Virtual Address Space

[Román Martínez]

Hi,

I compile main.s and it generates main.o. With objdump I can see:

0000000000000000 <_start>:
   0:	b8 01 00 00 00       	mov    $0x1,%eax
   5:	bb 00 00 00 00       	mov    $0x0,%ebx
   a:	cd 80                	        int    $0x80

After link main.o it generates main. With objdump I now can see:

0000000000400078 <_start>:
  400078:	b8 01 00 00 00       	mov    $0x1,%eax
  40007d:	bb 00 00 00 00       	mov    $0x0,%ebx
  400082:	cd 80                	        int    $0x80

So, linker generates virtual address, doesn't it? But why it starts at
400078 and not in other any location? Is there any logic here? A
virtual address can start at 0?

Regards.

El mié, 28-09-2016 a las 19:02 +0530, Gadre Nayan escribió:
> > > Virtual addresses will be used only in case MMU is enabled, otherwise
for a processor an address is something it can put on the bus,
irrespective of physical or virtual.
> > So when your PC increments virtual address for a MMU enabled system,
they will get translated to physical.
> > > On 28 Sep 2016 4:29 p.m., "Prabhunath G" <gprabhunath at gmail.com>
wrote:
> > > > > > > > > > > > > > > > > > The virtual addresses what you see in the output of objdump is
given/associated by Linker to every instruction and data symbol in
the data/bss section except for symbols in the stack section. It is
wrong to use generated in the context of Linker. When you initiate
$./a.out for execution, the kernel will take your start address
from the ELF header of a.out and place it on the PC (program
counter) or IP (instruction pointer) of the CPU, thereafter CPU
will start incrementing or generating virtual address for every
subsequent instructions. 
> > Regards,
> > Prabhu
> > 
> > 
> > > > > > On Wed, Sep 28, 2016 at 3:41 PM, Madhu K <madhu.sk89 at gmail.com>
wrote:
> > > Hi Arun,
> > > Thanks for your response.
> > > 
> > > > > > > > > > > > > > > I will elaborate my question.Assume I have test.c file, I
compiled test.c and generated the a.out ( Executable for linux ),
when I do objdump of a.out, we can see addresses( virtual address
) associated with each instruction, these instructions are
generated by whom?
> > > 
> > > Thanks
> > > 
> > > 
> > > > > > On Wed, Sep 28, 2016 at 11:35 AM, Arun Sudhilal <getarunks at gmail.
com> wrote:
> > > > Hello Madhu,
> > > > 
> > > > 
> > > > > > > > On Wed, Sep 28, 2016 at 10:36 AM, Madhu K <madhu.sk89 at gmail.com
> wrote:
> > > > 
> > > > > Hi All,
> > > > 
> > > > >
> > > > 
> > > > > > > > > This is to understand the Virtual address space.Basically who
generates the
> > > > 
> > > > > virtual addresses CPU or GNU compiler?
> > > > 
> > > > 
> > > > 
> > > > I didn't really get your question.
> > > > 
> > > > 
> > > > 
> > > > > > > > Linux kernel starts at a fixed location in virtual space. This
is
> > > > 
> > > > > > > > called PAGE_OFFSET. On a kernel split of 3GB/1GB, 32 system,
its is
> > > > 
> > > > > > > > 0xC000_0000. You can have a look at system.map file after
compiling
> > > > 
> > > > your kernel.
> > > > 
> > > > When cpu runs with MMU on, your cpu generates virtual address.
> > > > 
> > > > 
> > > > 
> > > > Regards,
> > > > 
> > > > Arun
> > > > 
> > > > 
> > > > 
> > > > >
> > > > 
> > > > > Thanks
> > > > 
> > > > > Madhu
> > > > 
> > > > >
> > > > 
> > > > > _______________________________________________
> > > > 
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> > > > 
> > > > > > > > > https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbie
s
> > > > 
> > > > >
> > > > 
> > > > 
> > > 
> > > 
> > > 
> > > 
> > > 
> > > _______________________________________________
> > > 
> > > Kernelnewbies mailing list
> > > 
> > > Kernelnewbies at kernelnewbies.org
> > > 
> > > https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> > > 
> > > 
> > 
> > 
> > 
> > -- 
> > Regards,
> > Prabhunath G
> > Linux Trainer
> > Bangalore
> > 
> > 
> > 
> > 
> > 
> > 
> > _______________________________________________
> > 
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> > 
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