566,567



     same as VCOUNT register for two-line resolution (see 54283;
     $D40B). Both light pen values are modified when the trigger is
     pressed (pulled low). The light pen positions are not the same as
     the normal screen row and column positions. There are 96
     vertical positions, numbered from 16 at the top to 111 at the
     bottom, each one equivalent to a scan line. Horizontal positions
     are marked in color clocks. There are 228 horizontal positions,
     numbered from 67 at the left. When the LPENH value reaches
     255, it is reset to zero and begins counting again by one to the
     rightmost edge, which has a value of seven.

     Obviously, because of the number of positions readable and the
     small size of each, a certain leeway must be given by the
     programmer when using light pen readouts on a program. At the
     time of this writing, Atari had not yet released its light pen onto
     the market, although other companies have.

566,567        236,237        BRKKY
     BREAK key interrupt vector. This vector is available only with
     the version "B" OS ROMs, not the earlier version. You can use
     this vector to write your own BREAK key interrupt routine.
     Initialized to 59220 ($E754).

568,569        238,239        ....
     Two spare bytes.

570            23A            CDEVIC
     Four-byte command frame buffer (CFB) address for a device --
     used by SIO while performing serial I/O, not for user access.
     CDEVIC is used for the SIO bus ID number The other three CFB
     bytes are:

571            23B            CCOMND
     The SIO bus command code.

572            23C            CAUX1
     Command auxiliary byte one, loaded from location 778 ($30A)
     by SIO.

573            23D            CAUX2
     Command auxiliary byte two, loaded from location 779 ($30B) by
     SIO.

574            23E            TEMP
     Temporary RAM register for SIO.

575            23F            ERRFLG
     SIO error flag; any device error except the timeout error (time
     equals zero).