Ah, thanks for your investigation! How is this expansion structured? How are the 16K blocks placed? Maybe my 4x16K "find" routine is failing in some cases?
It is a 256K (8x41256 chips), at 40-4Fh.
But I think not a software problem, because at 4Mhz working.
But what a strange hw problem which is not discovered under about 22 years?
If least two segments free from this area (4Eh,4Fh, other RAMs at other location not continous) then also freeze on turbo, if one (4Fh), then working.
Can you specify what the SymbOS do with these segments in these situations?
I guess when the only one free then it is not used. When two free it is used for Bank 0?
Can you compile me two test version, BFh port value with 0 and another with 4? Try to play with different wait states settings