This is the official Data Sheet for the Cirrus Logic 8900a NIC

For RR-Net programming you also need a 2nd datasheet which describes the 8-bit mode a bit further. Some important information from the datasheet:

If you receive a frame from the streaming registers, you have to read the first two 16 bit words in hi/lo order, and all following words in lo/hi order. If you don't do it like this, you will have strange byte skipping effects.

Code example:

LDA $DE09
STA TYPE+1
LDA $DE08
STA TYPE

LDA $DE09
STA SIZE+1
LDA $DE08
STA SIZE

LDA $DE08
...
LDA $DE09
...

etc

Very interesting! I had no idea about this! Perhaps that could explain some of the mysterious problems i had with my UDP/IP code...

PS. I've added the Documentation on 8-bit mode! Thanks for pointing it out