Path: utzoo!attcan!uunet!samsung!usc!jarthur!mwilkins
From: mwilkins@jarthur.Claremont.EDU (Mark Wilkins)
Newsgroups: comp.sys.mac.system
Subject: Re: There are 2 different 32-bit modes, folks!  (Was Re: System 7...)
Message-ID: <7285@jarthur.Claremont.EDU>
Date: 27 May 90 04:58:46 GMT
References: <7258@jarthur.Claremont.EDU> <9390003@hp-ptp.HP.COM>
Organization: Harvey Mudd College, Claremont, CA 91711
Lines: 51

In article <9390003@hp-ptp.HP.COM> davew@hp-ptp.HP.COM (Dave_Waller) writes:
>mwilkins@jarthur.Claremont.EDU (Mark Wilkins) writes:
>> 
>>     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.
>
>Let me see if I have this straight -- My IIcx (purchased in March... I
>assume it has the latest ROMs available for the IIcx) uses a memory
>manager that has 24 bits of addressing. This is 16MB of address space.
>Does that mean that I can address 16MB of Virtual Memory on my 4MB
>system?

  If I implied that the NuBus cards and ROM took up the bottom several
megabytes of the address space, I wasn't clear enough.

  "each NuBus card takes up 1 megabyte of the bottom 16 in the address space, 
and that the ROM takes up another."  That means that of the address space
which can be accessed with 24-bit addressing, you lose 1 MB for each card
and 1 for ROM.  The addresses you lose are at the high end of the 16MB
address space.  Actually, under the current version of System 7, you lose
1MB for each SLOT, not each CARD.  However, Connectix' Virtual 2.0 solves
this problem and later versions of Apple's VM might also.

  The addressing space available to your programs, on a IIcx, therefore, is 
0x000000 to  0xBFFFFF.  0xC00000 to 0xFFFFFF is allocated to NuBus slots and
ROM.  Yes, it's a hardware thing.  However, it MUST take up part of the
virtual address space because user programs access that memory directly, and
user programs have no means of bypassing the PMMU's memory mapping.  If you
had 16 MB of virtual RAM, there would be no part of the 24-bit address space
left which a program could use to get directly at video RAM, say, or the
ROM.

>My IIcx has the same damn HARDWARE as the IIci in terms of the
>integrated PMMU in the '030. Why is it not possible for Apple to offer a
>ROM upgrade to the IIcx that supports Clean, full 32-bit Memory
>managment? I would appreciate a clear answer to this from someone that
>knows... Don't worry about getting too technical.


  It sure is possible.  Apple just hasn't chosen to.  Quite possibly, they
are working on it.  However, if they were to announce such a thing, I would
guess that they'd wait until System 7 was announced to do so, because under
6.0.5 there's no reason to have a 32-bit clean memory manager.
  In fact, they might even come up with a better, software solution.  I'm
not contending that such a thing can't be done, I'm just saying that it
hasn't been announced and that there's no way to get the 32-bit memory
manager using EXISTING UPGRADES unless you go to a IIci or IIfx.

-- Mark Wilkins