Complex sentence: New device drivers, protocol stacks, file systems and other low-level systems were being developed all the time, code that was normally located in the monolithic kernel, and thus required considerable work and careful code management to work on.
Split into simpler sentences: New device drivers, protocol stacks, file systems and other low-level systems were being developed all the time. This code was normally located in the monolithic kernel, and thus required considerable work and careful code management to work on.