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From: daver@hpindda.HP.COM (Dave Richards)
Newsgroups: comp.os.minix
Subject: Re: Controlling processer status -- Minix v. Xinu
Message-ID: <3580004@hpindda.HP.COM>
Date: Sat, 3-Oct-87 18:51:10 EDT
Article-I.D.: hpindda.3580004
Posted: Sat Oct  3 18:51:10 1987
Date-Received: Wed, 7-Oct-87 06:38:54 EDT
References: <87@rocksvax.UUCP>
Organization: Hewlett Packard, Cupertino
Lines: 46

> Minix uses two routines -- lock()/restore() which saves the processor 
> status in a common private variable (lockvar).

> If found this way of coding to often leads to problems when you want to
> nest subroutines which each require the interrupts be turned off,
> and you don't want to write the code assuming the low-level subroutine is
> running with the interrupts turned off.

> Xinu uses two subroutines -- disable()/restore().  Disable returns the
> processor status and restore gets passed the new processor status -- the
> psw should be stored in local memory of the calling function.

> I've used variants of the lock/restore/unlock variant for several years and
> have gotten bitten too many times than I care to remember.  I'm going to
> change all references of lock/restore to disable/restore (I just got bitten
> again doing some hacking).

> Any comments?

I have Xinu running on a 68000 based system.  I found that even disable/
restore was too limiting!  I didn't like the idea of turning off my
clock interrupts unnecessarily, (since some useful things get done
by my clock interrupt routine).  So I went to the spl() mechanisms of
UNIX.  In spl you specified the level of interrupt you want to disable.
For example spl4() disables a set of interrupts, LAN card, tty devices
while allowing clock interrupts to occur.  spl7() turns off everything
save the non-maskable interrupt.  Like disable/restore you can do the
following:

	s = spl4();

	/* critical region */

	splx(s);

I plan on using this scheme in Minix, as well.  The different levels
will allow different interrupts (via the interrupt chip).  Disable/
restore is definitely cheaper to implement on the 80*86 and for your
needs may be exactly what you want.  When you start looking at
more than a couple interrupting devices you may find that you don't
want to lock out all interrupts all the time.

Just something to think about.

	Dave Richards
	HP Information Networks Division