54287



     59316 ($E7B4) to determine their cause.

     Bit          7    6     5    4   3   2   1   0
     Interrupt:  DLI  VBI  RESET  .... unused .....   

54287          D40F           NMIRES
     (W) Reset for NMIST (below); clears the interrupt request
     register; resets all of the NMI status together.

                              NMIST
     (R) NMI status; holds cause for the NMI interrupt in BITs 5, 6 and
     7; corresponding to the same bits in NMIEN above. If a DLI is
     pending, a jump is made through the global RAM vector
     VDSLST (512; $200). The OS doesn't use DLI's, so 512 is
     initialized to point to an RTI instruction and must be changed by
     the user before a DLI is allowed.

     If the interrupt is not a DLI, then a test is made to see if the
     interrupt was caused by pressing RESET key and, if so, a jump is
     made to 58484 ($E474). If not a RESET interrupt, then the system
     assumes the interrupt was a VBLANK interrupt, and a jump is
     made through VVBLKI at 546 ($222), which normally points to
     the stage one VBLANK processor. From there it checks the flag at
     CRITIC (66; $42) and, if not from a critical section, jumps
     through VVBLKD at 548 ($224), which normally points to the
     VBLANK exit routine. On powerup, the VBLANK interrupts are
     enabled while the display list interrupts are disabled. See the end
     of the memory map for a description of the VBLANK procedures.
     For IRQ interrupts, see location 53744 ($D20E).

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Locations 54288 to 54303 ($D410 to $D41F) are repeats of locations
54272 to 54287 ($D400 to $D40F).

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Locations 54784 to 55295 ($D600 to $D7FF) are unused but not empty
nor user alterable. See the note at 53504 ($D100).

---------------------------------------------------------------------------
OPERATING SYSTEM ROM

Locations 55296 to 65535 ($D800 to $FFFF) are the OS ROM.
These locations are contained in the 10K ROM cartridge, which sits in
the front slot of the Atari 800 or inside the Atari 400. The OS is
identical for both computers.

The locations given here are for the "A" version of the OS ROMs.
There are changes in the new "B" version ROMs, which are explained
in the appendix. Most of the changes affect the interrupt handler
routines and SIO. In making these changes, Atari cured some bugs