsystems, including graphics, hard disk access, and communications.[7] While it may have been theoretically possible to load Z80 binary code into the Rainbow to execute alongside 8088 code, this procedure has never been demonstrated.


The 100A model shipped with 64 kB[9] memory on the motherboard, while the 100B had 128 KB memory on the motherboard. Daughterboards were available from Digital Equipment Corporation that could increase system memory with up to an additional 768 kB for a total 892 kB.


Floppy disk drives[edit]

The floppy disk drives, known as the RX50, accepted proprietary 400 kB[10] single-sided, quad-density 5¼-inch diskettes. Initial versions of the operating systems on the Rainbow did not allow for low-level formatting, requiring users to purchase RX50 media from Digital Equipment Corporation. The high cost of media ($5 per disk) led to accusations of vendor "lock-in" against Digital. However, later versions of MS-DOS and CP/M allowed formatting of diskettes.

Of note was the single motor used to drive both disk drives via a common spindle, which were arranged one on top of the other. That meant that one disk went underneath the first but inserted upside-down. This earned the diskette drive the nickname "toaster".[citation needed] The unusual orientation confused many first-time users, who would complain that the machine would not read the disk.

Hard disks[edit]

Digital Equipment Corporation produced a Winchester disk controller for the Rainbow capable of controlling hard disks compatible with the ST-506 interface. The controller, however, was limited to a single drive with up to and including 8 heads and 1024 cylinders, limiting storage to a maximum of 67 MB.[11]

Third-party hard-disk controllers were also available.


The base Rainbow system was capable of displaying text in 80×24- or 132×24-character format in monochrome only. The system could apply attributes to text including bolding, double-width, and double-height-double-width.

The graphics option was a user-installable module that added graphics and color display capabilities to the Rainbow system. The Graphic module was based on a NEC 7220 graphic display controller (GDC) and an 8×64 KB DRAM video memory.

Due to the design of the graphics system, the Rainbow was capable of controlling two monitors simultaneously, one displaying graphics and another displaying text.[12]


The base Rainbow system generates a TTL composite-video signal in monochrome mode. With the inclusion of the graphics option, the Rainbow could also output sync-on-green RGB video signals at TTL levels. The Rainbow was most often coupled with the 12-inch VR201 monochrome monitor or the 13-inch VR241 color monitor, both produced by Digital Equipment Corporation.


The Rainbow 100 and the other two microcomputers which DEC announced at the same time (DECmate II and Pro-350) had two quirks that annoyed conservative users. The LK201 keyboard used a new layout that made some Teletype Model 33 and VT100 users unhappy. However, the VT220 style of this keyboard can clearly be seen in the layout of the enhanced 101-key keyboard adopted by IBM in 1985.


Third-party upgrades were also available, including an 80286 (286) processor upgrade (Turbow-286),[13] a 3.5-inch disk adapter kit (IDrive), and a battery-backed clock chip (ClikClok), all from Suitable Solutions.

In 1984, the first computer support for Native American languages began with the Rainbow 100. Rock Point Community School on the Navajo reservation in Arizona commissioned a ROM chip for the Navajo language, enabling the school to create bilingual computer programs.[14]


The Rainbow runs the CP/M 86/80 operating system, which detects whether software is written for 8-bit CP/M and 16-bit CP/M-86 and runs it on the appropriate processor.[15] DEC later released MS-DOS, but little DOS software was released on Rainbow media, and there were subtle differences between Rainbow's MS-DOS implementation and MS-DOS (or PC DOS) running on true IBM PC compatibles. Towards the end of its life, Rainbow users were able to run some IBM PC-compatible MS-DOS software using an emulation application called Code Blue, thought it only emulated the IBM PC's BIOS, and not the hardware, so programs that accessed the video cards directly would not work.

MS-DOS compatibility was added late in the design, so hardware interrupts and MS-DOS software interrupts overlapped. One DEC documentation pack for developers included a listing of Microsoft assembly code to handle this. The theory was that hardware interrupts would interrupt again but software interrupts would only happen once. The Rainbow 100B fixed this overlap in hardware by changes to the board design that allowed the OS to move the hardware interrupts. This means that the MS-DOS 3.10b version for the Rainbow can only run on the Rainbow 100B.

DEC itself ported Microsoft Windows 1.0 to the Rainbow. There was also a third-party upgrade that included a 286 processor board and a modified version of Windows 3.0.

