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RAM Charger 8 For Macintosh
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Why do I see delays redrawing the Mac OS 8 desktop?


(Tech0017A -- 03/26/98)


If you are running RAM Charger with Mac OS 8 with a desktop picture, then following the launch of _any_ application there will be a delay the first time any portion of the desktop needs to be redrawn. This is because during launch RAM Charger has forced the desktop picture to be "lost" and it must be restored - which takes time. There is _potential_ for this to happen at any time, with or without RAM Charger, should your memory be fairly full. However, this will happen every time following a launch when any currently available RAM Charger is enabled. This problem happens subsequent to all application launches when RAM Charging is installed, no matter what applications are RAM Charged.

This problem will be fixed our first release after 3/26/98 (either 8.1b3 or 8.1 final).


Desktop Pictures appears to keep a "pre-processed" copy of the desktop picture in memory, within the "free pool of memory" used by RAM Charger (the "free pool" is more technically referred to as the "ProcessZone" or "TempZone"). This "pre-processed" copy of the desktop picture is "purgable", which means that it may be discarded at any time if the memory is required for other operations.

If the "pre-processed" copy of the desktop picture is purged, then you will see a delay the first time the desktop picture is required for screen update. At such a time (called "lazy-evaluation"), the picture must be re-processed, and thus the pause. This is directly analogous to the sort of pauses you may occasionaly see with virtual memory, since "purgable" memory is a specialized implementation of virtual memory which requires no processor support. Purgable memory is supported at a higher level, in the memory manager and in applications.

In older versions of Mac OS, the free pool of memory was purged each time an application was launched (by a call MaxMem on the ProcessZone). In the past, RAM Charger has supplemented this launch-purge by gathering adjacent "purged" blocks into contiguous chunks. This allowed some applications to launch that would fail to launch the first time, but succeed when launched again since adjacent "purged" blocks would have been gathered late in the first launch. PrintMonitor showed this behavior under low memory conditions.

In system 8, Mac OS must no longer purge the free pool of memory each time an application is launched (macsbug: "atb maxmem; tvb maxmem"). If the system did, you would see the problem with Desktop pictures without RAM Charger. As a result, we have removed our enhancement to the Mac OS 8 "launch-purge". Apparently, it is no longer required.

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Copyright © 1995-98 Jump Development Group, Inc. All rights reserved. Jump, OptiMem, RAM Charger, and More About This Mac are trademarks of Jump Development Group, Inc. Apple and Macintosh are registered trademarks of Apple Computer, Inc. All other trademarks are the property of their respective holders.