Oct 20 1999.

    Virtual Memory Management on the Macintosh

    89/04/15 J M McIntosh

    Product description:

      "VIRTUAL" is a 3rd party system software application that exploits the memory management hardware provided by the Motorola 68030 CPU to provide you with a usable memory size of 8 Megabytes (8192K). This CPU chip is used in the Macintosh IIx, IIcx and SE/30 hardware platforms. With the addition of a 68851 Paged Memory Management Unit (PMMU), existing MAC IIs can also use a special version of this software to achieve the same memory size.

      The usage of VIRTUAL now allows us to defer the purchase of real RAM memory which is still quite expensive, costing us $500 per MB. Installing VIRTUAL versus purchasing 4MB of real memory per SE030 & IIcx, saves us about $900 per machine. For MAC IIs requiring upgrades to 8MB, savings are about $1200 per machine.

      This software is not usable on the older SE 20 or 40MB Hard Drive models that use the Motorola 68000 CPU which has no hardware PMMU support.

      To confirm VIRTUAL is working you should see the following ICON appear in the lower left of your screen when starting your Macintosh.

       VIRTUAL is ON.

       

      If for some reason VIRTUAL encounters an error when starting, or if you hold the "esc" key (the upper left key on the keyboard) down to turn VIRTUAL off, you will see the following ICON appear in the lower left of your screen when starting your Macintosh.

      VIRTUAL is OFF.

       

    Performance Issues:

      THRASHING

      Like all virtual memory management systems, performance degradation experienced from paging delays is directly related to the amount of real memory installed in the machine. If the amount of real memory installed is less than the required amount of memory for the task at hand, a phenomenon called "thrashing" can occur. This is exhibited when the machine spends most of its time moving pages to and from disk versus doing real work.

      In our current working environment, the amount of memory required to use the standard applications is less than 2MB. However, you can cause thrashing to occur if, for example, you create a 38 page (6000K) SuperPaint document and attempt to work with it. If thrashing occurs as part of your normal work the solution is to install more real memory in your machine.

    ADDRESS TRANSLATION OVERHEAD

      When a read/write request to memory is encountered the PMMU hardware translates the "virtual" address to a "physical" address, this process takes time. Benchmark testing has shown that an SE/30 running VIRTUAL can be upto 11% slower than a SE/30 running without VIRTUAL.

    Known problems:

      1) Apple's "FONT/DA MOVER", which is used to install or remove fonts and Desk Accessories.

      To avoid any problems with this application:

      a) Turn MultiFinder OFF. This can be done by selecting "Set Startup..." from the FINDER'S "Special" menu and picking Start up "SYSTEM" with FINDER.

      b) Select selecting "Restart" from the FINDER'S "Special" menu, and hold the "esc" key (the upper left key on the keyboard) down to turn VIRTUAL off.

      2) Alert Sound "Simple Beep" does not work 30% of the time.

        "Simple Beep" is played by a ROM routine which directly uses the hardware to create the sound, versus other digitized sounds which are played back by using the Apple/Sony stereo sound chip. VIRTUAL cause timing problems for the "Simple Beep" routine. The solution is to choose another sound via the Control Panel.

      4) The Macintosh may crash if the RAM Cache is set too high.

        Testing has shown that if the RAM cache is greater than the amount of real memory installed, the machine may crash. For normal usage a Cache size of 128K is fine, expand at own risk.

    Recommendations:

    Application Memory Sizes:

      Following is recommendations for application memory sizes to give the best memory usage. This information is based from End User experiences and vendor recommendations. Larger application sizes provide faster application throughput and less application related disk I/O, although this is dependent on the application. In many cases, we previously used a smaller memory size in order to Þt more of the standard applications into a 2.5MB SE.

      Ram Cache 128K For better performance.

      HyperCard 1000K As recommend by APPLE.

      MS Word V3.02 512K End User experiences, for better performance.

      SuperPaint V2.0 1300K End User experiences, to allow 3 page documents.

      Excel V1.5 512K Current Standard, no change.

      Finder 200K End User experiences, Þxes occasional memory problems.