PicC 9.50 dsPIC Manual.pdf

C Language FeaturesSupported Data Types and Variables3.4.9 Special Type QualifiersHI-TECH dsPICCsupports the special type qualifiers to allow the user to control placement ofstatic and extern class variables into particular address spaces.3.4.9.1 Persistent Type QualifierBy default, any C variables that are not explicitly initialised are cleared to zero on startup. This isconsistent with the definition of the C language. However, there are occasions where it is desired forsome data to be preserved across resets or even power cycles (on-off-on).The persistent type qualifier is used to qualify variables that should not be cleared on startup.In addition, any persistent variables will be stored in a different area of memory to other variables.persistent objects are placed within the psect nvram.This type qualifier may not be used on variables of class auto; if used on variables local to afunction they must be combined with the static keyword. For example, you may not write:void test(void){persistent int intvar; /* WRONG! */.. other code ..}because intvar is of class auto. To declare intvar as a persistent variable local to functiontest(), write:static persistent int intvar;If the DSPICC option, --STRICT is used, this type qualifier is changed to __persistent.There are some library routines provided to check and initialise persistent data - see A formore information, and for an example of using persistent data.3.4.9.2 YData Type QualifierThe dsPIC memory map for RAM is divided into two parts: x-data and y-data. Some dsPIC instructioncan only operate will addresses to object in the y-data range. Qualifying an object as ydatacause the object to be placed into the ranges of memory designated for ydata.3.4.10 Pointer TypesThere are two basic pointer types supported by HI-TECH C: data pointers and function pointers.Data pointers hold the address of data objects which can be indirectly read and possibly written by37