I haven read this in detail, and I dont know how much is 64 specific, but heres a great article/tutorial on writing for GEOS:

http://cbm.csbruce.com/~csbruce/cbm/transactor/v9/i1/p056.html

also:

http://cbm.csbruce.com/~csbruce/cbm/transactor/v9/i5/p021.html

and:

http://cbm.csbruce.com/~csbruce/cbm/transactor/v9/i5/p056.html

Another article specific to GEOS 128:

http://cbm.csbruce.com/~csbruce/cbm/transactor/v9/i4/p029.html

I just wanted to add that after reading that last article you posted here, the article following that one in the same magazine is also about GEOS and seems to have some good information. Here it is:

Easy GEOS Info Sectors
http://cbm.csbruce.com/~csbruce/cbm/transactor/v9/i4/p034.html