54283



     next TV line. It is used to synchronize the VBI's or DLI's with the
     screen display.
     To see the effect of the WSYNC register, type in the second
     example of a Display List Interrupt at location 512. RUN it and
     observe that it causes a clean separation of the colors at the
     change boundary. Now change line 50 to:

     50  DATA 72,169,222,234,234,234,141,24,208,104,64

     This eliminates the WSYNC command. RUN it and see the
     difference in the boundary line.

     The keyboard handler sets WSYNC repeatedly while generating
     the keyboard click on the console speaker at 53279 ($D01F).
     When interrupts are generated during the WSYNC period, they
     get delayed by one scan line. To bypass this, examine the
     VCOUNT register below and delay the interrupt processing by
     one line when no WSYNC delay has occurred.

54283          D40B           VCOUNT
     (R) Vertical line counter. Used to keep track of which line is
     currently being generated on the screen. Used during Display
     List Interrupts to change color or graphics modes. PEEKing here
     returns the line count divided by two, ranging from zero to 130
     ($82; zero to 155 on the PAL system; see 53268; $D014) for the
     262 lines per TV frame.

54284          D40C           PENH
     (R) Light pen horizontal position (564). Holds the horizontal color
     clock count when the pen trigger is pressed.

54285          D40D           PENV
     (R) Light pen vertical position (565). Holds the VCOUNT value
     (above) when the pen trigger is pressed. See the Hardware
     Manual, p. II-32, for a description of light pen operation.

54286          D40E           NMIEN
     (W) Non-maskable interrupt (NMI) enable. POKE with 192 to
     enable the Display List Interrupts. When BIT 7 is set to one, it
     means DL instruction interrupt; any display list instruction where
     BIT 7 equals one will cause this interrupt to be enabled at the
     start of the last video line displayed by that instruction. When BIT
     6 equals one, it allows the Vertical Blank Interrupt and when BIT
     5 equals one, it allows the RESET button interrupt. The RESET
     interrupt is never disabled by the OS. You should never press
     RESET during powerup since it will be acted upon.

     NMIEN is set to 64 ($40) by the OS IRQ code on powerup,
     enabling VBI's, but disabling DLI's. All NMI interrupts are
     vectored through 65530 ($FFFA) to the NMI service routine at