Without going into details, it can be said that Base Memory is the most used memory. 640 kB is a must these days. If you buy a machine with 1 MB of RAM, always make sure you get 640 kB of base memory rather than just 512 kB and 512 kB of extended memory. 

Extended memory can be used by a small number of pro-grams only and therefore has limited applications. 

Expanded memory can be accessed by many more programs but still by a very small minority of the available programs. 

Main uses extended memory are: 

RAM-disks
 
A RAM-disk is a chunk of semiconductor memory that behaves like an ordinary disk but is extremely fast. It also loses its data instantly once power is turned off but is great for temporary files such as index files, extracted data from Lotus to be imported into another application etc. Example: VDISK.SYS, supplied as part of DOS 3.x. 

To create a RAM-disk, add one line in your config.sys such as DEVICE = \DOS\VDISK.SYS 256 /E. This will look in the DOS subdirectory for the driver, create a 256 kB RAM-disk. The /E parameter will place it into the extended memory.

Disk caches 

A disk cache is a program to speed up disk access by storing the most frequently use information in the computer's memory and reading ahead from the disk in anticipation. With floppy disks, the time saved can be spectacular. Writes are almost always performed to the disk to prevent loss of data in case of power failure. Example: PC-CACHE, as supplied with PC-Tools. A shareware product is EMMCACHE.

Print spoolers 

A print spooler utilises the computer's memory as a high speed buffer so that a fast computer is not slowed down by a slow printer. For example you can print a 100 page database report and then load a spread sheet program, print reports and graphs, then use your wordprocessor while the database report is still printing. Print spoolers that use extended memory usually come with the memory card. The AST SUPERSPL is a good example of a spooler with lots of options. A shareware product is EXTSPL.

OS/2 

The operating system OS/2 can make good use of extended memory. Considering how memory-hungry OS/2 is, you'll need it too! 

UNIX

UNIX is another operating system that can use extended memory. 

Some DOS programs that use extended memory are:

• FrameWork II and FrameWork III, particularly when using spreadsheets
• SPF-PC (a programmer's editor for those who use it on a mainframe, with terrific column and hexadecimal editing facilities)

• Remove any memory resident programs. Reduce or re-move any conventional memory RAM-disk, reduce the number of files and buffers in the CONFIG.SYS file (reducing buffers will slow any disk access; reducing the files will eventually prevent some programs from running, but until then there is no performance degradation.)
• Install memory cards which can be configured for both expanded and extended) memory. ONLY DO THIS if your application can utilise the extra memory. Most cannot.

• Load your device drivers and resident utilities into high RAM (works only for 80386 machines with 1 MB of RAM on the motherboard). LoadHi will do the trick; it costs about $100. 386MAX is another good product. Quarterdeck have released a program for the 286 computers.
• Install a disk-ram utility. This is the opposite of a RAM-disk: It makes part of your hand disk appear as RAM, allowing you to load even the largest spreadsheet. Don't hold your breath while your spreadsheet is recalculated - performance is pretty awful. However, given the choice between manual pro-cessing and very slow machine processing, the decision is not difficult. ONLY DO THIS if your application can utilise the extra EMS memory. Most cannot. Example: ABOVE DISK for approx $150, or the shareware program, VRAM.