Own C compiler? IIRC, that's not quite true. I worked on the C/C++ compiler team at Microsoft from 1991 to 2007, and I don't remember us producing a drop of the compiler just for Excel. I do think there was a special compiler flag for them, though, that fixed the order in which global variables were laid out to duplicate the source order. It was either Excel or Word that saved state by taking the addresses of the first and last global variables of interest, then dumping all memory in that range to disk. The flag was something like -bzalign. The bz stood for Bozo, which hints at the compiler team's thoughts on the practice.