[3][4] Due to design of the IBM PC, DOS suffered from what was known as the 640 KB barrier.
It was a scarce resource as many applications demanded a large part of this basic memory fragment at runtime.
To load TSRs high within CONFIG.SYS, the INSTALLHIGH directive must be used instead of the LOADHIGH command.
[4] 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.