Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!nstn.ns.ca!news
From: maceache@fox.nstn.ns.ca (Tim Maceachern)
Newsgroups: comp.arch
Subject: 68xxx memory addressing
Summary: idea on instructions using altered 68000 addressing
Keywords: tag 68000
Message-ID: <1991Jan16.210201.7962@nstn.ns.ca>
Date: 16 Jan 91 21:02:01 GMT
Sender: news@nstn.ns.ca
Distribution: comp
Organization: NSTN Network Operations Centre, Nova Scotia, Canada
Lines: 14

This is an idea I had a while ago.  I believe that most applications do
not need a 32 bit address space, and most computers won't have that much
real RAM anyway.  Of course there will be those who want to run huge
applications in a 32 bit virtual memory, but...
Why not have a tagged jump instruction in the 68000 set.  It would take
the top 4 bits of the address register (or memory value) and use it as 
an indexed jump into the next 16 memory locations, each of which would
be a jump to a subroutine or such.  (Convert byte addresses to word if
necessary to create the 16-entry jump table.)

This type of instruction would be efficient for interpreters where the
type of an operand is not known at compile time.  The loss of the 4
top bits is not significant in most applications.  The variac
(whoops) variable would use a 28 bit address.