LOADHIGH (abbreviated 'LH') is an internal DOS command that is used to load a program into the upper memory area (UMA) instead of conventional memory. Due to design of the IBM PC, DOS suffered from what was known as the 640 KiB barrier. The size of this memory area, known as conventional memory, was fixed and independent of the amount of system memory actually installed. Various schemes 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 runtime. 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). DR-DOS, which introduced built-in high-load methods, also supports HILOAD and HIDEVICE, respectively.
Most modern operating systems now run in protected mode, with support for an unsegmented (flat) memory model, and do not have a 640 KB constraint. LOADHIGH and other methods of freeing conventional memory have largely become obsolete. This command is no longer available in the command interpreter of newer Windows operating systems.