7548-MEMLO



     Non-resident portion of DUP.SYS, DOS utility routines. DUP
     provides the utilities chosen from the DOS menu page, not from
     BASIC. It is not resident in RAM when you are using BASIC or
     another cartridge; rather it is loaded when DOS is called from
     BASIC or on autoboot powerup (and no cartridge supersedes it).
     When DUP is loaded, it overwrites the lower portion of memory.
     If you wish to save your program from destruction, you must have
     created a MEM.SAV file on disk before you called DOS from your
     program. See the DOS Reference Manual.

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Locations 1792 to 2047 ($700 to $7FF; page seven) are the user boot
area. MEMLO and LOMEM point to 1792 when no DOS or DUP
program is loaded. This area can then be used for your BASIC or
machine language programs. The lowest free memory address is 1792,
and programs may extend upwards from here. There is a one-page
buffer before the program space used for the tokenization of BASIC
statements, pointed to by locations 128, 129 ($80, $81). Actually a
program may start from any address above 1792 and below the screen
display list as long as it does not overwrite this buffer if it is a BASIC
program. Also, 1792 is the start of the FMS portion of DOS when
resident.

When software is booted, the MEMLO pointer at 743,744 ($2E7,$2E8)
in the OS data base (locations 512 to 1151; $512 to $47F) points to the
first free memory location above that software; otherwise, it points to
1792. The DUP portion of DOS is partly resident here, starting at 5440
($1540) and running to 13062 ($1540 to $3306). The location of the OS
disk boot entry routine (DOBOOT) is 62189 ($F2ED). The standard
Atari DOS 2.OS takes up sectors one through 83 ($53) on a disk. Sector
one is the boot sector. Sectors two through 40 ($28) are the FMS
portion, and sectors 41 ($29) through 83 are the DUP.SYS portion of
DOS. For more information, see the DOS and OS source listings and
Inside Atari DOS.

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FMS, DOS.SYS and DUP.SYS
     Disk boot records (sector one on a disk) are read into 1792 ($700).
     Starting from $700 (1792), the format is:

     Byte    Hex       Label and use
     0       700       BFLAG: Boot flag equals zero (unused).
     1       701       BRCNT: Number of consecutive sectors to
                       read (if the file is DOS, then BRCNT equals
                       one).
     2,3     702,703   BLDADR: Boot sector load address ($700).
     4,5     704,705   BIWTARR: Initialization address.
     6       706       JMP XBCONT: Boot continuation vector; $4C