APPENDIX C
The following program fragment demonstrates how you
would print the character A on the console screen using
GEMDOS command 2:
move.w #'A ', — (sp) * push the character value on stack
move.w # 2, — (sp) * push GEMDOS command number on
* stack
trap #1 * call GEMDOS handler
addq.l #4,sp * pop parameters (4 bytes) off stack
Calling the GEMDOS routines from C is much simpler.
Most C compilers come with a library routine called gem-
dos(), which stacks the parameters and executes the TRAP
#1 instruction. For example, the sample call illustrated above
could be accomplished in C by the single statement:
gemdos(2, 'A');
Since it's easier to remember a command name than a
command number, most C compilers include a header file
called OSBIND.H that defines macros for all of the GEMDOS
functions. For example, the macro definition for GEMDOS
command 2 is
#define Cconout(a) gemdos(0x2,a)
Therefore, after you #include OSBIND.H in your pro
gram, you can call the sample function like this:
Cconout('A');
Since this format is the most readable, most reference
books and sample programs use the C macro notation. Re
member, however, that in order to use GEMDOS functions
in your C programs, you must link your program with the
compiler library that contains the gemdos() function, and
you must #include OSBIND.H if you use the macros.
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