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From: tdn@spice.cs.cmu.edu.ARPA (Thomas Newton)
Newsgroups: net.micro.mac
Subject: Re: 1.5 Meg Upgrade from MacMemory
Message-ID: <465@spice.cs.cmu.edu.ARPA>
Date: Sun, 13-Oct-85 18:36:18 EDT
Article-I.D.: spice.465
Posted: Sun Oct 13 18:36:18 1985
Date-Received: Tue, 15-Oct-85 07:36:34 EDT
Organization: Carnegie-Mellon University, CS/RI
Lines: 57

> 	It is my understanding that the location of screen memory is
> determined by a ScreenBase pointer that is placed in low memory.
> I'm not sure how it gets there, but I would guess that there is a ROM
> routine that figures out on boot up whether you have a 128K or 512K and
> then puts the appropriate value in the pointer location. I don't see
> why the Apple ROM couldn't figure out that you have 1 Meg, 2 Meg
> or whatever and calculate where screen memory should be. Obviously Levco
> does something like this with its ROM.

The ScreenBase pointer does not determine the location of screen memory;
instead, the screen locations supported by the hardware determine ScreenBase.
It's important that the two correspond.   I think that the current Apple ROMs
assume that you have either a 128K Mac or a 512K Mac and set ScreenBase for
one of these two configurations.

In one of the MAUG conferences, Andy Hertzfield said that it was important
for people upgrading to more than 512K of memory to get upgrades that placed
the screen at the top of memory.  Currently, locating screen memory at the
top of a >512K Mac requires patching the ROMs to set ScreenBase correctly.
The new Apple ROMs should set ScreenBase correctly (= pointing to the top
of memory) for Macs with more than 512K of memory.  But the new ROMs won't
help you (and in fact will hurt you) if you have a >512K upgrade that leaves
the screen at the top of the first 512K of memory.

I called MassTech about their upgrades, and it sounds like they're doing
things right (the extra memory is contiguous with the standard memory, the
screen is located at the top of the address space, and the board contains
patches to the current ROM to let it know about the extra memory).  From the
little I've heard, it sounds like Levco is using a similar technique.  But I
have seen some ads for >512K upgrades that I wouldn't touch with a ten-foot
pole.  A few questions to ask:

    (1) Is the extra memory contiguous with the Mac's built-in memory?
        (the answer to this one should be YES; if the answer is NO, then
	 you'll only be able to use the extra memory as a ramdisk)
	 
    (2) Is the screen relocated to the top of memory?
        (the answer to this one should be YES; according to Hertzfield,
         this is necessary for compatability with the new ROMs)

    (3) Are the current ROMs patched to know about the extra memory?
        (the answer to this one should be YES)

    (4) Does the upgrade require permanent modifications to the Mac?  If so,
        is it possible to buy service on a modified Mac?  (One of the places
        I called modifies the Mac permanently, but will only do service work
        on their own board.  Since regular Apple dealers won't take modified
        digital boards as trade-ins, you'd be in real trouble if you got one
	of their upgrades and your Mac's digital board died.)

    (5) What is done about heat and/or power supply problems?  (MassTech
        installs a fan with all >512K upgrades and installs a modification
	to the power supply with 2MB upgrades on older Macs; I think that
        Levco also installs a fan and possibly a power supply mod).
	
                                        -- Thomas Newton
                                           Thomas.Newton@spice.cs.cmu.edu