Start symbol for an compressed kernel

[zhaobin xv]

2013/8/17 Russell King - ARM Linux <linux at arm.linux.org.uk>:
> On Sat, Aug 17, 2013 at 07:39:46PM +0800, zhaobin xv wrote:
>> how to find Start symbol for an compressed kernel for arm
>
> The start address of a compressed kernel is the first byte of the image.
>
> Symbolically, that is 'start', which is a non-global symbol.  However,
> this symbol is useless since it normally is zero, because the entire
> image is relocatable and therefore built for address zero.
>
> However, it is normal to load the image not at address zero.

why do U define ENTRY(_start)?
Is _start the Start symbol?
how to explain _start