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117 lines
2.7 KiB
C
117 lines
2.7 KiB
C
/*
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* Minimalistic overlay demo program.
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*
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* 2009-10-02, Oliver Schmidt (ol.sc@web.de)
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*
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*/
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#include <stdio.h>
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#include <conio.h>
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#include <fcntl.h>
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#include <unistd.h>
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extern void _OVERLAY1_LOAD__[], _OVERLAY1_SIZE__[];
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extern void _OVERLAY2_LOAD__[], _OVERLAY2_SIZE__[];
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extern void _OVERLAY3_LOAD__[], _OVERLAY3_SIZE__[];
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/* Functions resident in an overlay can call back functions resident in the
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* main program at any time without any precautions. The function log() is
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* an example for such a function resident in the main program.
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*/
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void log (char *msg)
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{
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printf ("Log: %s\n", msg);
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}
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/* In a real-world overlay program one would probably not use a #pragma but
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* rather place the all the code of certain source files into the overlay by
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* compiling them with --code-name OVERLAY1.
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*/
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#pragma code-name (push, "OVERLAY1");
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void foo (void)
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{
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/* Functions resident in an overlay can access all program variables and
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* constants at any time without any precautions because those are never
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* placed in overlays. The string constant below is an example for such
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* a constant resident in the main program.
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*/
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log ("Calling main from overlay 1");
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}
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#pragma code-name (pop);
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#pragma code-name (push, "OVERLAY2");
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void bar (void)
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{
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log ("Calling main from overlay 2");
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}
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#pragma code-name (pop);
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#pragma code-name (push, "OVERLAY3");
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void foobar (void)
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{
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log ("Calling main from overlay 3");
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}
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#pragma code-name(pop);
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unsigned char loadfile (char *name, void *addr, void *size)
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{
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int file = open (name, O_RDONLY);
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if (file == -1) {
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log ("Opening overlay file failed");
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return 0;
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}
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read (file, addr, (unsigned) size);
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close (file);
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return 1;
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}
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void main (void)
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{
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log ("Calling overlay 1 from main");
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/* The symbols _OVERLAY1_LOAD__ and _OVERLAY1_SIZE__ were generated by the
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* linker. They contain the overlay area address and size specific to a
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* certain program.
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*/
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if (loadfile ("ovrldemo.1", _OVERLAY1_LOAD__, _OVERLAY1_SIZE__)) {
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/* The linker makes sure that the call to foo() ends up at the right mem
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* addr. However it's up to user to make sure that the - right - overlay
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* is actually loaded before making the the call.
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*/
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foo ();
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}
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log ("Calling overlay 2 from main");
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/* Replacing one overlay with another one can only happen from the main
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* program. This implies that an overlay can never load another overlay.
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*/
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if (loadfile ("ovrldemo.2", _OVERLAY2_LOAD__, _OVERLAY2_SIZE__)) {
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bar ();
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}
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log ("Calling overlay 3 from main");
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if (loadfile ("ovrldemo.3", _OVERLAY3_LOAD__, _OVERLAY3_SIZE__)) {
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foobar ();
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}
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cgetc ();
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}
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