Comment by Paul Collins (pi314aussie[at]yahoo.com): Got one of these suckers out of an IBM PS/2 system model 60. Put it into 287 slot in a 286-20MHz M/Board. I learned that the main CPU socket and the 287co-pro socket run independantly of each other. Using utility software, I have found the 287 socket runs a 10MHz turbo, and 8MHz un-turbo, and that the main CPU socket runs at 20MHz turbo, and 8MHz un-turbo. I changed the clock-crystal from a 40MHz rock to a 50MHz rock, and now the main CPU runs at 25MHz, and the co-pro socket still runs at 10MHz. Good thing too, as the filched D80287-10 runs hot at its rated 10. I wouldn't like it to run any faster.