Path: utzoo!attcan!uunet!husc6!mailrus!ames!oliveb!pyramid!prls!mips!earl
From: earl@mips.COM (Earl Killian)
Newsgroups: comp.arch
Subject: Re: Is the Intel memory model safe from NO-ONE ?!?
Message-ID: <2183@gumby.mips.COM>
Date: 14 May 88 01:03:19 GMT
References: <353@cf-cm.UUCP> <3095@edm.UUCP>, <20618@think.UUCP> <1988May12.162207.16764@utzoo.uucp>
Lines: 22
In-reply-to: henry@utzoo.uucp's message of 12 May 88 16:22:07 GMT


   From: henry@utzoo.uucp (Henry Spencer)
   Newsgroups: comp.arch
   Date: 12 May 88 16:22:07 GMT
   Organization: U of Toronto Zoology

   > ... it traps into SUPERVISOR mode, even though the program
   > that executed the divide instruction was running in USER mode.  Why
   > should a zero-divide need to be handled by the protected kernel,
   > rather than simply trapping to a user handler?

   Probably because practically every machine in existence routes *all*
   traps and interrupts to the kernel, which can pass them on to the user
   if it pleases.  I know of no machine, offhand, whose hardware has any
   notion of a "user handler".

The PDP-10's LUUO instructions trap to a handler in the user address
space.  They are commonly used in PDP-10 programs to extend the
instruction set.
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