I got the Rainbow to go a bit further - the Z80 now boots up, copies it's boot image to private RAM, checksums it, and reports the correct checksum back to the 8088. So that gets us past that test, now we fail on error 10, which is "Video VFR" according to the manual. Appears that's the 60 Hz vblank IRQ...
For peeps following along at home, ftp://ftp.update.uu.se/pub/rainbow/doc/rainbow-docs/pc100tm1.pdf
is the Technical Manual, which is quite a bit less error-prone and much better written than the System Specification.
Last edited by R. Belmont; 05/06/12 09:22 PM.