53775



     VINTER at 516 ($204). If due to a 6502 BRK instruction, then vector
     through VBREAK at 518 ($206).

     Timers relate to audio dividers of the same number (an interrupt is
     processed when the dividers count down to zero). These bits in
     IRQEN are not set on powerup and must be initiated by the user
     program before enabling the processor IRQ.
     There	are two other interrupts, processed by PIA, generated over
     the serial bus Proceed and Interrupt lines, set by the bits in the
     PACTL and PBCTL registers (54018 and 54019; $D302, $D303):

     Bit Decimal Locution Interrupt
     0       1   PACTL    Peripheral A (PORTA) interrupt enable
                          bit.
     7     128   PACTL    Peripheral A interrupt status bit.
     0       1   PBCTL    Peripheral B (PORTB) interrupt enable
                          bit.
     7     128   PBCTL    Peripheral B interrupt status bit.

     The latter PORT interrupts are automatically disabled on powerup.
     Only the BREAK key and data key interrupts are enabled on
     powerup. The shadow register is 16 ($10).

                              IRQST
     (R) Interrupt request status. Bit functions are the same as IRQEN
     except that they register the interrupt request when it is zero rather
     than the enable when a bit equals one. IRQST is used to determine
     the cause of interrupt request with IRQEN, PACTL and PBCTL as
     above.

     All IRQ interrupts are normally vectored through 65534 ($FFFE) to
     the IRQ service routine at 59123 ($E6F3), which determines the
     cause of the interrupt. The IRQ global RAM vector VIMIRQ at 534
     ($216) ordinarily points to the IRQ processor at 59126 ($E6F6). The
     processor then examines 53774 ($D20E) and the PIA registers at
     54018 and 54019 to determine the interrupt cause. Once
     determined, the routine vectors through one of the IRQ RAM
     vectors in locations 514 to 526 ($202 to $20E). For Non-Maskable
     Interrupts (NMI's), see locations 54286 to 54287 ($D40E; $D40F).
     See the OS User's Manual for complete details.

53775          D20F           SKCTL
     (W) Serial port control. Holds the value 255 ($255) if no key is
     pressed, 251 ($FB) for most other keys pressed, 247 ($F7) for
     SHIFT key pressed (*M). See the (R) mode below for an
     explanation of the bit functions. POKE with three to stop the
     occasional noise from cassette after I/O to bring POKEY out of the
     two-tone mode. (562).

                              SKSTAT