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By 1990, the GNU system was almost complete; the only major missing component was the kernel. We had decided to implement our kernel as a collection of server processes running on top of Mach. Mach is a microkernel developed at Carnegie Mellon University and then at the University of Utah; the GNU HURD is a collection of servers (or ``herd of gnus'') that run on top of Mach, and do the various jobs of the Unix kernel. The start of development was delayed as we waited for Mach to be released as free software, as had been promised.
One reason for choosing this design was to avoid what seemed to be the hardest part of the job: debugging a kernel program without a source-level debugger to do it with. This part of the job had been done already, in Mach, and we expected to debug the HURD servers as user programs, with the GNU debugger (GDB). But it took a long time to make that possible, and the multi-threaded servers that send messages to each other have turned out to be very hard to debug. Making the HURD work solidly has stretched on for many years.