Apple II
Technical Notes
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                                                  Developer Technical Support


ProDOS 8
#1:    The GETLN Buffer and a ProDOS Clock Card

Revised by:    Matt Deatherage                                  November 1988
Revised by:    Pete McDonald                                    November 1985

This Technical Note describes the effect of a clock card on the GETLN buffer.
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ProDOS automatically supports a ThunderClock(TM) or compatible clock card when 
the system identifies it as being installed.  When programming under ProDOS, 
always consider the impact of a clock card on the GETLN input buffer ($200 - 
$2FF).  ProDOS can support other clocks which may also use this space.

When ProDOS calls a clock card, the card deposits an ASCII string in the GETLN 
input buffer in the form:  07,04,14,22,46,57.  This string translates as the 
following:

    07 = The month, July (01=Jan,...,12=Dec)
    04 = The day of the week, Thurs.(00=Sun,...,06=Sat)
    14 = The date (00 to 31)
    22 = The hour, 10 p.m. (00 to 23)
    46 = The minute (00 to 59)
    57 = The second (00 to 59)
    
ProDOS calls the clock card as part of many of its routines.  Anything in the 
first 17 bytes of the GETLN input buffer is subject to loss if a clock card is 
installed and is called.

In general, it has been the practice of programmers to use the GETLN input 
buffer for every conceivable purpose.  Therefore, an application should never 
store anything there.  If your application has a future need to know about the 
contents of the $200 - $2FF space, you should transfer it to some other 
location to guarantee that it will remain intact, particularly under ProDOS, 
where a clock card may regularly be overwriting the first 17 bytes.

The ProDOS 8 Technical Reference Manual contains more information on the clock 
driver, including the necessary identification bytes, how the ProDOS driver 
calls the card, and how you may replace this routine with your own.


Further Reference
o    ProDOS 8 Technical Reference Manual