530,531



530,531        212,213        VTIMR2
     POKEY timer two vector for AUDF2 (53762, $D202), initialized to
     59314 ($E7B2). IRQ.

532,533        214,215        VTIMR4
     POKEY timer four vector for AUDF4 (53766, $D206), initialized
     to 59314 ($E7B2). This IRQ is only vectored in the "B" version of
     the OS ROMs.

534,535        216,217        VIMIRQ
     The IRQ immediate vector (general). Initialized to 59126
     ($E6F6). JMP through here to determine cause of the IRQ
     interrupt. Note that with the new ("B") OS ROMs, there is a
     BREAK key interrupt vector at locations 566, 567 ($236, $237).
     See 53774 ($D20E) for more information on IRQ interrupts.

     The new "B" version OS ROMs change the vectors above as
     follows:

     VDSLST     59280 ($E790)
     VPRCED     59279 ($E78F)
     VINTER     59279 ($E78F)
     VBREAK     59279 ($E78F)
     VKEYBD     NO CHANGE
     VSERIN     60175 ($EB0F)
     VSEROR     NO CHANGE
     VSEROC     60111 ($EACF)
     VTIMR 1-4  59279 ($E78F)
     VIMIRQ     59142 ($E706)
     VVBLKI     59310 ($E7AE)
     VVBLKD     59653 ($E905)

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The locations from 536 to 558 ($218 to $22E) are used for the system
software timers. Hardware timers are located in the POKEY chip and
use the AUDF registers. These timers count backwards every 1/60
second (stage one VBLANK) or 1/30 second (stage two VBLANK)
interval until they reach zero. If the VBLANK process is disabled or
intercepted, the timers will not be updated. See De Re Atari for
information regarding setting these timers in an assembly routine
using the SETVBV register (58460; $E45C). These locations are user-
accessible and can be made to count time for music duration, game
I/O, game clock and other functions.
Software timers are used for durations greater than one VBLANK
interval (1/60 second). For periods of shorter duration, use the
hardware registers.

536,537        218,219        CDTMV1
     System timer one value. Counts backwards from 255. This SIO