Editing Conventional memory (section)

=== Driver and TSR optimization ===
Hardware expansion boards could use any of the upper memory area for ROM addressing, so the upper memory blocks were of variable size and in different locations for each computer, depending on the hardware installed. Some windows of upper memory could be large and others small. Loading drivers and TSRs high would pick a block and try to fit the program into it, until a block was found where it fit, or it would go into conventional memory.

An unusual aspect of drivers and TSRs is that they would use different amounts of conventional and/or upper memory, based on the order they were loaded. This could be used to advantage if the programs were repeatedly loaded in different orders, and checking to see how much memory was free after each permutation. For example, if there was a 50&nbsp;KB UMB and a 10&nbsp;KB UMB, and programs needing 8&nbsp;KB and 45&nbsp;KB were loaded, the 8&nbsp;KB might go into the 50&nbsp;KB UMB, preventing the second from loading. Later versions of DOS allowed the use of a specific load address for a driver or TSR, to fit drivers/TSRs more tightly together.

In MS-DOS 6.0, Microsoft introduced <code>[[MEMMAKER]]</code>, which automated this process of block matching, matching the functionality third-party [[DOS memory management|memory managers]] offered. This automatic optimization often still did not provide the same result as doing it by hand, in the sense of providing the greatest free conventional memory.

Also in some cases third-party companies wrote special multi-function drivers that would combine the capabilities of several standard DOS drivers and TSRs into a single very compact program that used just a few kilobytes of memory. For example, the functions of mouse driver, CD-ROM driver, ANSI support, DOSKEY command recall, and disk caching would all be combined together in one program, consuming just 1 – 2 kilobytes of conventional memory for normal driver/interrupt access, and storing the rest of the multi-function program code in EMS or XMS memory.