Need help with the Uncompressed kernel be overriden after decompress_kernel called?

Hi all.

In arch/x86/boot/compressed/head_32.S

before the call of decompress_kernel()
it load the address of z_extract_offset_negative(%ebx) as the output address
and actually the output_len of the decompressed kernel is larger than extract_offset so the decompressed data will override the uncompressed data begin at input_data Is that the case?
How does it worked as the the input buffer is covered by the output buffer while uncompressing? It confused me a lot.
or I'd made some misunderstanding on the decompressing routine?

Thanks.

2011-08-27
jiangtao.jit