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From: davet@oakhill.UUCP (Dave Trissel)
Newsgroups: net.micro.68k
Subject: Call for 68K architectural change evaluations
Message-ID: <415@oakhill.UUCP>
Date: Sat, 11-May-85 09:08:47 EDT
Article-I.D.: oakhill.415
Posted: Sat May 11 09:08:47 1985
Date-Received: Mon, 10-Jun-85 04:39:14 EDT
Organization: Motorola Inc. Austin, Tx
Lines: 43


OK all you 68K gurus out there let's have your opinion on this architectural
change to the family.

Some of us have been considering allowing writes to PC space.  The MOVE
instruction with the destination ea of any PC usage is now illegal.  This    .
means we could easily allow all of the PC modes here. I haven't checked on
the other instructions yet.

So far I have come up with the following conclusions on the matter:

 1) MMU systems may have compatibility problems since they may be unprepared
    to handle an error on a write to code space.  Of course, if they always
    set MMU control bits such that code space writes were invalid anyway
    they wouldn't notice any difference.  Non-MMU systems don't depend on
    any protection but they may get tripped up in the bus error
    handler if they make the no-write assumption there (such as assuming
    a write error must be data space.)

 2) High-Level Languages will not be affected in the least nor will there be
    any need for them to utilize the newly available modes. This is because
    they divide the code and data into two different sections anyway and so
    never have a need to perform writes to the code area to begin with.

    This leaves the real use in the ballpark of assembler programmers.

 3) Many, indeed most, 68K systems do not make a distinction between code
    and data space function codes even if an MMU is utilized.  (As an example
    the following code will work on these systems:
		       LEA  THIS,An       Point to code space item
		       CLR  (an)          Clear word there
    since the address map is not split between code and data.)  Or better said
    they doubly map the code space into both code and data space function
    codes.  This would indicate the additional modes would be allowing
    functionality already allowed in most systems anyway.

This is all I've come up with so far.  Any comments on these or ideas of
your own will be appreciated.  Since I presume everyone on this newsgroup
would be interested go ahead and post them here.  We especially want to hear
from anyone thinking that this may be a bad idea.

Motorola Semiconductor             Dave Trissel
Austin, Texas             {ihnp4,seismo,gatech}!ut-sally!oakhill!davet