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Loadhigh (abbreviated 'lh') is an internal MS-DOS command that is used to load a program into the upper memory area (UMA) instead of conventional memory. Due to limitations of the early 80x86 architecture, MS-DOS (and other DOS-compatible operating systems of the day) suffered from what was known as the 640 KiB barrier. The size of this memory area (640 kibibytes), known as conventional memory was fixed and independent of the amount of system memory actually installed. Later, as DOS-based configurations came to be equipped with more and more memory, various schemas were developed to support extra memory (see also XMS, EMS and DOS extender), but conventional memory was still an issue due to compatibility issues. It was a scarce resource as many applications demanded a large part of this basic memory fragment at run-time. Therefore, it was often necessary to move some TSR programs like the mouse driver or the SMARTDRV disk caching driver prior to running a memory-hungry application. This was achieved by using loadhigh called with the program's name as the parameter.
The equivalent of loadhigh for device drivers is DEVICEHIGH (usable only within CONFIG.SYS).
After the introduction of modern operating systems (stemming from MS-DOS) which run in protected mode, support an unsegmented (flat) memory model and do not ultimately suffer from the 640 kB constraint, loadhigh and other methods of freeing conventional memory have become obsolete and this command is no longer recognized within the command interpreter of newer Windows operating systems.
See also
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