That is exactly it. For example, on the Atari ST you display graphics by copying the bitmaps to the screen address. Much of the C code is used during precomput

That is exactly it. For example, on the Atari ST you display graphics by copying the bitmaps to the screen address. Much of the C code is used during precomputation of data before the actual time-critical code is run. This involves copying lots of data in order to set it up so that as little computation as possible is performed in the actual time-critical parts.