Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!mailrus!cornell!rochester!udel!mmdf
From: Leisner.Henr@xerox.com (marty)
Newsgroups: comp.os.minix
Subject: Re: critical sections
Message-ID: <3433@louie.udel.EDU>
Date: 22 Jul 88 18:09:24 GMT
Sender: mmdf@udel.EDU
Lines: 75

John,

I've been using the following similar functions and their equivalents on a
number of processor architectures (including National 32xxx, TI34010,  8085/Z80,
80[12]86.

Lock() returns the old PSW (processor status word) and disables interrupts.
Restore(psw) sets the psw.  Restore can also be used from C to do other things
to the PSW.  If anything I think the CTASK implementation is less advantageous.
The following is  easier  and faster to implement since no bit
shifting/twiddling is involved.
Generally programs don't care about the PSW.  But simple disable/enable doesn't
work because you don't know what the interrupts were doing on the way in.
It fits into the CTASK structure, since the data returned is not used by the
program.

I do like the syntax.

The following is the Aztec assembler  code for 80286 (procdef just provides
convenient ways to pass arguments):

; File: int80286.asm - created by Marty Leisner
; leisner.Henr         20-Jul-88 14:53:05

; Copyright (C) 1987, 1988 by Martin Leisner. All rights reserved.


	include lmacros.h
	
; C callable routines to handle critical sections similar to the Xinu
; approach.
;
; C program would execute
;	ps = disable();
;	....
;	restore(ps)
;  to alter disable interrupts without changing them at the outcome.
	procdef	disable
	pushf
	cli		
	pop	ax
	pret
	pend	disable
	
	procdef	restore, <<proc_status, word>>
	push	proc_status
	popf		; flags are according to proc_status
	pret
	pend	restore
	
	finish

	end	

For a TI34010 (to see another example), it looks like this:
unsigned        disable()
{
        asm("   getst   a8");   /* save status to a8, TI GSP C compiler returns
answers
                                 * in a8
                                 */
        asm("   dint");
}

void restore(ps)
register unsigned ps;   /* force C compiler to shove ps into A9 (!?! check) */
{
        asm("   putst   a9");
}

marty
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