Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!decvax!tektronix!tekecs!frip!andrew
From: andrew@frip.gwd.tek.com (Andrew Klossner)
Newsgroups: comp.arch
Subject: Re: Not-so RISCy
Message-ID: <11040@tekecs.TEK.COM>
Date: 20 Feb 89 16:46:10 GMT
References: <732@wpi.WPI.EDU>
Sender: andrew@tekecs.TEK.COM
Organization: Tektronix, Wilsonville, Oregon
Lines: 28

[]

	"Would it be a terrible hardship to only have two data sizes
	(perhaps character and word) and not allow words to cross word
	boundaries?"

Data sizes: you need do atomic 8-bit, 16-bit, and 32-bit loads and
stores in order to deal with all the sorts of device registers you
might meet.

No crossing boundaries:  absolutely.  My favorite machines prohibit
this.  I find it useful for detecting garbaged pointers while
debugging.  Also, it's quite convenient for the kernel if an
instruction can't reference more than one page, which wouldn't be so if
a load could refer to a word that starts in one page and ends in
another.

On the down side, this breaks a lot of existing code.  A pathological
case is the Fortran program that uses EQUIVALENCE statements to do its
own memory allocation (there being no such facility in the language),
and which "knows" that a double can be equivalenced to any integer.
This gives rise to double references at addresses that are not
multiples of eight bytes.  One workaround is to have the Fortran
compiler fall back to using two single-word operations to fetch or
store a double.

  -=- Andrew Klossner   (uunet!tektronix!orca!frip!andrew)      [UUCP]
                        (andrew%frip.wv.tek.com@relay.cs.net)   [ARPA]