MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

MPLAB C Compiler for PIC18 MCUs vs. 16-Bit Devices
D.21 BIT FIELDS
Bit fields in the PIC18 MCU Compiler cannot cross byte storage boundaries and, therefore,
cannot be greater than 8 bits in size.
The 16-Bit Device Compiler supports bit fields with any bit size, up to the size of the
underlying type. Any integral type can be made into a bit field. The allocation cannot
cross a bit boundary natural to the underlying type.
For example:
struct foo {
long long i:40;
int j:16;
char k:8;
} x;
struct bar {
long long I:40;
char J:8;
int K:16;
} y;
struct foo will have a size of 10 bytes using the 16-Bit Device Compiler. i will be
allocated at bit offset 0 (through 39). There will be 8 bits of padding before j, allocated
at bit offset 48. If j were allocated at the next available bit offset, 40, it would cross a
storage boundary for a 16 bit integer. k will be allocated after j, at bit offset 64. The
structure will contain 8 bits of padding at the end to maintain the required alignment in
the case of an array. The alignment is 2 bytes because the largest alignment in the
structure is 2 bytes.
struct bar will have a size of 8 bytes using the 16-Bit Device Compiler. I will be allocated
at bit offset 0 (through 39). There is no need to pad before J because it will not
cross a storage boundary for a char. J is allocated at bit offset 40. K can be allocated
starting at bit offset 48, completing the structure without wasting any space.
© 2008 Microchip Technology Inc. DS51284H-page 209