'align' instruction on MIPS

Question

What exactly do this instruction? I know that it try to align data with a multiple of a especific number but, why would you need to do this? Is there an equivalent instruccion in other assemblers?

Answer 1

You usually align data to get better performance. For most processors, memory access has some penalty when not accessing specific byte boundaries. For other assemblers, there often is some kind of pseudo-op .align for this. Most compilers also align their data structures (though you can disable it for debug purposes).

Also See this Wikipedia entry.

Note that non-emulated MIPS systems might even crash if you try to access unaligned memory cells (see here and here).

Answer 2

Is there an equivalent instruccion in other assemblers?

MASM has an Align directive: http://msdn.microsoft.com/en-us/library/dwa9fwef(VS.80).aspx

Answer 3

It aligns everything to the nth power of two. Its not an instruction, its a directive that will be translated into instructions

As for its usage,for exampe:

mips32 instructions are always 32 bytes long. So each instruction should start on a word boundary. Adding an .align directive before the code starts, aligns things to 32 bits. This has many benefits, including that it only takes 1 memory access for the instruction´s fetch, and that it will probably be benefitial on the instruction cache.