Epson 386SX - Manual (Page 141)

The size parameter allows you to specify the amount of extended
memory to be used as expanded memory. You specify the
amount of memory in kilobytes. If you do not specify a size, the
default value is 256KB.
This example tells the computer to use 1024KB of extended
memory as expanded memory:
DEVICE=EMM386.SYS 1024
The X:mmmm-nnnn parameter specifies a range of memory to
exclude from the EMM386.SYS command in hexadecimal
notation. EMM386.SYS does not locate its page frame or other
mappable pages in this memory range.
For example, to specify 1024KB of memory as expanded
memory and ensure that EMM386SYS does not locate any
pages in the address range C400 to C7FF, include this command
in your CONFIG.SYS file:
DEVICE=EMM386.SYS 1024 X:C400-C7FF
You can include more than one X: parameter in your
DEVICE=EMM386.SYS command to exclude more than one
range of memory.
Note
Do not use the X: parameter unless you experience a memory
conflict with a memory option card.
Using MS-DOS with Your Equity 386SX 4-45