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From: woody@tybalt.caltech.edu (William Edward Woody)
Newsgroups: comp.sys.mac.misc
Subject: Re: THIS IS INFO ON THE NEW MACS! (Unofficial, of course)
Message-ID: <1990Oct10.042854.12961@nntp-server.caltech.edu>
Date: 10 Oct 90 04:28:54 GMT
References: <149@wgate.UUCP> <5467@nisca.ircc.ohio-state.edu> <0093DEF5.46FCE8C0@uclapp.physics.ucla.edu>
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In article <0093DEF5.46FCE8C0@uclapp.physics.ucla.edu> jprice@uclapp.physics.ucla.edu (John Price) writes:
>>o Also, most will have 32-bit clean ROMs.
>
>	Dumb question...but just what is a "clean" ROM?

	Okay, kiddies, let's have a bed-time story, okay?

	Once upon a time, when the first Macintosh came out, it only had 128K
of RAM and a 68000 CPU (which used only the bottom 24 bits of the 32 bit
address).  Apple, in their infinite wisdom, desided to use the top 8 bits
of the address (which the hardware ignored) as 'flag bits' for the memory
manager.  This saved some memory (probably an average of 256 bytes in a
typical application), but at the cost of using address bits for things they
weren't supposed to be used for.

	When Apple desided to use the 68020 in the Macintosh II, they finally
had a processor which used all 32 bits of the address, but the operating system
still used the top 8 bits for address.  So the original Mac II used the
'AMMU', the Apple Memory Management Unit, (for machines without a PMMU) to
effectively ignore the top 8 bits of address.

	The upshot of this is in a CPU which can address up to 4 gigs of memory,
the Macintosh Operating System could address only 16 meg.

	Now a lot of applications desided that 'if it's good enough for Apple,
it's good enough for me,' and started using the top 8 bits of the address lines
for their own use.

	So, we now have the relm of the '32 bit clean' applications and ROMs,
and the '32 bit dirty' stuff.  The ROMs were rewritten (in a backwards
compatable way) to use a slightly different memory management system which
did not use the top 8 bits for random flags.  (That apple could do this
without seriously hurting 99% of the applications out there is testimony
of the original design of the Macintosh.)

	But there's still a lot of Macs out there which use the old (32 bit
dirty) ROMs, such as my Macintosh II.  If the application is 32 bit clean,
and the ROMs are 32 bit clean, then it should be possible to use up to
4 gigabytes to run the application.  If not, then you must live in 16 megs.

	Hope this clears the air a little...

--
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