User`s manual

334 digi.com Hints and Tips
• Reduce usage of root constants and string literals
Shortening literal strings and reusing them will save root space. The compiler automatically reuses iden-
tical string literals.
These two statements :
printf (“This is a literal string”);
sprintf (buf, “This is a literal string”);
will share the same literal string space whereas:
sprintf (buf, “this is a literal string”);
will use its own space since the string is different.
• Use xdata to declare large tables of initialized data
If you have large tables of initialized data, consider using the keyword xdata to declare them. The disad-
vantage is that data cannot be accessed directly with pointers. The function xmem2root() allows
xdata to be copied to a root buffer when needed.
// This uses root code space
const int root_tbl[8]={300,301,302,103,304,305,306,307};
// This does not
xdata xdata_table {300,301,302,103,304,305,306,307};
main(){
// this only uses temporary stack space
auto int table[8];
xmem2root(table, xdata_table, 16);
// now the xmem data can be accessed via a 16 bit pointer into the table
}
Both methods, const and xdata, create initialized data in flash at compile time, so the data cannot be
rewritten directly.
• Use xstring to declare a table of strings
The keyword xstring declares a table of strings in extended flash memory. The disadvantage is that
the strings cannot be accessed directly with pointers, since the table entries are 20-bit physical
addresses. As illustrated above, the function xmem2root() may be used to store the table in tempo-
rary stack space.
// This uses root code space
const char * name[] = {“string_1”, . . . “string_n”};
// This does not
xstring name {“string_1”, . . . “string_n”};
Both methods, const and xstring, create initialized data in flash at compile time, so the data cannot
be rewritten directly.
• Turn off selected debugging features
Watch expressions, breakpoints, and single stepping can be selectively disabled on the Debugger tab of
Project Options to save some root code space.