Due to design of the
IBM PC, DOS suffered from what was known as the
640 KB 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
EMS,
XMS) and
DOS extenders, 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 high some
TSR programs like the
mouse driver or the disk caching driver (like
SMARTDRV) prior to running a memory-hungry application. This was achieved by using called with the program's name as the parameter. To load TSRs high within
CONFIG.SYS, the
INSTALLHIGH directive must be used instead of the command. The equivalent of for
device drivers is
DEVICEHIGH (usable only within
CONFIG.SYS). These are also supported since DR DOS 6.0. DR DOS 5.0 and higher also support
HIINSTALL 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. and other methods of freeing conventional memory have largely become obsolete. is part of the
Windows XP MS-DOS subsystem to maintain MS-DOS and MS OS/2 version 1.x syntax compatibility only. It is not available at all on
Windows XP 64-Bit Edition and also no longer available in the
command interpreter of newer
Windows operating systems. ==See also==