Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!zrm
From: zrm@eddie.MIT.EDU (Zigurd R. Mednieks)
Newsgroups: comp.sys.mac
Subject: Re: Is Mac II flawed?
Keywords: Nubus architecture
Message-ID: <7630@eddie.MIT.EDU>
Date: 10 Dec 87 20:23:22 GMT
References:  <952@uokmax.UUCP>
Reply-To: zrm@eddie.MIT.EDU (Zigurd R. Mednieks)
Organization: MIT, EE/CS Computer Facilities, Cambridge, MA
Lines: 40

In article <952@uokmax.UUCP> jlperkin@uokmax.UUCP () writes:
	 I have been told about an article appearing in MacUser a few
months ago stating that there was a problem with the Nubus...
.... According to thissource the rom placement
makes this impossible because of the downward compatibility of the Mac
II with the SE. Therefore it would take a complete motherboardchange
to correct the problem. In other words, big bucks! It also means you
...
 > > >J. Les Perkins

The Mac II motherboard is a virtual NuBus slot, but that should not
stand in the way of an upgrade board "mapping in" the Mac II ROMs, or
more likely, copying them into faster memory. Motherboard assets like
the SCSI and seriial ports could also be mapped into any addressing
scheme from any NuBus slot.

The SE compatibility mode consists of making each NuBus slot a 1MB
partition in a 16MB SE-like address space. Why they bothered with
this, I don't really know, since no reasonable application really
cares about anything outside the application heap, nor even where the
application heap is. This makes life tough for high-end (e.g. 24-bit)
graphics cards because there isn't enough address space in each slot
under this mapping scheme. (Hey Apple, any good answers for this
seemingly backward-thinking design decision? And what about future MacII
OSs? Will they have full 32-bit NuBus addressing for Mac NuBus
peripherals?) 

None of this, however, nixes the idea of a NuBus upgrade card for a
68030 or other future processor. Nor is a multiprocessor setup out of
the question. In fact, a separate NuBus processor would not have to
limit itself to the 1MB per card addressing scheme of the current Mac
OS.

-Zigurd

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