Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!netcom!hill
From: hill@netcom.UUCP (Tom Hill)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Porting UNIX apps. to MS-DOS
Message-ID: <16602@netcom.UUCP>
Date: 11 Nov 90 00:58:22 GMT
References: <1990Oct26.223541.26634@NCoast.ORG> <1990Oct31.190520.11526@Octopus.COM> <1990Nov10.223333.4628@rti.rti.org>
Organization: Netcom- The Bay Area's Public Access Unix System {408 241-9760 guest}
Lines: 27

In article <1990Nov10.223333.4628@rti.rti.org> bcw@rti.rti.org (Bruce Wright) writes:
...
>> >  oldSS = _SS;
>> 	
>> 	You *MUST* disable interrupts during this operation. If an interrupt
>> 	occures during the stack segment transition, you will not return
>> 	from the interrupt correctly and crash the machine.
>
>This is misleading.  It's not necessary to disable interrupts when
>swapping stack segments if you do it right!  The point is that the
>80x86 processors disable interrupts for one instruction after loading
>the SS register (some of them disable interrupts for one instruction
>after loading ANY segment register, but later processors limited it
>to SS - I forget the exact details of which processors behave which
>way because it's usually not important).  So if the next instruction
>IMMEDIATELY after loading the new value into SS loads the new value
>into SP, then you're golden and don't need to mess around with manually
>disabling interrupts.

Sorry, but this is only correct for the later intel processors. The original
8088s did not disable the interrupts after a load of SS, thus requiring
thta interrupts be disabled. If you know your code will only run on
ATs & up, sequential loads of SS & SP work fine. If you want to be sure
your code will run on your occasional original PC, disable the interrupts.


Tom Hill