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From: meuchen@grad2.cis.upenn.edu (Paul Eric Menchen)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Where does video on the Mac II get its memory?
Summary: You can save memory by changing to 2-bit
Message-ID: <22344@netnews.upenn.edu>
Date: 27 Mar 90 21:11:07 GMT
References: <1990Mar26.100104.552@eng.umd.edu> <367@gazette.bcm.tmc.edu> <1990Mar27.035008.2705@chinet.chi.il.us>
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Reply-To: meuchen@grad1.cis.upenn.edu (Paul Eric Menchen)
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In article <1990Mar27.035008.2705@chinet.chi.il.us> henry@chinet.chi.il.us (Henry C. Schmitt) writes:
>In article <367@gazette.bcm.tmc.edu> dale@bcm.tmc.edu (Dale A. Samuelsen) writes:
>>In article <1990Mar26.100104.552@eng.umd.edu> allon@eng.umd.edu (Allon 
>>Stern) writes:
>>> and that you can save memory by switching to 2-color mode.
>>
...
>Please note that simply going to the Monitors cdev and changing to 2
>colors (or greys) will _not_ save you any memory, the video memory
>is allocated at startup and cannot be changed without rebooting.
>
>The way to do this is 1) open the monitors cdev, 2) hold down the
>option key and hit the options button (so much for intuitive), 3)
>choose the maximum colors you want memory allocated for, 4) reboot.
>
>I find many people have the mistaken idea that changing the number
>of colors at the top level of the Monitors cdev will allocate or
>deallocate memory for video.
...
By changing to two bit mode you won't get any more memory for system
system use from the video card, but you can save memory. Let me
explain.

First, an example (with an explanation to follow): I have a Mac II, 8
bit Apple video card, 1 meg memory (don't laugh, please). If I try to
execute Illustrator 88 in 8 bit mode, I get a "Not enough memory ..."
message. Putting it in 4 bit mode will allow it to run if I'm not
running Master Juggler.  Master Juggler also takes memory, as do other
CDEVs and INITs I have running.  When I have these running, I have to
change to 2 bit mode to get Illustrator 88 to run.  I can try to
execute Illustrator in 8 bit mode, get a "Not enought memory ..."
message, then change the pixel depth with the Control Panel to some
lower value and then run Illustrator successfully, without ever
rebooting.

Why? To start, a reboot is not necessary for the Control Panel
settings to take effect in terms of operation of the video card.  If
it were, it would be impossible to change to 256 colors after strating
up in 2 bit mode. I, however, do this all the time (o.k. every now and
then).

Further, while the video memory is independent of the main memory,
many (most?) programs use main memory for video purposes. If you want
to generate a pixel map/image/whatever, you'll do it in main memory.
Illustrator can generate a screen preview of whatever you're working
on. To do this it uses main memory. Illustrator generates the image
based on the pixel depth of the screen, and of course 2-bit images
take less memory than 8-bit images. If you have a program to display
GIF images, it uses main memory to decompress the image, and if the
program is smart it will only allocate enough memory based on the
pixel depth of your display.  

Just think about when you have multiple windows on the screen. You
may not be able to see part of one window, so it's not currently
taking any video memory, but as soon as you move the window covering
it to see what was covered, there it is. A write from main memory to
the video memory has occured.

Just about everything you see on the screen exists in main memory
somewhere in one form or another. There are a few exceptions to this,
but not many (one that comes to mind occurs in GIFfer, and it tells you
when this occurs.)

Paul Eric Menchen
meuchen@grad1.cis.upenn.edu