Compound that with unability to have separate user and kernel
mappings active at the same time (unlike e.g. Motorola 68000
family MMUs do), and the userspace can't have even that 4GB,
but is limited to at least 3.5/0.5 (user/kernel) split, more
commonly to 3.0/1.0 split.
How does FreeBSD do this? What about other OSs? Do they map out most
of userland on syscall entry and map it in as required for their
equivalents to copy_to/from_user? (Taking the performance hit in doing
so?)
--cw
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