Zilog Z8F0130 - Manual (Page 112)

Using the Integrated Development Environment UM013037-1212
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Zilog Developer Studio II – Z8 Encore!
User Manual
also be seen at the machine instruction level, where a separate instruction, LDC , must be
generated by the compiler to load data from the ROM space. That means that if constants
are placed in ROM, a different function must be called if a
const pointer is used as a
function parameter than the function that is called for a non- const pointer type.
For example, consider the following code:
char const *quote = “Ask not ...”;

char buffer[80];

rom_src_strcpy(buffer,quote); // OK if CONST = ROM;

// parameter type mismatch

// if not

strcpy(buffer,quote); // Parameter type mismatch

// if const = ROM, OK otherwise.
The top form shown here calls a function whose signature is
char * rom_src_strcpy (void * dest, rom void * source)
whereas the standard function strcpy has the more usual signature
char * strcpy (void * dest, const void * source)
The top form succeeds in this code snippet when the const=ROM option has been selected,
and fails otherwise (when const data is stored in RAM). The bottom form fails when
const=ROM but succeeds otherwise. There can never be a case when both calls succeed,
because the second pointer argument of rom_src_strcpy() is a fundamentally different
type, pointing into a different space, than the second pointer argument of strcpy() .
In short, the result of these architectural constraints is that if the Place Const Variables in
ROM checkbox is selected, it is impossible for the compiler to treat the const keyword in
a way that complies with the ANSI C Standard.
It is better to not select this deprecated option and let the compiler use const variables in
RAM when needed and to use the rom keyword explicitly to declare any data such as
tables that you really do want to locate in ROM.
Disable ANSI Promotions
The option of enabling or disabling ANSI promotions refers to promoting char and
short values to int s when doing computations, as described in more detail in this sec-
tion. Disabling the promotions was made a user option in earlier releases of ZDS II with
the goal of reducing code size because the promotions called for by the ANSI C standard
are often unnecessary and can lead to considerable code bloat. However, over time, sev-
eral problems were found in the compiler’s ability to apply this option consistently and
correctly in all cases. Therefore, Zilog no longer recommends the use of this feature and,
to address the original code size issue, has expended more effort to reduce code size and
remove truly unnecessary promotions while observing the ANSI standard. For this reason,
the
Disable ANSI Promotions checkbox is now available only as a deprecated feature. It
remains available because some users have carefully created working code that might