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SBASIC Manual - Page 58

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     SBasic User's Manual     SBasic Version 2.7             Page 58
     Printed:  December 5, 1999

     Generally, you should not use ASMFUNC to define labels used internally
     by SBasic run-time routines.  Doing so will cause the assembler that
     processes the resulting output file to report an error.  This happens
     because SBasic will create two identical labels in its output assembly
     language file, one label that you defined in your ASM section and a
     matching label in an SBasic library file.

     The exceptions to this rule involve the labels _INKEY and  _OUTCH.
     SBasic reserves these labels for internal routines that handle
     character I/O.  By default, the _OUTCH routine sends the character in
     the A-register to the 68hc11's SCI port, and the _INKEY routine
     returns a character from the SCI in the D-register.  SBasic
     automatically appends a library file containing the assembly language
     source for these routines whenever it processes a statement that
     requires them.

     In this one case, SBasic permits your source file to override the
     normal inclusion of a library file.  For example, if SBasic detects an
     ASMFUNC statement defining the label _OUTCH:

         asmfunc  _outch

     SBasic does not append the _OUTCH library file.  Instead, SBasic
     relies on whatever _OUTCH assembly language routine you define in your
     SBasic source file to handle all character output.

     This feature allows you to redirect the output from all SBasic PRINT
     and OUTCH statements to an alternate device.  Similarly, you can
     redirect the input for the SBasic INKEY function from an alternate
     device.  This makes it easy to add SBasic support for formatted output
     to devices such as LCDs, or character input from custom keyboards.
     For example:

          asmfunc  _outch

          staa      $1004               send char in A to port b

          print "2 + 2 ="; 2+3

     This sample program sends a mathematical statement to port B that
     should convince anyone your computer is loony.