Need help understanding memory models, cpu modes and address translation

Need help understanding memory models, cpu modes and address translation

[Vaibhav Jain]

Hi,

I am eager to understand the basics of Memory models  (flat, segmented etc)
, CPU modes (real,protected)
and address translation (physical to logical etc.) and how all of them work
together. I am very confused
about this and would really appreciate if someone could provide good
references to these topics.

Thanks
Vaibhav Jain
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Need help understanding memory models, cpu modes and address translation

[Daniel Baluta]

Hi,

> I am eager to understand the basics of Memory models? (flat, segmented etc)
> , CPU modes (real,protected)
> and address translation (physical to logical etc.) and how all of them work
> together. I am very confused
> about this and would really appreciate if someone could provide good
> references to these topics.

You may find useful information inside i366 Programmers Manual.
Anyhow, reading materials is the first step in understanding these
concepts. You will have to actually  read/write/debug pieces of code
related to them.

thanks,
Daniel.

[1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf

Need help understanding memory models, cpu modes and address translation

[Vaibhav Jain]

Hi,

Thanks for the link.I really appreciate but I need something more basic and
something that
explains these concepts from a broader perspective and not in the context of
a
particular cpu architecture.Please send me more such links if you come
across any.

Thanks
Vaibhav Jain

On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>wrote:

> Hi,
>
> > I am eager to understand the basics of Memory models  (flat, segmented
> etc)
> > , CPU modes (real,protected)
> > and address translation (physical to logical etc.) and how all of them
> work
> > together. I am very confused
> > about this and would really appreciate if someone could provide good
> > references to these topics.
>
> You may find useful information inside i366 Programmers Manual.
> Anyhow, reading materials is the first step in understanding these
> concepts. You will have to actually  read/write/debug pieces of code
> related to them.
>
> thanks,
> Daniel.
>
> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf
>
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Need help understanding memory models, cpu modes and address translation

[amit mehta]

Tons of resource
http://wiki.osdev.org/

On Wed, Jul 13, 2011 at 2:07 PM, Vaibhav Jain <vjoss197@gmail.com> wrote:
> Hi,
>
> Thanks for the link.I really appreciate but I need something more basic and
> something that
> explains these concepts from a broader perspective and not in the context of
> a
> particular cpu architecture.Please send me more such links if you come
> across any.
>
> Thanks
> Vaibhav Jain
>
> On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>
> wrote:
>>
>> Hi,
>>
>> > I am eager to understand the basics of Memory models? (flat, segmented
>> > etc)
>> > , CPU modes (real,protected)
>> > and address translation (physical to logical etc.) and how all of them
>> > work
>> > together. I am very confused
>> > about this and would really appreciate if someone could provide good
>> > references to these topics.
>>
>> You may find useful information inside i366 Programmers Manual.
>> Anyhow, reading materials is the first step in understanding these
>> concepts. You will have to actually ?read/write/debug pieces of code
>> related to them.
>>
>> thanks,
>> Daniel.
>>
>> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf
>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>



-- 
Life is elsewhere. Cross Frontiers. Fly away.

- Salman Rushdie

Need help understanding memory models, cpu modes and address translation

[Paraneetharan Chandrasekaran]

The document and osdev are very good and useful. Thanks a lot.

- Paraneetharan C

On 13 July 2011 14:23, amit mehta <gmate.amit@gmail.com> wrote:

