Xref: utzoo unix-pc.general:1512 comp.sys.att:4471
Path: utzoo!attcan!uunet!cbmvax!ditto
From: ditto@cbmvax.UUCP (Michael "Ford" Ditto)
Newsgroups: unix-pc.general,comp.sys.att
Subject: Re: Those LEDs in the UNIXpc, one last time; (HwNote01)
Summary: (*MCR_ADDR = mcr_save = foo) is OkeyDokey.
Keywords: 7300 LED drivers kernel
Message-ID: <4975@cbmvax.UUCP>
Date: 11 Oct 88 18:01:46 GMT
References: <357@uncle.UUCP>
Reply-To: ditto@cbmvax.UUCP (Michael "Ford" Ditto)
Organization: Commodore Technology, West Chester, PA
Lines: 20

In article <357@uncle.UUCP> jbm@uncle.UUCP (John B. Milton) writes:
>As as an example, "led_on(LED0)" produces the code:
>	and.w	&65279,mcr_save
>	mov.w	mcr_save,4849664
>Note that since there is no locking here, an interrupt routine that uses the
>MCR could run between these two instructions.

But also note that this is not a problem if the interrupt routines only
*write* to the mcr and mcr_save (which should be the case) because
mcr_save is updated first.  The only potential problem is if an interrupt
routine *reads* mcr_save assuming that the hardware is in the same
state.  I don't know of any need to read mcr_save, so I doubt that this
is ever done.
-- 
					-=] Ford [=-

"The number of Unix installations	(In Real Life:  Mike Ditto)
has grown to 10, with more expected."	ford@kenobi.cts.com
- The Unix Programmer's Manual,		...!sdcsvax!crash!elgar!ford
  2nd Edition, June, 1972.		ditto@cbmvax.commodore.com