Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!bloom-beacon!athena.mit.edu!dcw
From: dcw@athena.mit.edu (David C. Whitney)
Newsgroups: comp.sys.apple
Subject: Re: II gs Caracteristics
Message-ID: <15488@bloom-beacon.MIT.EDU>
Date: 29 Oct 89 14:13:39 GMT
References: <8910161202.aa01399@SMOKE.BRL.MIL> <8193@microsoft.UUCP> <36001@apple.Apple.COM> <3442@mace.cc.purdue.edu>
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Reply-To: dcw@athena.mit.edu (David C. Whitney)
Organization: Massachusetts Institute of Technology
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In article <3442@mace.cc.purdue.edu> asd@mace.cc.purdue.edu (Kareth) writes:
>In article <36001@apple.Apple.COM> mattd@Apple.COM (Matt Deatherage) writes:
>
>>brianw@microsoft.UUCP (Brian Willoughby) writes:
>>>>>- How many memory it can adress ?
>>>>   Up to 8 Mbytes
>>>
>>>Make that 16 Mbytes.  Both video RAM and ROM memory are above the 8 Meg
>
>>That's a little misleading.  The processor can address 16 MB, and the ROM and
>Very.
>
>>slow RAM in the system are both in the high part of that address space.
>>However, the hardware can't address fast RAM above the 8 MB mark, and the
>>Memory Manager doesn't look for it.  This has an effect such that if you put
>>more than a total of 8 MB of RAM in the system through an expansion card, the
>>first 8 MB are all you'll get to use.
>
>I'd heard (way back in the beginning of the dawn of the IIgs) that Apple
>had specifically put aside 8 meg of ROM space for their personal use in
>the future. [...]
>  Matt's explanation seems to make more sense than the
>one I heard.  Of course, then the question is, why did Apple choose to
>limit the memory space by over half?  Owell, the world may never know.
>
>-kareth.

Actually, it only addresses 8mb because it's a port of some sort of
the mac memory manager. Now, the Mac II is currently "crippled" to
recognize only 8mb of RAM, even the though the processor can reach
something like 4gb. As I understand it, the top address lines are not
used to address memory, but rather as hardware switches. Whenever the
CPU is asking for very high addresses, the rest of the hardware routes
that request to the ROM or other hardware (like peripherals - meaning
the NuBUS).

Even with the virtual memory support of system 7, you won't be getting
a full 32 bits of address space as RAM. You only get 31 bits. That top
bit means "select the ROM." Wow, 31 bits of ROM space. They'll never
fill it up.

At any rate, did ya ever notice that hardware-related addresses in the
GS are all in high-memory? Perhaps we see a similar design here? Hmm?
Cost effective? Hmm? Could it be ..... SATAN??? (sorry, couldn't resist :-)


Dave Whitney
dcw@sun-bear.lcs.mit.edu  ...!mit-eddie!sun-bear!dcw  dcw@athena.mit.edu
My employer pays me well. This, however, does not mean he agrees with me.
I wrote Z-Link & BinSCII. Send me bug reports. I use a //GS. Send me Tech Info.