Dave Pitts' aforementioned page links to some 990 stuff at bitsavers. The stuff there is largely system level stuff. People whose interests are closer to chip level might want to have a look at the 9900 Microprocessor Series Family Systems Design Databook available at archive.org, if they're not already aware: https://archive.org/details/9900MicroprocessorSeriesFamilySystemsDesignData… A 2 inch thick book that contained pretty much everything you were likely to need to know about the 9900 family. One of the few other useful 9900 resources I vaguely remember was a 3rd party 9900 book which contained the source for an "instant input assembler" - a tiny line by line assembler which you could add to your PROM-based 9900 debug monitor so you could enter small programs as source rather than hex. Neat. Can't remember name of author or of book, can't quickly find them either. I worked with 990/4 systems many years ago and there was almost no 9900 family glue logic inside. As far as I remember, the /4 was designed before (most of?) the 99xx support chips came to market. So a serial card in a /4 had a handful of chips doing what a 9902 would eventually do in one. Not sure about the video controller but the same probably applied. I have a vague recollection that the interweb has some decent writeups of the innards of the 99/4 home computer stuff (sorry, no pointers); from those, it might be possible to work out if the kit of parts on eBay is a plausible match for a 99/4 or similar volume market product. Have a lot of fun John Wallace