[TUHS] 2.9BSD on an actual rl02 - swap confusion

Clem Cole clemc at ccc.com
Mon Feb 9 08:03:17 AEST 2015


On Sun, Feb 8, 2015 at 3:42 PM, Noel Chiappa <jnc at mercury.lcs.mit.edu>
wrote:

> I don't know how the later systems work, but in V6, the swap device, and
> the
> start block / # of blocks are specified in the c.c configuration file (i.e.
> they are compiled into the system). So you can take one partition, and by
> specifying less than the full size to 'mkfs', you can use the end of the
> partition for swap space (which is presumably what's happening with
> /dev/rl0
> here).
>
‚ÄčAh Noel - Thanks for the refresher course.   That's right.  I now remember
it.  I knew it was compiled into the kernel but I had forgotten the details.

It was not until much later that the swap image became less screwed
down/more reflexible.   You first needed to get to larger disks (rp05/rp06)
which had to be partitioned because they overflowed a 16 bit integer‚Äč.
Once people started to partition them, then all sort of new things occurred
and I that's when the idea of a dedicated swap partition came up.   I've
forgotten if that was a BSDism or UNIX/TS.   Certainly by the time of Sam's
4.1A?? configuration tool that created conf.c and low.s it had already been
in for a while.

As I recall in V6 and I think V7, the process was first placed in the swap
image before the exec (or at least space reserved for it).   So you had to
have a swap space to boot because to fork the "init" it needed to assigned
to the swap space (chick/egg issue). When demand support was added to the
kernel, the process did not have to have that requirement, so it meant swap
set up could be a post initial program load operation for the start
sequence.

Clem


Clem
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