19 Sep
2024
19 Sep
'24
1:38 a.m.
I would prefer old kernel to new kernel handoff if it can be made to work reliably.
Nowadays there are a lot of things that run before the kernel gets control.
On Sep 18, 2024, at 3:38 PM, ron minnich
<rminnich(a)gmail.com> wrote:
Interesting about the amiga. I'm assuming their firmware zeros memory on reset, so
you have to do handoff from kernel to kernel, not via a reset and so on?
What was particularly nice about the V6/PDP-11 case: we were able to yank reset, which
let us cleanly reset/disable devices, because everything was in memory when we got back. I
miss the simplicity of the old machines.
On Wed, Sep 18, 2024 at 3:07 PM Christian Hopps <chopps(a)chopps.org
<mailto:chopps@chopps.org>> wrote:
>
> We had/have this functionality in the Amiga port of NetBSD.
>
> It is implemented as `/dev/reload` device and you copy a kernel image to it. In
locore.s there's code that copies the kernel image over top of the running kernel and
then restarts. I believe for it to work nothing below the copy code in locore.s can change
:)
>
> Thanks,
> Chris.
>
> Phil Budne <phil(a)ultimate.com <mailto:phil@ultimate.com>> writes:
>
> > ron minnich wrote:
> >> But I'm wondering: is Ed's work in 1977 the first "kernel
boots kernel" or
> >> was there something before?
> >
> > There was! The PDP-7 UNIX listings contain a program trysys.s
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/sys/trysys.s
> > that reboots the system by reading a.out into user memory (in the high
> > 4K of core), then copies it to low memory and jumping to the entry
> > point. The name suggests its original intended use was to test a new
> > system (kernel).
> >
> > P.S.
> > Normal bootable system images seem to have been stored in reserved
> > tracks of the (fixed head) disk (that are inacessible via system calls):
> >
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/sys/maksys.s
> > reads a.out and uses I/O instructions to write it out.
> >
> > P.P.S.
> > Accordingly, I put together a "paper tape" for booting the system:
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/other/pbboot.s
> >
> > P.P.P.S.
> > The system (kernel) is 3K words, the last 1K of low memory
> > used for the character table for the vector graphics controller.
> >
> > The definitions for the table are compiled by
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/cmd/cas.s
> > from definition file
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/sys/cas.in
> > (after, ISTR, figuring out the ordering of the listing pages!)
> >
> > I don't think we ever figured out how the initial character table
> > is loaded into core. One thing that was missing from the table
> > was the dispatch array, which I recreated:
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/other/chrtbl.s
> >
> > The system (kernel) could be built for a "cold start", reloading the
> > disk (prone to head crashes?) from paper tape? But I don't think
> > anyone ever reconstructed the procedure for rebuilding a disk that way.
> >
> > The disk was two sided, and the running system only used one side:
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/cmd/dsksav.s
> > https://github.com/DoctorWkt/pdp7-unix/blob/master/src/cmd/dskres.s
> > appear to be programs to save and restore the filesystem from the
> > "other" side of the disk.
>