[TUHS] kernel boots kernel in 1977

[ron minnich]

well, yes, on many systems, there's a lot that runs before the kernel. But
if you have a risc-v system with oreboot, you own the system. The problem
is that on most of these systems a reset will stop the dram clock for a
little bit, or glitch clock enable, or dram power, or whatever. New systems
are not designed to allow this.

Ideally, we could force a reset of everything save memory, but modern
systems are not designed in this way. Most annoying.

On Wed, Sep 18, 2024 at 4:38 PM Bakul Shah <bakul at iitbombay.org> wrote:

> 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 at 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 at 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 at 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.
>>
>>
>
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