Syslinux - Jun 2008 - mysteries of memdisk code

Hello,

 

I was browsing the code for memdisk (I want to adjust it to my hardware)
and noticed a couple of things that I am struggling to understand.  I
would appreciate if someone could clarify the following mysteries to me:

 

1)       _binary_memdisk_bin_start[], _binary_memdisk_bin_end[] and
_binary_memdisk_bin_size[]; These variables are declared (as extern
const chat in 'setup.c') but never defined in the code. Yet, it seems
that somehow they are initialized with the size and start address of
memdisk. How can it be?

2)       What is driverptr (declared in setup())? It is defined as
driverptr = driverseg  << 16; Why is it shifted by 16-bits?

 

Thanks.

 

Best regards,

Alex

Hallo,

some wild guesses which may help you right now and may be clarified by
hpa later on:

* Loginov Alexander <aloginov at asmpt.com> [20080618
08:11]:
> 1)       _binary_memdisk_bin_start[], _binary_memdisk_bin_end[] and
> _binary_memdisk_bin_size[]; These variables are declared (as extern
> const chat in 'setup.c') but never defined in the code. Yet, it
seems
> that somehow they are initialized with the size and start address of
> memdisk. How can it be?
most likely, the linker does that ... have a look at the linker script
and what happens in that area :)
> 2)       What is driverptr (declared in setup())? It is defined as
> driverptr = driverseg  << 16; Why is it shifted by 16-bits?
no idea what it is, but name and shifting seem to be related to
http://en.wikipedia.org/wiki/X86_memory_segmentation

Hope that helps you for the moment ...

   Andreas

-- 
flatline IT services - A.Kotes - IT-consulting for ISPs & Internet services

Loginov Alexander wrote:
> 
> 1)       _binary_memdisk_bin_start[], _binary_memdisk_bin_end[] and
> _binary_memdisk_bin_size[]; These variables are declared (as extern
> const chat in 'setup.c') but never defined in the code. Yet, it
seems
> that somehow they are initialized with the size and start address of
> memdisk. How can it be?
> 
GNU ld can convert a binary file to an object file (.o), and then it 
automatically create those symbols assuming that the original file was 
named memdisk.bin.
	
	-hpa