APPENDIX SEVENTEEN
DOS 2.5 And The 1050 Drive
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:


1792          	700          	BFLG
     Boot flag; always equals 0.

1793          	701          	BRCNT
     Number of sectors in the disk boot; three--the first three on the
     disk.

1794,1795          	702,703          	BLDADDR
     Boot load address; where DOS is loaded into memory; always
     1792 ($700).

1796,1797          	704,705          	BINTAD
     DOS initialization address; always 5440 ($1540).

1798-1800          	706-708          	BCONT
     JMP instruction to jump to the address where the boot program
     continues execution; 1812 ($714).

1801          	709          	SABYTE
     Maximum number of concurrently open files--usually three.

1802          	70A          	DRVBYT
     Drive allocation byte; one bit per drive.

1803          	70B          	SAFBFW
     Unused.

1804,1805          	70C,70D          	SASA
     Buffer allocation address for drives and files.

1806          	70E          	DFSFLG
     Reads zero if there is no DOS.SYS on disk, nonzero if present.

1807,1808          	70F,710          	DFLINK
     Points to first sector of the DOS.SYS file.

1809          	711          	BLDISP
     Number of displacement bytes to sector link bytes (last three);
     always 125 ($7D).

1810,1811          	712,713          	DFLADDR
     Address of the FMS (D:) handler table; 1995 ($7CB).

1812          	714          	XBCONT
     Boot program begins here.

1900          	76C          	BSIO
     BASIC SIO routines.

1906          	772          	BSIOR
     FMS disk handler routines.

1913          	779          	....
     Write verify flag; 80 ($50) turns it off, 87 ($57) turns it on.

1995          	7CB          	DFMSDH
     FMS handler table. Has data in it different from 2.0 handler.

2016          	7E0          	DINIT
     DOS initialization routine.

4993          	1381          	FBC
     Start of the FMS file control blocks; first of eight.

5121          	1401          	FILDIR
     128-byte buffer for a disk directory sector.

5439          	153F          	....
     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.

5440          	1540          	MINIDUP
     Start of permanently resident portion of DUP.SYS.

5540          	154A          	SFLOAD
     Entry to DUP.SYS's routine to load binary files.

5542          	15A6          	STLOAD
     Used with SFLOAD.

5545          	15A9          	LOAD
     Used with SFLOAD.