The Unix Heritage Society mailing list
 help / Atom feed
From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Warner Losh <imp@bsdimp.com>
Cc: The Unix Heritage Society <tuhs@minnie.tuhs.org>,
	Grant Taylor <gtaylor@tnetconsulting.net>
Subject: Re: [TUHS] Pre-init initialization
Date: Thu, 8 Aug 2019 09:31:24 -0400
Message-ID: <20190808133124.GA3340@mit.edu> (raw)
In-Reply-To: <CANCZdfpPbnB+8GGbzT65dRigDEX=i4qQEsy9MKyV3TnxS8-v2w@mail.gmail.com>

On Wed, Aug 07, 2019 at 10:40:06AM -0600, Warner Losh wrote:
> The move into RAM disks happened as memories grew. This obviated the need
> to copy a filesystem to a new disk's swap space and then use that to really
> install the system.
> 
> Kernels with bundled RAM disks were a thing in BSD for a long time, though
> I'm unsure when that functionality entered the code stream. I know the
> early versions of FreeBSD's installer ran out of a RAM disk loaded off
> floppies, though that's not the same as initrd since there never was a
> pivot to something else: just a reboot to boot the real thing.

I implemented loading the root file system as a ramdisk from a floppy
in 1991 for Linux.  This predates the publication of 386BSD, and I
wasn't aware of how commercial Unix systems did their initial
installation and setup.  So as far as I know I came up on it on my
own, but none of this was especially earthshaking or subtle.

Linux's ramdisk was implemented because most PC's of the day (I was
using a 40 MHz 386 with only a megabyte of memory) only had a single
floppy drive (and if they had a second it was a 5.25" drive).  So the
very first implementation which I did assumed the kernel would be no
larger than 512k, and looked for Minix file system signature at 512k.
If it existed, it would load it into the ramdisk and use that as the
boot region.

Before that, the kernel was loaded from the first floppy, and then
kernel would prompt the user to eject the boot disk and insert the
root disk, and hit any key to continue, and then it would mount the
root disk.  But then the only floppy drive would be tied up with the
root file system, so that meant there needed to be a second kernel on
the root floppy, and everything else needed to set up the root image
would have to be copied to the hard disk, and then you'd have to
reboot to release the floppy drive so you could install other programs
that couldn't fit on that initial 1.44MB root floppy.

For people who are interested in the history of initrd used as part of
the boot process in Linux, please refer to Werner Almesberger's 2000
paper, "Booting Linux: The History and the Future"[1], presented at the
Ottawa Linux Symposium.

[1] http://www.almesberger.net/cv/papers/ols2k-9.ps.gz

Cheers,

					- Ted

  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-06 23:46 Grant Taylor via TUHS
2019-08-07  1:22 ` Andrew Warkentin
2019-08-07 13:04 ` Clem Cole
2019-08-07 16:40   ` Warner Losh
2019-08-08 13:31     ` Theodore Y. Ts'o [this message]
2019-08-09  1:52       ` Grant Taylor via TUHS
2019-08-10  0:23         ` Theodore Y. Ts'o
2019-08-10  2:28           ` Grant Taylor via TUHS
2019-08-10  4:21             ` Kevin Bowling
2019-08-10  4:50               ` Grant Taylor via TUHS
2019-08-10  4:52               ` Adam Thornton
2019-08-08 13:43     ` Clem Cole
2019-08-08 18:59       ` Warner Losh
2019-08-09  1:46   ` Grant Taylor via TUHS
2019-08-09  2:15     ` Warner Losh
2019-08-09  6:43       ` Kevin Bowling
2019-08-10  1:45 ` Chris Hanson

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190808133124.GA3340@mit.edu \
    --to=tytso@mit.edu \
    --cc=gtaylor@tnetconsulting.net \
    --cc=imp@bsdimp.com \
    --cc=tuhs@minnie.tuhs.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

The Unix Heritage Society mailing list

Archives are clonable: git clone --mirror http://inbox.vuxu.org/tuhs

Newsgroup available over NNTP:
	nntp://inbox.vuxu.org/vuxu.archive.tuhs


AGPL code for this site: git clone https://public-inbox.org/ public-inbox