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In the FLAT and PROTECTED builds, the file descriptors are maitained maintained in a table internal to the OS.  In PROTECTED mode, this requires a system call to access the file descriptors.  In the KERNEL build mode, however, this system call overhead is wasteful.  Ideally, the file descriptor table would be moved out of the OS and into the process address space where it can be accessed directly.

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Currently, the user-process virtual address space and the kernel-mode virtual address space do not overlapped.  This is an odd arrangement and forces the user address space into awkward regions.  This was done so that a single address environment can support both user- and kernel- mode operation.

More correctly, the user address space should include the entire virtual address space (other that regions that may have specific hardware functionality such as vector tables).  And the user address space should overlap the kernel address space.

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