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From: mwilkins@jarthur.Claremont.EDU (Mark Wilkins)
Newsgroups: comp.sys.mac.system
Subject: There are 2 different 32-bit modes, folks!  (Was Re: System 7...)
Keywords: System 7, Virtual Memory, 32 Bit Clean
Message-ID: <7258@jarthur.Claremont.EDU>
Date: 25 May 90 10:07:09 GMT
References: <145253@felix.UUCP> <1990May23.052032.26750@Neon.Stanford.EDU> <2609@wrgate.WR.TEK.COM> <742@earth.cs.utexas.edu> <7249@jarthur.Claremont.EDU> <744@lovelady.cs.utexas.edu>
Organization: Harvey Mudd College, Claremont, CA 91711
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  I will try to clear up the raging confusion about 32-bit mode and Virtual
Memory in System 7.

  All Macs have the 24-bit memory manager.  What this means is that when
your program requests memory, it is pointed to by an address for which only
the bottom 24 bits are valid.

  The Mac IIci and the Mac IIfx ALSO have a 32-bit memory manager.  With
this memory manager, all 32 bits are valid for every address, and the stuff
that was kept in the other 8 are pushed off elsewhere.  This memory manager
is ONLY available on a IIci or IIfx.  The ONLY upgrade path which will get
you there is the one which takes you to those machines.  This upgrade is NOT
free.

  (Bear with me, Ted, I'm getting to you.)

  However, all Mac II class machines as well as the SE/30 have what is known
as 32-bit addressing mode.  This means that if the processor is accessing an
address, it TRIES to use all 32 bits.  On any machine other than the IIci or
the IIfx, this can (generally) only be used to access the NuBus slots.  If
you turn this mode on and send the processor a 24-bit address, the machine
crashes.

  Virtual memory, by the way, doesn't need 32-bit addressing of any kind to
work.  That's why it works great on Mac IIs and SE/30s.

  On with the circus...

In an earlier article ted@cs.utexas.edu (Ted Woodward) challenges:

>Just like the 24 bit mem managers in ROM in the older machines, this is fixed
>in software.  Or I guess that really wasn't sys 7 I played with on that SE/30.
>Or AUX (1.0, but still 32bit) on that IIx...

    A/UX doesn't use the ROM memory manager at all.  It has its own memory
    manager, designed just like that of the very UNIX system you're probably
    using.  However, this type of software solution will not be built into
    System 7.

    As for the SE/30 question, see below.

>My boss (service manager for the Texas Union Microcenter here at UT Austin,
>which has just won some Apple service award or somesuch) has told me that
>early Mac II's have ROMS that aren't compatable, and there is a FREE (I think
>he said free) ROM upgrade.

    Early Mac IIs don't like 32-bit mode, new ones can tolerate it.  That 
    doesn't mean that they have a 32-bit memory manager.  True, that fix is
    free.  It is, however, separate from the PMMU wiring problem.  You can
    have the newer ROMs and your PMMU still won't work because some of the 
    connectors are just hanging there.

>And yes, that SE/30 mentioned earlier was using VM, for a total of 11MB
>(not enough space on the disk partitioned for more...sigh)

    Note that Virtual Memory can get you up to 16 megabytes WITHOUT 32-bit
    cleanliness of any sort.  (What's 2^24?  Anyone?  Anyone?)  Note that
    each NuBus card takes up 1 megabyte of the bottom 16 in the address
    space, and that the ROM takes up another.

I guess my point is that needless upgrading can be a waste of money.  There
is a lot of misinformation about this stuff.  So before Mr. Woodward sells
you his IIx, please learn the facts and decide for yourself whether another
hardware platform will better suit your needs.  :-)

-- Mark