512,513



interrupts, peripheral and serial bus interrupts, BREAK and other key
interrupts, and 6502 BRK instruction interrupts. They can usually be
used to vector to user routines. See NMIST 54287 ($D40F) and IRQEN
53774 ($D20E) for more information. NMI interrupt vectors are marked
NMI; IRQ interrupt vectors are marked IRQ.
Refer to the chart below location 534 for a list of the interrupt vectors in
the new OS "B" version ROMs.

512,513        200,201        VDSLST
     The vector for NMI Display List Interrupts (DLI): containing the
     address of the instructions to be executed during a DLI (DLI's are
     used to interrupt the processor flow for a few microseconds at the
     particular screen display line where the bit was set, allowing you
     to do another short routine such as music, changing graphics
     modes, etc.). The OS doesn't use DLI's; they must be user-
     enabled, written and vectored through here. The NMI status
     register at 54287 ($D40F) first tests to see if an interrupt was
     caused by a DLI and, if so, jumps through VDSLST to the routine
     written by the user. DLI's are disabled on powerup, but VBI's are
     enabled (see 546 to 549; $222 to $225).

     VDSLST is initialized to point to 59315 ($E7B3), which is merely
     an RTI instruction. To enable DLI's, you must first POKE 54286
     ($D40E) with 192 ($C0); otherwise, ANTIC will ignore your
     request. You then POKE 512 and 513 with the address (LSB/MSB)
     of the first assembly language routine to execute during the DLI.
     You must then set BIT 7 of the Display List instruction(s) where
     the DLI is to occur. You have only between 14 and 61 machine
     cycles available for your DLI, depending on your GRAPHICS
     mode. You must first push any 6502 registers onto the stack, and
     you must end your DLI with an RTI instruction. Because you are
     dealing with machine language for your DLI, you can POKE
     directly into the hardware registers you plan to change, rather
     than using the shadow registers that BASIC uses.

     There is, unfortunately, only one DLI vector address. If you use
     more than one DLI and they are to perform different activities,
     then changing the vectoring to point to a different routine must
     be done by the previous DLI's themselves.

     Another way to accomplish interrupts is during the VBLANK
     interval with a VBI. One small problem with using DLI's is that
     the keyboard "click" routine interferes with the DLI by throwing
     off the timing, since the click is provided by several calls to the
     WSYNC register at 54282 ($D40A). Chris Crawford discusses
     several solutions in De Re Atari, but the easiest of them is not to
     allow input from the keyboard! See Micro, December 1981,
     Creative Computing, July 1981 and December 1981.