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At 1:35 PM -0400 2001-07-10, Richard B. Johnson wrote:
>Unlike some OS (like VMS), a context-switch does not occur
>when the kernel provides services for the calling task.
>Therefore, it was most reasonable to have the kernel exist within
>each tasks address space. With modern processors, it doesn't make
>very much difference, you could have user space start at virtual
>address 0 and extend to virtual address 0xffffffff. However, this would
>not be Unix. It would also force the kernel to use additional
>CPU cycles when addressing a tasks virtual address space,
>i.e., when data are copied to/from user to kernel space.

Certainly the shared space is convenient, but in what sense would a
separate kernel space "not be Unix"? I'm quite sure that back in the
AT&T days that there were Unix ports with separate kernel (vs user)
address spaces, as well as processors with special instructions for
doing the copies (move to/from user space). Having separate system &
user base page table pointers makes this relatively practical.
/Jonathan Lundell.
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