APPENDIX ONE
VBLANK Processes
The VBLANK routines are all documented in the OS listings, pages 35
to 38. In the "A" ROMs, they are processed in locations 59345 to 59665
($E7D1 to $E911). In the "B" ROMs, they are processed at 59310 to
59628 ($E7AE to $E8EC). See also De Re Atari for more explanation.

Stage 1 VBLANK:
Performed every VBI:
1) Increment the realtime clock at 18 - 20 ($12-$14)
2) Process the attract mode variables (location 77; $4D)
3) Decrement system timer one at 536 ($218) and if zero JSR through
   550 ($226).

Stage 2 VBLANK:
Performed every VBI which does not interrupt critical sections:
1) Update the hardware registers from the shadows as follows:

Shadow:        Hardware:            Update reason:
SDLISTL/H      DLISTL/H             DISPLAY LIST END
SDMCTL         DMACTL
CHBAS          CHBASE
CHACT          CHACTL
GPRIOR         PRIOR
COLOR0-4       COLPF0-4,BAK         ATTRACT MODE
PCOL0-3        COLPM0-3
LPCNV/H        PENV/H               LIGHT PEN
STICK0-1       PORTA                JOYSTICKS
PTRIG0-3       PORTA                PADDLE TRIGGERS
STICK2-3       PORTB
PTRIG4-7       PORTB
PADDL0-7       POT0-7               PADDLES
STRIG0-3       TRIG0-3              JOYSTICK TRIGGERS
....           CONSOL               CONSOLE SPEAKER OFF

2) System timers two to five (locations 540,542,544; $21C,$21E,$220)
   are decremented and if the value is zero, the corresponding flags
   are set. A JSR is made through 552 ($228) if timer two equals zero.
3) A character is read from POKEY keyboard register at 53769
   ($D209) and read into CH at 764 ($2FC) if the auto-repeat is active.
4) The keyboard debounce counter is decremented by one if it is not
   zero and if no key is being pressed.
5) Keyboard auto-repeat logic is processed.
6) Exit the VBLANK routine through 58466 ($E462).