Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!munnari.oz.au!bruce!cechew
From: cechew@bruce.cs.monash.OZ.AU (Earl Chew)
Newsgroups: comp.os.minix
Subject: Re: Loading in protected mode... (was Re: Received:)
Message-ID: <3517@bruce.cs.monash.OZ.AU>
Date: 22 Dec 90 22:52:47 GMT
References: <39678@nigel.ee.udel.edu> <3492@bruce.cs.monash.OZ.AU> <1990Dec21.061152.22251@menudo.uh.edu>
Organization: Monash Uni. Computer Science, Australia
Lines: 67

In <1990Dec21.061152.22251@menudo.uh.edu> kevin@nuchat.UUCP (Kevin Brown) writes:

>>you have more than 640k machine as most do these days). This limits the size of
>>kernels to about 586k.

>This is not strictly true.  The segment registers have a resolution of 16
>bytes.  That is, they form the upper 16 bits of a 20 bit base address (for
>8088 mode, that is).  With this in mind, you can place the loader such that 
>its end is within 16 bytes of the end of 640K (but make sure you have room 
>for any data space the loader might want to use at the end of the loader 
>image!).  This is what I have effectively done.

Yes. What you say is true. For shoelace I had to invoke the `64k dma rule'
hence 640k-64k=586k.

>Once I have 16 meg, the "need" for a protected-mode loader which dynamically
>allocates uninitialized data segments will be greater.

>Now, now, Earl...no need to be jealous.  :-) :-) :-)

I suppose that until I get another 1-3Mb, my need won't be so great --- so you
might have to be patient :-)

>Have you guys patched up the kernel in such a way that this behavior is
>portable, i.e. not dependent on the existence of shoelace?  It would be
>very nice to be able to allocate arbitrarily large buffer caches and such.  
>In fact, it would be real nice to be able to grow the process table on 
>demand, too.

I'm not really in a position to answer these are Bruce's mods and I am not
using them yet (remember, I've got the `little' machine).

>I was under the impression that, once Minix got started, it didn't use anything
>in the BIOS space.  I know it needs it for initialization, though, but that's
>about it as far as I know.

bios_wini uses the bios. tty uses the video page(s).

>You can probably get away with using memory below 1Mb as uninitialized data
>space for some of the kernel structures.  Or you can cause mm to realize that
>there's free memory there and it will allocate it when it can.

This is painful. PC memory is already fragmented. Using paging to provide a
clean map is easiest (until VM comes).

>Well, how does the kernel switch to protected mode to begin with?  I would
>think you could use the same technique.  Pardon my simplicity, but why can't
>you simply move the code to do this from the kernel bootstrap procedure to
>the loader?

I suppose so, but there may be complications when the kernel wants to set up
page tables then switch to paged mode when it's already in paged mode!

>Ouch.  No memory above 1M?  I don't know if I would be able to live with
>that.  I can barely live with 8 meg.  :-)

I used to live in 16k many years ago. 1M is comparative bliss. Actually, those
with C&T 386 chipsets can reclaim lots of shadow ram and get most of their
lower 1M back.

Merry Christmas

Earl
-- 
Earl Chew, Dept of Computer Science, Monash University, Australia 3168
EMAIL: cechew@bruce.cs.monash.edu.au PHONE: 03 5655447 FAX: 03 5655146
----------------------------------------------------------------------