A question about decompressing kernel on boot

Hi list, In the source code arch/x86/boot/compressed/head_64.S, it described how compressed kernel is copied from the origin place to another pointed by rbx register. But I'm confused about the copying code, why it needs to minus 8 from _bss when calculating _bss original and new addresses. This is the source code. /* * Copy the compressed kernel to the end of our buffer * where decompression in place becomes safe. */ pushq %rsi leaq (_bss-8)(%rip), %rsi leaq (_bss-8)(%rbx), %rdi movq $_bss /* - $startup_32 */, %rcx shrq $3, %rcx std rep movsq cld popq %rsi Thanks, Larry