What is this area 0x0000_0000

corone · 07-10-2012, 01:09 PM

Hi,

The start address of the Code section (or Text section) is 0x0804_8000.

Code:

struct mm_struct *mm = NULL;

printk("mm->start_code: %x\n", (unsigned int)mm->start_code);

the output is the following...

Code:

mm->start_code: 8048000

and refer to the following...

Code:

# cat /proc/1/maps
08048000-08050000 r-xp 00000000 03:01 1673       /sbin/init
08050000-08051000 rw-p 00007000 03:01 1673       /sbin/init
08051000-08072000 rw-p 08051000 00:00 0          [heap]
...

# pmap -x 1
1:   init [2]
Address   Kbytes     RSS    Anon  Locked Mode   Mapping
08048000      32       -       -       - r-x--  init
08050000       4       -       -       - rw---  init
08051000     132       -       -       - rw---    [ anon ]
...

#

The Code section (or Text section) starts at 0x0804_8000.

Then, what is in the area 0x0000_0000 - 0x0804_8000?
What is this area 0x0000_0000 - 0x0804_8000 for?

Thank you.

business_kid · 07-10-2012, 02:07 PM

Quote:

Then, what is in the area 0x0000_0000 - 0x0804_8000?
What is this area 0x0000_0000 - 0x0804_8000 for?

In the bad old days, when '640k was enough memory for anyone - ever' there was Dos and everything was open in this 640k, and the later added upper memory -->1M. Weird things were done with A20 (for above 1M) because it was never going to be needed for memory:-//.

Nowadays, because of backward compatibility this and the bios area under 64k can be security hazards which hackers aim at, so the kernel keeps stuff out of the bottom 64k. Processors, architectures etc have all changed but in single steps and everything

HTH.