13 May
2025
13 May
'25
11:05 p.m.
On Mon, 12 May 2025, Noel Chiappa wrote:
I found a /jnk directory in MIT-CSR's root; it
had a lot of interesting
stuff in it. One particularly interesting one was 'kov':
The idea of this kernel overlay scheme is to increase the amount of
code that can be included in the UNIX kernel. This is done by reserving
one of the I space segmentation registers (the highest free, usually
KISA5 for non-split systems) and changing its value dynamically so as
to map in the appropriate code as needed. I chose to use only one page
register (limiting each KOV to 4Kw), in order to minimize the mapping
overhead.
Hammering KISA5 on 40-class machines seemed to have been a common trick in
those days. For example, the AUSAM buffering scheme at UNSW had a global
kernel symbol "b" (analogous to "u") sitting at 100000 (?) that
pointed to
the current buffer header; you could have as many buffers as physical
memory allowed, ending the dreaded buffer-hang problem.
-- Dave