The latest version of DOS (Disk Operating System) for the XL and XE
computers is 2.5. It offers several advantages over the earlier ver-
sions (including the ill-received DOS 3.0), including dual-density
formatting, new XIO formatting commands available from BASIC, a
RAMDISK program for the 130XE, and greater compatibility with DOS
2.0. If you use DOS 3.0, I suggest you get a copy of 2.5 as soon as you
can.
DOS 2.5 formats a track with 26 sectors instead of the 18 DOS 2.0 han-
dles; this means a disk with 1010 sectors free instead of 707 (leaving
931 free sectors with DOS and DUP.SYS files on a disk). The 1050 (not
the 810) drive can automatically sense which density the disk in the
drive is using. DOS 2.0 can read a 2.5 disk but the additional sectors
are invisible to it.
New BASIC Commands for DOS 2.5
When you OPEN a disk from BASIC to get a directory read (see loca-
tion 1792; $700 in the Addenda section), you normally use OPEN
#1,6,0,"D:*.*." Now, if you use OPEN #1,7,0,"D:*.*," DOS will specify
files which occupy disk sectors that can't be accessed by 2.0 with
angle brackets, like . These files are invisible to
DOS 2.0 when reading a directory; they can't be loaded, nor do they
show up in the directory.
Formatting the disk by the XIO command is enhanced. The usual
method is XIO 254, #1,0,0,"D1:." This will format the disk, trying first
for dual density, and if the drive doesn't support it, formatting in
single (2.0) density. XIO 253, #1,0,0,"D1:" formats a disk with single
density only (a new option--P--has been added to the DOS menu to
format in single density as well). XIO 253, #1,34,0,"D1:" will format a
disk in dual density only.
RAMdisk for the 130XE
DOS 2.5 includes a special program called RAMDISK.SYS. This loads
up when the disk is booted and determines if your computer is a
130XE. If so, it runs a small program which creates a "disk drive" out
of the 64K extended memory bank. The RAMdisk acts just like a real
disk, except that it's faster. It is formatted into 499 sectors and a direc-
tory and has the drive number D8:. DOS 2.5 supports drives 1-8, but
is initialized to drives 1, 2, and 8, so if you have other drives, change
location 1802 ($70A); that is, if you have three drives and the
RAMdisk, POKE 1802, 135. All bits in location 1802 now represent pos-
sible drives.
When it runs, RAMDISK.SYS copies MEM.SAV and DUPSYS to the
RAMdisk, then modifies a location so that you call up DUP.SYS from
the RAMdisk rather than D1:. This brings up DOS almost immediately
when you leave BASIC. However, if you want to delete DUP.SYS from
the memory drive and call it up from drive 1 as usual, type POKE
5439, ASC("1"), this points DOS back to the original drive. You can
also delete MEM.SAV from D8: if you don't need it.
DOS 2.5 Boot Sector and Memory Map
Locations 1792-1812 ($700-$714) are loaded directly into RAM from
the boot sector (sector 1) on a disk. Refer back to the section in the
400/800 memory map tar more explanation. These are from an article
by Neil Harris in the Atari Explorer; they are locations Atari promises
to support in the future:
POKE with 49 (ASC("1")) to reroute DOS to call DUP.SYS from D1:
rather than D8: when using the RAMdisk--you can then delete
DUP.SYS and MEM.SAV from the RAMdisk for extra space. See
location 1923 ($783) in the Addenda.