> Tons of resource
> http://wiki.osdev.org/
>
> On Wed, Jul 13, 2011 at 2:07 PM, Vaibhav Jain <vjoss197@gmail.com> wrote:
> > Hi,
> >
> > Thanks for the link.I really appreciate but I need something more basic
> and
> > something that
> > explains these concepts from a broader perspective and not in the context
> of
> > a
> > particular cpu architecture.Please send me more such links if you come
> > across any.
> >
> > Thanks
> > Vaibhav Jain
> >
> > On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>
> > wrote:
> >>
> >> Hi,
> >>
> >> > I am eager to understand the basics of Memory models  (flat, segmented
> >> > etc)
> >> > , CPU modes (real,protected)
> >> > and address translation (physical to logical etc.) and how all of them
> >> > work
> >> > together. I am very confused
> >> > about this and would really appreciate if someone could provide good
> >> > references to these topics.
> >>
> >> You may find useful information inside i366 Programmers Manual.
> >> Anyhow, reading materials is the first step in understanding these
> >> concepts. You will have to actually  read/write/debug pieces of code
> >> related to them.
> >>
> >> thanks,
> >> Daniel.
> >>
> >> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf
> >
> >
> > _______________________________________________
> > Kernelnewbies mailing list
> > Kernelnewbies at kernelnewbies.org
> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> >
> >
>
>
>
> --
> Life is elsewhere. Cross Frontiers. Fly away.
>
> - Salman Rushdie
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
Regards,
Paraneetharan C
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Need help understanding memory models, cpu modes and address translation

[Jeff Haran]

 

 

From: kernelnewbies-bounces@kernelnewbies.org
[mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Vaibhav
Jain
Sent: Wednesday, July 13, 2011 1:38 AM
To: Daniel Baluta
Cc: kernelnewbies at kernelnewbies.org
Subject: Re: Need help understanding memory models,cpu modes and address
translation

 

Hi,

Thanks for the link.I really appreciate but I need something more basic
and something that 
explains these concepts from a broader perspective and not in the
context of a 
particular cpu architecture.Please send me more such links if you come
across any.

Thanks
Vaibhav Jain

On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>
wrote:

Hi,

> I am eager to understand the basics of Memory models  (flat, segmented
etc)
> , CPU modes (real,protected)
> and address translation (physical to logical etc.) and how all of them
work
> together. I am very confused
> about this and would really appreciate if someone could provide good
> references to these topics.

You may find useful information inside i366 Programmers Manual.
Anyhow, reading materials is the first step in understanding these
concepts. You will have to actually  read/write/debug pieces of code
related to them.

thanks,
Daniel.

[1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf

 

You might want to try "Understanding the Linux Virtual Memory Manager"
by Mel Gorman. It's freely available on the web.

 

It's Linux specific, but since Linux needs to run on a multitude of
different processors with different memory management hardware, the
approach it takes to doing so in inherently broad.

 

 


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Need help understanding memory models, cpu modes and address translation

[Anuz Pratap Singh Tomar]

On Wed, Jul 13, 2011 at 6:29 PM, Jeff Haran <jharan@bytemobile.com> wrote:

> ** **
>
> ** **
>
> *From:* kernelnewbies-bounces at kernelnewbies.org [mailto:
> kernelnewbies-bounces at kernelnewbies.org] *On Behalf Of *Vaibhav Jain
> *Sent:* Wednesday, July 13, 2011 1:38 AM
> *To:* Daniel Baluta
> *Cc:* kernelnewbies at kernelnewbies.org
> *Subject:* Re: Need help understanding memory models,cpu modes and address
> translation****
>
> ** **
>
> Hi,
>
> Thanks for the link.I really appreciate but I need something more basic and
> something that
> explains these concepts from a broader perspective and not in the context
> of a
> particular cpu architecture.Please send me more such links if you come
> across any.
>
> Thanks
> Vaibhav Jain****
>
> On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>
> wrote:****
>
> Hi,
>
> > I am eager to understand the basics of Memory models  (flat, segmented
> etc)
> > , CPU modes (real,protected)
> > and address translation (physical to logical etc.) and how all of them
> work
> > together. I am very confused
> > about this and would really appreciate if someone could provide good
> > references to these topics.****
>
> You may find useful information inside i366 Programmers Manual.
> Anyhow, reading materials is the first step in understanding these
> concepts. You will have to actually  read/write/debug pieces of code
> related to them.
>
> thanks,
> Daniel.
>
> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf****
>
> ** **
>
> You might want to try ?Understanding the Linux Virtual Memory Manager? by
> Mel Gorman. It?s freely available on the web.****
>
> ** **
>
> It?s Linux specific, but since Linux needs to run on a multitude of
> different processors with different memory management hardware, the approach
> it takes to doing so in inherently broad.****
>
> ** **
>
>
>
>
> A lot of basic concepts can be learnt from JIm Turley's book on 8086
Other book is modern operating systems by Tanenbaum which explains
everything in much detail with  code snippets.
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Need help understanding memory models, cpu modes and address translation

