[lkml]   [2001]   [Jul]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
On Tue, 10 Jul 2001, Jesse Pollard wrote:
> "Richard B. Johnson" <>
> > 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.
> I believe the VAX/VMS implementation shared OS and user space:
> p0 - user application 0
> p1 - system shared libraries 0x3fffffff
> p2 - kernel 0x7fffffff
> rest was I/O, cache memory 0xffffffff
> It was a hardware design, not a function of the software.

Correct, except that "p2" is called S0. IIRC it is the top quarter of the
address space, and there's a reserved S1 region. The command interpreter
is also mapped into P2. The very top of memory is reserved for device

> UNIX origins were on a PDP-11. there were two sets of addressing registers
> 1 kernel, 1 user (except on 11/45 - 1 kernel, 1 user, 1 "executive"
> (never used except in some really strange form of extented shared library)

Early '11s didn't have the Ispace/Dspace split. My PDP11 databook is at
home, and I don't trust my memory far enough to say which model introduced
the split. My recollection is that the earliest implementation was an
add-on board which monitored the Unibus address lines and fired interrupts
when it thought its programmed memory access rules were violated.

Mark H. Wood, Lead System Programmer mwood@IUPUI.Edu
Make a good day.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-03-22 12:56    [W:0.052 / U:0.816 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site