Software
A benefit of the Rainbow was that it could continue to run 8-bit CP/M software as users moved into the 16-bit world of MS-DOS. The disadvantages were that 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 IBM PC-compatible MS-DOS software using an emulation application called Code Blue.
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.
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.
Read more about this topic: Rainbow 100