360 model 20 Hardware Architecture & Functional Characteristics

'-Mike' How compatible was the 360-25 in '1401 mode'? Was it as 'simple' as setting the 360 up with the 1401 microcode and then running unchanged 1401 code? I can see how the microcode would have made it possible to run 1401 object code, but it's less clear to me how it would have made the I-O devices compatible as well.

I don't remember the answer to that question. By "how it would have made the I-O devices compatible as well" do you mean plug compatible at the interface, or compatible in that the I-O routine object code could remain unchanged. The whole reason for existence of the 360-25 was immediate ability to run 1401 programs. It'd only make economic sense for one off programs that had poor or non-existent documentation, perhaps not even source code, and that used the most basic of I-O. Certainly that could have been handled in micro code. (It wasn't like I-O routine for the 1401 consisted of more than a handful of instructions.) I am sure that the Honeywell Series 200 was the impetus for the IBM 360-25 since the Series 200 was a superset of the 1401. IBM had translation programs to convert 1401 source code to 360 source code, so the niche for the 360-25 was pretty small. The only one I ever worked with was in an IBM Field Systems Service Center, and even then the only work I ever did related to 1401 programs was to convert to System 360 from Series 200 that had been converted from 1401. Not a happy path, and too far down the road for the 360-25 to be of use. But just the idea of a meta progamable programable computer was sorta exciting.

Phil Weldon