Atari DOS 2.5: XF551 Owner's Manual page 118

Note t o ATARI1 30XE Computer owners: Keep in mind that if
DOS 2.5 is using the additional64K of memory in a 130XE as a
Ram Disk, the Ram Disk is always Drive 8 (08 :). See Appendix K
and your 130XE Owner 's Manual for more details.
Technical Notes
In theory, either DOS 2.0S or DOS 2.5 can support up to eight
disk drives and up to eight files open at the same time. In prac-
tice , there are several limitations on these numbers: (1) at a mini-
mum , most programs must reserve IOCB number zero for the
keyboard and screen (the E: device) ; (2) ATAR I does not make a
disk drive that can be assigned a device number greater than 4;
(3) because OUP.SYS loads into memory at a particular location
under DOS 2.0S , and because DOS 2.5 keeps DUP.SYS at that
same location for compatibility reasons , there is a real limit to the
amount of buffer room available when DUP.SYS is active.
The practical result of these limitations is that you are limited to
five disk drives (including the RamDisk if you have an ATAR I
130XE Computer) and seven files open at one time when using
BASIC and other languages and programs that pay attention to
the system LOMEM pointer (at location $02E7). If, however, you
wish to be able to use the DOS Menu, you are limited to a maxi-
mum of four drives (again including the Ram Disk). Because the
DOS Menu never uses more than two files at a time, you must
provide for at least that many concurrently open files, but you
may use more if you wish.
Another practical li mitation on how many drives and buffers you
may use is that many programs currently available assume both
a particular value for LOM EM (the contents of location $OE27)
and a minimal configu ration of two drives and three concurrently
ope n files . Since LOME M's value depends on the number of
buffers you have chosen, the following comparison chart lists
buffer allocation information for both DOS 2.0S and DOS 2.5.
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