Software bundled with Dec Rainbow floor model included:


  • Rainbow 100 Technical Manual
  • Getting Started with Rainbow 100
  • MBasic-86 User's Guide Digital Equipment Corporation Distributed
  • WordStar User's Guide for Rainbow 100
  • License and Software Product Description

These documents come in booklets contained inside two hard case boxes, with the DEC logo.


The Rainbow was MS-DOS compatible, but not completely software- or hardware-compatible with the IBM PC. At the time this seemed reasonable, with expectation being that the interface that programs would target would be MS-DOS, not the underlying hardware. However, many significant commercial software products were written directly to the hardware, for a variety of reasons including performance. Very soon the market expectation was that all MS-DOS computers would be fully IBM PC compatible. While "Code Blue" did a good job at emulating the IBM BIOS, its inability to trap references to the video and other hardware limited what would run on the Rainbow. The FOSSIL TSR allowed several terminal programs and editors to run on the IBM-PC, Rainbow and other early 8088/8086 computers, but its limited adoption limited its usefulness. Various other hacks allowed popular programs like Turbo Pascal, Turbo C to run, while patches circulated for a while, but new releases made this difficult to keep up with and over time these hacks dried up (the online archives have very little new after 1991 or 1992).


In popular culture[edit]

The Rainbow 100 plays a small but important role in the creation of the FidoNet system. A computer club in St. Louis was in the process of setting up a BBS system using CBBS on CP/M when they learned that DEC would be giving the club a Rainbow 100 for free. The group planned on starting the BBS on this machine as soon as it arrived, but when it did so they found that the Z80 did not have access to the serial ports. Casting about for a new BBS platform that ran on DOS, they learned of FidoBBS and arranged to have its developer port the serial drivers to the Rainbow platform. This ran up considerable phone bills sending emails and file transfers between St. Louis and the developer in San Francisco, and the developer produced FidoNet as a method of automating exchanges late at night when phone rates were lower.[16]

A television advertisement for the DEC Rainbow appears in The Philadelphia Experiment.

The DEC Rainbow 100 can be seen in the 1984 film Ghostbusters. Janine uses the Rainbow at the Ghostbusters' reception. Except it was wired up wrong. You can also spot it in the 1984 film Beverly Hills Cop. Several of them are used in the police office.

Piers Anthony wrote many of his books from the '80s on his beloved DEC Rainbow.[17]


  1. ^ a b Ryan, Chris. "Digital Rainbow 100". Old-Computers.com. Retrieved 4 January 2010. 
  2. ^ Klein, Erik S. "DEC Rainbow 100". The Vintage Computer. Retrieved 4 January 2010. 
  3. ^ a b Convert Rainbows from A to B (contained in zip file).
  4. ^ Digital Equipment Corporation. Illustrated Parts Breakdown: PC100 Rainbow 100 System Unit (Part #EK-SB100-IP-003) 1985. p. 11/12, 25/26.
  5. ^ Digital Equipment Corporation. Illustrated Parts Breakdown: PC100 Rainbow 100 System Unit (Part #EK-SB100-IP-003) 1985. p. 15/16, 32.
  6. ^ Digital Equipment Corporation. uu.se - Illustrated Parts Breakdown: PC100 Rainbow 100 System Unit (Part #EK-SB100-IP-003) 1985. p. A-21, A-23/24.
  7. ^ a b Digital Equipment Corporation. Rainbow 100 Technical Manual. 1st ed. May 1984. p. 4-2.
  8. ^ a b Digital Equipment Corporation. Rainbow 100 Technical Manual. 1st ed. May 1984. p. 4-1.
  9. ^ In the context of semiconductor memory, 1 kilobyte = 1,024 bytes.
  10. ^ In the context of floppy disks, 1 KB = 1,000 bytes or 1,024 bytes.
  11. ^ WUTIL Users Manual (Contained in archive)
  12. ^ Hartley, R.B. "Dual Monitors on a Rainbow." Within the RBGIF distribution
  13. ^ Pictures of the Turbow-286 Rainbow Accelerator
  14. ^ Christal, Mark. "Virtual Museum Projects for Culturally Responsive Teaching in American Indian Education." Dissertation. 2003: [1]
  15. ^ Snyder, John J. Ph.D. (June 1983). "A DEC on Every Desk?". BYTE. pp. 104–106. Retrieved 5 February 2015. 
  16. ^ Ben Baker, "Fidonet History", 2 May 1987
  17. ^ http://www.hipiers.com/02dec.html


External links[edit]