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From: valentin@cbmvax.commodore.com (Valentin Pepelea)
Newsgroups: comp.sys.atari.st
Subject: Re: Atari TT 030 Launched!
Keywords: Atari TT
Message-ID: <12559@cbmvax.commodore.com>
Date: 11 Jun 90 21:39:55 GMT
References: <1990Jun5.143231.4977@watserv1.waterloo.edu> <13266@wpi.wpi.edu> <81214@tut.cis.ohio-state.edu> <1990Jun6.044350.20403@cbnewsh.att.com> <5503@jhunix.HCF.JHU.EDU> <12551@cbmvax.commodore.com>
Reply-To: valentin@cbmvax (Valentin Pepelea)
Distribution: usa
Organization: Commodore, West Chester, PA
Lines: 36

In article <12551@cbmvax.commodore.com> daveh@cbmvax (Dave Haynie) writes:
>In article <5503@jhunix.HCF.JHU.EDU> jheddy@jhunix.UUCP (Jared Brennan) writes:
>
>> However, I believe that the line A traps are real instructions
>> on the 68030, while they are invalid on the 68000, which allows them to be
>> used for ST graphics primitives.  
>
> No, line A still forces an exception on the 68030.  Mac uses them too, though
> I don't really know why; an exception is considerably more expensive than a
> subroutine call, even indirect subroutine calls such as used in Amiga 
> libraries.

The advantage of A-line instructions is that they drop you into supervisor
mode instantly. Thus functions that need access to supervisor-only registers
do not need to call the Supervisor() function like our operating system does.
Should the writers of these operating systems then decide to implement memory
protection, the function call methodology is already properly implemented.
The Amiga on the other hand would have to either insert an A-line instruction
as the first instruction of each function call, or call the Supervisor()
function.

All this is needed for parameter checking, which has to be done within a
protected supervisor-only piece of code. So while A-line is apparently a
clumsy way of initiating OS functions calls, when memory protection is
implemented A-line instructions prove to be a preferable. A future Mac
operating system is rumored to have memory protection, and yes, they also
use A-line instruction for OS calls.

> Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"

Valentin
-- 
The Goddess of democracy? "The tyrants     Name:    Valentin Pepelea
may distroy a statue,  but they cannot     Phone:   (215) 431-9327
kill a god."                               UseNet:  cbmvax!valentin@uunet.uu.net
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