This tidbit from the DOS 2.0 README.txt (section 5) is fascinating:
The user manual contains some significant errors. Most of these are
due to last minute changes to achieve a greater degree of compatibility
with IBM's implementation of MS-DOS (PC DOS). This includes the use
of "\" instead of "/" as the path separator, and "/" instead of "-"
as the switch character. For transporting of batch files across
machines, Microsoft encourages the use of "\" and "/" respectively
in the U.S. market. (See DOSPATCH.TXT for how you can overide this.
The user guide explains how the end-user can override this in CONFIG.SYS).
AL Function
-- --------
0 DL, on return, will contain the DOS switch
character. On most systems this will default to
'-'.
1 Set the switch character to the character in DL.
2 Read the device availability byte into DL. If
this byte is 0, then devices must be accessed in
file I/O calls by /dev/device. If this byte is
non-zero, then the devices are available at every
node of the directory tree (i.e. CON is the
console device not the file CON). This byte is
generally 0.
3 Set the device availability byte to the value in
DL.