550,551



     $E905), the exit for the VBLANK routine. NMI.

     These two VBLANK vectors point to interrupt routines that occur
     at the beginning of the VBLANK time interval. The stage one
     VBLANK routine is executed; then location 66 ($42) is tested for
     the time-critical nature of the interrupt and, if a critical code
     section has been interrupted, the stage two VBLANK routine is
     not executed with a JMP made through the immediate vector
     VVBLKI. If not critical, the deferred interrupt VVBLKD is used.
     Normally the VBLANK interrupt bits are enabled (BIT 6 at
     location 54286; $D40E is set to one). To disable them, clear BIT 6
     (set to zero).

     The normal seguence for VBLANK interrupt events is: after the
     OS test, JMP to the user immediate VBLANK interrupt routine
     through the vector at 546, 547 (above), then through SYSVBV at
     58463 ($E45F). This is directed by the OS through the VBLANK
     interrupt service routine at 59345 ($E7D1) and then on to the
     user-deferred VBLANK interrupt routine vectored at 548, 549. it
     then exits the VBLANK interrupt routine through 58466 ($E462)
     and an RTI instruction.

     If you are changing the VBLANK vectors during the interrupt
     routine, use the SETVBV routine at 58460 ($E45C). An
     immediate VBI has about 3800 machine cycles of time to use a
     deferred VBI has about 20,000 cycles. Since many of these cycles
     are executed while the electron beam is being drawn, it is
     suggested that you do not execute graphics routines in deferred
     VBI's. See the table of VBLANK processes at the end of the map
     area.

     if you create your own VBI's, terminate an immediate VBI with a
     JMP to 58463 ($E45F) and a deferred VBI with a JMP to 58466
     ($E462). To bypass the OS VBI routine at 59345 ($E7D1) entirely,
     terminate your immediate VBI with a JMP to 58466 ($E462).

     Here's an example of using a VBI to create a flashing cursor. It
     will also blink any text you display in inverse mode.

     10   FOR BLINK = 1664 TO 1680: READ B
        YTE: POKE BLINK, BYTE: NEXT BLINK
     20   POKE 548,128: POKE 549,6
     30   DATA 8,72,165,20,41,16,74,74,74,
        141
     40   DATA 243,2,104,40,76,62,233

     To restore the normal cursor and display, POKE 548,62 and
     POKE 549,233.

550,551        226,227        CDTMA1
     System timer one jump address, initialized to 60400 ($EBF0).