Alignment requirements

To speed memory accessing,the modern processor required to store base data from special address.The base rule is start address must can be divisible by size of corresponding base data.For example, ARM-32 processor,byte data start address must be divisible by 1,half word data start address must be divisible by 2 and word data start address must be divisible by 4.

Padding

For satisfying the alignment requirements,padding rules occurs to solve several base data arrangement in continue memory segment,such as c structrue ,c function stack even c bitfield.

The detail of padding rule is just to satisfy alignment in continue memory segment.The start base data of the continue memory segment is aligned and same to the all after it.