ALIGN in Linker Scripts - linker

Question

What does the ALIGN keyword do in linker scripts? I read many tutorials about linker scripts but I cant understand what really ALIGN do. Can any one explain it simply. Thanks!

Answer 1

A typical usage is
. = ALIGN(8);
This means: insert padding bytes until current location becomes aligned on 8-byte boundary. That is:
while ((current_location & 7) != 0)
*current_location++ = padding_value;

Answer 2

The ALIGN() instructions tell the linker that section(bss, text) should be this much aligned.
For a typical idea, you can take a look here (4.6.3 "Output Section Description")
e.g.
//.data is aligned by word size on the 32-bit architecture and direct it to the data section
For a 32-bit machine, it typically needs to be word aligned
.data : ALIGN(4)
{
*(.data*)
} > data_sdram

Answer 3

. = ALIGN(8)
Corresponds to the following (working link script example using operators):
data = .;
. = ((data + 0x8 - 1) & ~(0x8 - 1)) - data;

Answer 4

There are two typical uses for ALIGN.
To align the start of a section to a required boundary
To pad a section to a required size.
/* . = ALIGN(BEGIN) ; equivalent to below, if VMA==LMA */
.section : ALIGN(BEGIN)
{
/* . = ALIGN(BEGIN) ; !!Not equivalent to above!! */
...
. = ALIGN(END);
}
The first ALIGN ensure the starting value is a multiple of BEGIN as a power of 2. The 2nd ALIGN will ensure that the size of a multiple of END. Normally you will want them to be equal and powers of two which indicate how many address bits are needed and can help with caching.
For instance copy loops can be optimized with large transfer sizes if the size is prohibited by the linker script to have fixed size multiples.
The ALIGN(BEGIN) inside the '.section' would result in some junk at the beginning of the sections. See: FILL() and =fill for ways to control what is written. See below on reason why not to use this; it actually does nothing.
Some example of structure that need to be aligned are,
a vector table
an mmu table
time critical routines that need cache fills
.bss and .initdata for initialization efficiency
The Gnu ld syntax allows you the ability to use ALIGN anywhere.
If you were to custom code structures with BYTE, LONG, you might need to align tables/structures in a section. This is fairly obtuse use of a linker scripts, but is possible and an exception to the opening two uses. However, for most beginning to understand ld, that two uses are almost always the desired use.
Operation inside a section works as if all of the addresses are relative. So the start of the section is zero (and the not equivalent doesn't work). As the last thing is . = ALIGN(...);, this sets the size of the section because it is zero based. This will even work if the start address of the section is not aligned.