Texas Instruments 4000M - Manual (Page 256)

Memory Area s
MS-DOS directly accesses up to 640 KB of RAM for the
execution of programs and commands and for storing
temporary data.
MS-DOS cannot directly access memory
beyond this 640-KB limit. The amount of standard default
RAM (640 KB) in the computer is displayed on the Setup
Program menu as Standard (Memory), and it can be
changed in 64-KB increments if required by your
application.
The following memory map shows the available memory
and how
MS-DOS uses it.
Memory Areas
F-2 Configuring Memory