simplify the design of the hardware forming the interface between the processor and thememory system

Computer Systems A Programmer's Perspective Second Edition

Many computer systems place restrictions on the allowable addresses for the

primitive data types, requiring that the address for some type of object must be a

multiple of some value K (typically 2, 4, or 8). Such alignment restrictions simplify

the design of the hardware forming the interface between the processor and the

memory system. For example, suppose a processor always fetches 8 bytes from

memory with an address that must be a multiple of 8. If we can guarantee that any

double will be aligned to have its address be a multiple of 8, then the value can

be read or written with a single memory operation.Otherwise, we may need to 

perform two memory accesses, since the object might be split across two 8-byte

memory blocks.

 

Linux follows an alignment policy where 2-byte data types (e.g.,

short) must have an address that is a multiple of 2, while any larger data types

(e.g.,int,int *,float, and double) must have an address that is a multiple of 4.

Note that this requirement means that the least significant bit of the address of

an object of type short must equal zero. Similarly, any object of type int, or any

pointer, must be at an address having the low-order 2 bits equal to zero.

 

Microsoft Windows imposes a stronger alignment requirement—any primitive object of

K bytes, for K=2, 4, or 8, must have an address that is a multiple of K. In particular,

it requires that the address of a double or a long long be a multiple of 8. This

requirement enhances the memory performance at the expense of some wasted space.

The Linux convention, where 8-byte values are aligned on 4-byte boundaries was probably

good for the i386, back when memory was scarce and memory interfaces were

only 4 bytes wide. With modern processors, Microsoft’s alignment is a better design decision.

Data type long double, for which gcc generates IA32 code allocating 12 bytes (even though

the actual data type requires only 10 bytes) has a 4-byte alignment requirement with

both Windows and Linux.