Question

I’m currently in C course and we are working on bitshifting to pack a date. I understand the concept of shifting but I am at a loss for the rudimentary stuff like how many places to shift and how to formulate the mask, so I can pull them back out. Any direction or recommended reading would be great.

I’ve got this far 🙁

 #define DAY_MASK  0x???

 #define DAY_OFFSET  ?

 #define MONTH_MASK  0x???

 #define MONTH_OFFSET  ?

 #define YEAR_MASK  0x???

 #define YEAR_OFFSET  ?

Thanks

Answer 1

Assume you want 5 bits to days, 4 bits to month, and all the rest to year, something like:

#define DAY_OFFSET 0
#define MONTH_OFFSET 5
#define YEAR_OFFSET 9
#define DAY_MASK 0x1F
#define MONTH_MASK 0x1E0
#define YEAR_MASK (~0x1FF)

note that I use the ~ operator in year, so it will work on any machine (with no care to the size of int except that it should be large enough to contain the date)

edit: If you (like me) feel not natural with hex numbers, you can use binary:

#define DAY_MASK 0b11111
#define MONTH_MASK 0b111100000
#define YEAR_MASK (~0b111111111)

and then you can clearly see which bits every field use.