CCS C Compiler Manual PCB / PCM / PCH

Using Program Memory for Data
CCS C Compiler provides a few different ways to use program memory for data. The different
ways are discussed below:
Constant Data:
The const qualifier will place the variables into program memory. If the keyword const is used
before the identifier, the identifier is treated as a constant. Constants should be initialized and
may not be changed at run-time. This is an easy way to create lookup tables.
The rom Qualifier puts data in program memory with 3 bytes per instruction space. The address
used for ROM data is not a physical address but rather a true byte address. The & operator can
be used on ROM variables however the address is logical not physical.
The syntax is:
const type id[cexpr] = {value}
For example:
Placing data into ROM
const int table[16]={0,1,2...15}
Placing a string into ROM
const char cstring[6]={"hello"}
Creating pointers to constants
const char *cptr;
cptr = string;
The #org preprocessor can be used to place the constant to specified address blocks.
For example:
The constant ID will be at 1C00.
#ORG 0x1C00, 0x1C0F
CONST CHAR ID[10]= {"123456789"};
Note: Some extra code will precede the 123456789.
The function label_address can be used to get the address of the constant. The constant
variable can be accessed in the code. This is a great way of storing constant data in large
programs. Variable length constant strings can be stored into program memory.
A special method allows the use of pointers to ROM. This method does not contain extra code
at the start of the structure as does constant.
For example:
char rom commands[] = {“put|get|status|shutdown”};
The compiler allows a non-standard C feature to implement a constant array of variable length
strings.
The syntax is:
const char id[n] [*] = { "string", "string" ...};
Where n is optional and id is the table identifier.
For example:
const char colors[] [*] = {"Red", "Green", "Blue"};
#ROM directive:
Another method is to use #rom to assign data to program memory.
The syntax is:
#rom address = {data, data, … , data}
34