Kernel Definition

[Gregory TRAVIS]

In <48JBS77 at xds13.ferranti.com> peter at ficc.ferranti.com (Peter da Silva) writes:

>In article <29696 at hydra.gatech.EDU> ken at dali.cc.gatech.edu (Ken Seefried iii) writes:
>> Your friend has absolutely no idea what he's talking about.  In
>> any traditional Unix system (V6, V7, Sys[35], BSD), the kernel
>> is never swapped.

>Weren't there some varieties of 2BSD that had an overlayed kernel on the
>PDP-11? But that was pretty late in the life of 2BSD, when they were putting
>all sorts of 4BSD stuff in it. I think it was 2.11 that started doing this.

>(when was the last time you ran into an overlayed program?)

Not overlayed in the traditional sense.  From 2.8BSD and beyond, the kernel
text (program) size had bloated beyond the 64K barrier.  The solution was to
modify the linker so that multiple "overlays" of the kernel were linked
to begin at the same spot.  What you ended up with was a microkernel
whos size was a multiple of 8K.  Then you had a whole bunch of chunks
of the rest of the kernel laying around that were 64K - <size of microkernel>
into which you put the various other parts of the kernel.

The PDP-11 segment registers allowed the 64K program space to be divided up
into eight 8K segments.  With the above scheme, the kernel could be
300K of code - the crt0.o code and the code tacked onto the beginning
of every function took care of modifying the segment registers
on the fly to map in the appropriate code for whatever syscall was going
on.

For performance reasons you usually wanted to make sure that stuff like
machdep.c, the trap code, the tty driver, the disk drivers, and the
scheduler all lived in the microkernel.  There was no function call overhead
when calling routines in the microkernel.  You also wanted to put routines
that were likely to call each other in the same "overlay."  The highest
overhead was encountered when a routine in overlay A called a routine
in overlay B.

Swapping out parts of the kernel was never part of the plan.  All of the
code for the kernel, even if it was 300K, was locked into core from system
startup to shutdown.  All in continguous memory.  Only the hardware
segment registers were modified to bring the appropriate pieces of this
code into the 64K "execution window."

This overlay scheme was also used at the application level.  Big Emacs would
have been impossible without it!  And it was almost completely transparent
to the programmer.

greg



--
Gregory R. Travis                Indiana University, Bloomington IN 47405
greg at cica.indiana.edu  		 Center for Innovative Computer Applications