The Rainbow 100 was a microcomputer introduced by Digital Equipment Corporation (DEC) in 1982 to compete in the IBM PC market. This desktop unit had the video-terminal display circuitry from the VT102, a video monitor similar to the VT220 in a box with both Z80 and 8088 CPUs. The Rainbow 100 was a triple-boot machine: VT102 mode, CP/M mode (using the Z80), and CP/M-86 or MS-DOS mode using the 8088.
The Rainbow came in three models, the 100A, 100B and 100+. The 'A' model didn't allow for a hard disk controller, whereas the 'B' allowed this option and the '+' shipped with the controller and a hard drive. 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 100B has this selection built into the boot up firmware.
The '100+' model is actually a marketing designation signifying that the system ships with a hard drive installed; the 100+ and 100B are identical in all other respects. The '100B' (and '100+' by definition) had the option to boot from the hard disk (or Winchester drive, as it appeared in the boot menu), while the '100A' firmware did not support booting directly from the hard disk.
The '100A' model shipped with 128 KB memory on the motherboard, while the '100B' had 192 KB memory on the motherboard.