Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!cis.ohio-state.edu!tut.cis.ohio-state.edu!ucbvax!agate!usenet.ins.cwru.edu!ncoast!allbery
From: allbery@NCoast.ORG (Brandon S. Allbery KF8NH)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: 8-bit death
Message-ID: <1991May13.224027.9132@NCoast.ORG>
Date: 13 May 91 22:40:27 GMT
References: <3626.tnews@templar.actrix.gen.nz> <1991May12.181436.20304@NCoast.ORG> <3691.tnews@templar.actrix.gen.nz>
Reply-To: allbery@ncoast.ORG (Brandon S. Allbery KF8NH)
Followup-To: comp.sys.amiga.advocacy
Organization: North Coast Public Access Un*x (ncoast)
Lines: 50

As quoted from <3691.tnews@templar.actrix.gen.nz> by jbickers@templar.actrix.gen.nz (John Bickers):
+---------------
| Quoted from <1991May12.181436.20304@NCoast.ORG> by allbery@NCoast.ORG (Brandon S. Allbery KF8NH):
| > TRAP is slower, but JSR doesn't switch the processor to supervisor state.
| > Yes, Unix runs the kernel in supervisor and user programs in user mode.  Why
| > do you think Unix programs core dump instead of guru-ing?
| 
|     I thought part of that had to do with using MMUs. Except for the fact
|     that Amiga programs can blat into sensitive places (the Blitter,
|     etc), why not control reads/writes with the MMU, and use the TRAPs
|     for trapping what they're supposed to trap like GOMF does already?
+---------------

They're both needed.  If programs run in supervisor mode, they can defeat the
MMU; if they run in user mode without MMU memory protection, they can still
hode things up good for the system or other programs.

+---------------
| > Actually, 680x0 Unix usually puts the system call in d0.
| 
|     Nevertheless, hidden. I haven't noticed an intfoo() function in
|     a Unix compiler yet.
+---------------

Later MS-DOS C compilers come with real libraries, so int() isn't necessary
for system calls.  BIOS calls are another matter, but they're a prime symptom
of brain-damage anyway.

+---------------
|     Does the Amiga? I'm as sure as I ever am about anything that it
|     doesn't.
+---------------

So user processes run in user mode.  But the system has to run in supervisor
mode to manipulate the MMU... oh, that's right, the Amiga doesn't use the MMU.
(Yes, I know about compatibility and the 68000.)  So does the Amiga ever use
supervisor mode?  And if it does, how does it get there without a trap?

And using a non-interrupt entry point runs the risk of somebody accidentally
typo'ing a #define and making every program in existence break as a result.
That's why people moved from JSR's to interrupts in the first place (I recall
LDOS trumpeting their svc interface as an alternative to CALLs to invoke OS
functions, waaaaay back when).

++Brandon
-- 
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