Rockwell Automation 9323-S5500D A.I. SERIES MICROLOGIX 1000 AND PLC-500 SOFTW - Manual (Page 509)

MicroLogix 1000 and PLC-500 A.I. Series Software Reference
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Change DOS versions. DOS 4.xx occupies more RAM than any other version.
DOS 5.0 occupies relatively little RAM. In systems with more than 1 Megabyte of
RAM, DOS 5.0 can be configured to use memory outside of the base 640K area.
Memory beyond 640K
DOS applications run in base memory, the first 640K. Most computers are now sold
with one or more Megabytes installed. Several mechanisms are available for using the
extra memory:
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RAM Disks
Extra RAM can be used to emulate an extremely fast disk drive for temporary
storage. Note that the files on a RAM disk will be lost when the computer is
booted.
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Loading Resident programs (TSRs) and device drivers outside of base memory
Memory management software has been developed which allows you to load TSRs
and device drivers into the extra memory, allowing you to use more of the base
640K for your applications. See your software dealer for details.