DEFINITIONS AND BASIC CONCEPTS

Memory: When you start your system, a variety of software is Loaded in RAM in order for your computer to run useful applications. The software for your system is loaded according to this general Memory Map. In order for any program to run, a copy is pulled off your hard drive and loaded into your RAM (basically computer chips "inside the box") because accesing it is very fast! RAM is considered "volatile"memory-when you turn off the computer the contents of RAM disappears. Your hard drive is considered "non-volatile" memory-when you turn off the computer and turn it back on it's still there.

Now let's talk about what's loaded in the various areas of your RAM:

• Conventional (or Lower) Memory: Refers to the first 640KB of your RAM. This where DOS applications load and run. For best DOS performance, you must maximize the amount of "free conventional memory" (RAM) in this area.
• Upper Memory: Refers to the area from 640KB up to 1MB in your RAM Loading software into this area is called "loading high. " Loading programs and drivers "high" (DOS 5.0 and higher) on startup allows you to have more free conventional memory noted above. this loading high is accomplished by loading HIMEM.SYS, EMM386.EXE (RAM or NOEMS) and having DOS=UMB in your config.sys file.
• High Memory Area (HMA): The area of RAM from 1MB to 1MB+64KB. Used to load parts of DOS in DOS versions 5.0 and later by use of the command DOS=HIGH in your config.sys file. With the above upper memory tip the command to actually use is: DOS=HIGH,UMB.
• Expanded Memory (EMS): One of the first industry standards that defined how to use the RAM above 1MB. Used by many DOS games, old versions of Lotus, etc. Requires a 64KB "Page Frame" in upper memory.
• Extended Memory (XMS): Another, newer standard developed to use RAM above 1MB. Windows needs XMS to run. The driver file HIMEM.SYS loaded in your config,sys file creates Extended Memory. (Note: Windows 95 loads HIMEM.SYS even if the driver is not shown in your config.sys)
• Virtual Memory: In Windows, this refers to the sum of your RAM plus the size of the Windows swap file on your hard drive. Win3.x running in the "386 Enhanced Mode" and Windows 95 use the swap file as an extension of your RAM. If you keep loading applications and data in Windows and your RAM begins to fill up then Windows will begin to "swap" programs and data to the swap file on the hard drive. The swap file in Windows 95 is "dynamic." It will shrink and grow as needed.

Note: If you get "out of memory" errors running Windows applications it can be a RAM problem or it can mean you do not have enough free hard drive space for the Win95 swap file to grow. You should have on the order of 50MB of free space on your Windows 95 drive.

Disk Cache:

Generally (Sometimes just called "caching") refers to that portion of your RAM that is "fenced off" to keep a copy of data that was earlier read from or written to your harddrive ("Disk"). This technique is commonly used in DOS and Windows. If properly sized, it makes your system response much faster because accessing data from the disk cache in RAM is far faster than accessing the same data from the hard drive.

The disk cache in RAM stores your most recently used (from the hard drive) programs or data, limited of course, by it's size. Another little known fact is: If you make a disk cache in RAM too large, the CPU will spend far too much of its time trying to keep the huge cache filled up at the detriment of the rest of the system!

The operating system (DOS or Windows) will check this cache in RAM every time you ask for data-and if it is in the cache-it will copy it from there into the normal part of RAM.

In Windows 95 this cache is called "VCache" or "Virtual Cache." This cache performs the same function that SMARTDRV.EXE does in DOS. Windows 95, by default, tries to set aside a vcache of 1/4 (or more!) of your total RAM. See my Win95 Tips for tips on adjusting your Win95 VCache setting in SYSTEM.INI. Note: VCache is NOT "Virtual Memory!" In Windows "virtual memory" refers to the swap file Windows uses on your hard drive.

To see your disk cache in action: Run a common application, then close it. Run it a second and even third time.

You should note that on the second and third tries the program comes up much faster because the program is loading itself to run from the cache in RAM rather then from your hard drive

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