Texas Instruments 4000M - Manual (Page 261)

Some applications may require “backfill” memory, which is
the unused area of standard memory that can be used by
EMM386.EXE as Expanded memory. For example, an
application may require only 256 KB or 512 KB of standard
memory, leaving 384 KB and 128 KB of backfill memory
space, respectively, for use as Expanded memory.
The Shadow ROM memory that you define with the Setup
Program enables mapping the internal system BIOS, Video
BIOS, and option BIOS into high-speed memory for faster
operation. When not selected, the shadow memory is
available for use as Extended memory. The shadow memory
area is limited to 384 KB.
Your computer provides 4 MB of RAM (main memory)
standard. The first MB (1024 KB) of the standard 4 MB is
the system base memory (640 KB) and shadow memory
(384 KB) and cannot be increased beyond 1024 KB. You
can optionally add 4 MB or 16 MB of memory to use as
Extended and/or Expanded memory. This added memory
when combined with the remaining standard 3 MB (4 MB -
1024 KB) of system memory (not used for base or shadow
memory) provides either 7 MB or 19 MB, respectively, of
Extended or Expanded memory.
See the Microsoft MS-DOS User’s Guide and Reference for
detailed instructions on installing and configuring
expanded memory.
HIMEM.SYS
HIMEM.SYS is an eXtended Memory Manager ( XMM )
conforming to eXtended Memory Specifications ( XMS ), version
3.04. HIMEM.SYS uses 64 KB of the high-memory area ( HMA )
at the beginning of Extended memory to store a single TSR
program or device driver, or it uses this area for data storage.
This effectively increases the size of standard memory for use
by your main application.
Memory Device Drivers
Configuring Memory F-7