User manual

mikroC PRO for dsPIC
MikroElektronika
221
Bit Fields
Bit elds are specied numbers of bits that may or may not have an associated identier. Bit elds offer a way of
subdividing structures into named parts of user-dened sizes.
Structures and unions can contain bit elds that can be up to 16 bits.
You cannot take the address of a bit eld.
Note: If you need to handle specic bits of 8-bit variables (char and unsigned short) or registers, you don’t
need to declare bit elds.
Much more elegant solution is to use the mikroC PRO for dsPIC30/33 and PIC24’s intrinsic ability for
individual bit access — see Accessing Individual Bits for more information.
Bit Fields Declaration
Bit elds can be declared only in structures and unions. Declare a structure normally and assign individual elds like
this (elds need to be unsigned):
struct tag {
unsigned biteld-declarator-list;
}
Here, tag is an optional name of the structure; biteld-declarator-list is a list of bit elds. Each component
identifer requires a colon and its width in bits to be explicitly specied. Total width of all components cannot exceed two
bytes (16 bits).
As an object, bit elds structure takes two bytes. Individual elds are packed within two bytes from right to left. In
biteld-declarator-list, you can omit identier(s) to create an articial “padding”, thus skipping irrelevant bits.
For example, if there is a need to manipulate only bits 2–4 of a register as one block, create a structure like this:
struct {
unsigned : 2, // Skip bits 0 and 1, no identier here
mybits : 3; // Relevant bits 2, 3 and 4
// Bits 5, 6 and 7 are implicitly left out
} myreg;
Here is an example:
typedef struct {
lo_nibble : 4;
hi_nibble : 4;
high_byte : 8;} myunsigned;
which declares the structured type myunsigned containing three components: lo_nibble (bits 3..0), hi_nibble
(bits 7..4) and high_byte (bits 15..8).