[Vaibhav Jain]

Hi,

Thanks a lot for the resources! I just had a cursory look at them and could
see that they explain virtual memory and
address translation in depth. Just wanted to ask if they explain the
concepts Memory models (flat, segmented ) and cpu modes
(real,protected) equally well. I am more confused about
these concepts.

Thanks
Vaibhav Jain

On Wed, Jul 13, 2011 at 10:40 AM, Anuz Pratap Singh Tomar <
chambilkethakur@gmail.com> wrote:

>
>
>   On Wed, Jul 13, 2011 at 6:29 PM, Jeff Haran <jharan@bytemobile.com>wrote:
>
>>  ** **
>>
>> ** **
>>
>> *From:* kernelnewbies-bounces at kernelnewbies.org [mailto:
>> kernelnewbies-bounces at kernelnewbies.org] *On Behalf Of *Vaibhav Jain
>> *Sent:* Wednesday, July 13, 2011 1:38 AM
>> *To:* Daniel Baluta
>> *Cc:* kernelnewbies at kernelnewbies.org
>> *Subject:* Re: Need help understanding memory models,cpu modes and
>> address translation****
>>
>> ** **
>>
>> Hi,
>>
>> Thanks for the link.I really appreciate but I need something more basic
>> and something that
>> explains these concepts from a broader perspective and not in the context
>> of a
>> particular cpu architecture.Please send me more such links if you come
>> across any.
>>
>> Thanks
>> Vaibhav Jain****
>>
>> On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>
>> wrote:****
>>
>> Hi,
>>
>> > I am eager to understand the basics of Memory models  (flat, segmented
>> etc)
>> > , CPU modes (real,protected)
>> > and address translation (physical to logical etc.) and how all of them
>> work
>> > together. I am very confused
>> > about this and would really appreciate if someone could provide good
>> > references to these topics.****
>>
>> You may find useful information inside i366 Programmers Manual.
>> Anyhow, reading materials is the first step in understanding these
>> concepts. You will have to actually  read/write/debug pieces of code
>> related to them.
>>
>> thanks,
>> Daniel.
>>
>> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf****
>>
>> ** **
>>
>> You might want to try ?Understanding the Linux Virtual Memory Manager? by
>> Mel Gorman. It?s freely available on the web.****
>>
>> ** **
>>
>> It?s Linux specific, but since Linux needs to run on a multitude of
>> different processors with different memory management hardware, the approach
>> it takes to doing so in inherently broad.****
>>
>> ** **
>>
>>
>>
>>
>> A lot of basic concepts can be learnt from JIm Turley's book on 8086
> Other book is modern operating systems by Tanenbaum which explains
> everything in much detail with  code snippets.
>
>
>
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Need help understanding memory models, cpu modes and address translation

[Mulyadi Santosa]

On Wed, Jul 13, 2011 at 14:33, Vaibhav Jain <vjoss197@gmail.com> wrote:
> Hi,
>
> I am eager to understand the basics of Memory models? (flat, segmented etc)
> , CPU modes (real,protected)
> and address translation (physical to logical etc.) and how all of them work
> together.

maybe something like this?

http://tldp.org/HOWTO/KernelAnalysis-HOWTO.html

that's outdated, but many rules are stay the same.

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

Need help understanding memory models, cpu modes and address translation

[Joel A Fernandes]

On Wed, Jul 13, 2011 at 9:41 PM, Mulyadi Santosa
<mulyadi.santosa@gmail.com> wrote:
> On Wed, Jul 13, 2011 at 14:33, Vaibhav Jain <vjoss197@gmail.com> wrote:
>> Hi,
>>
>> I am eager to understand the basics of Memory models? (flat, segmented etc)
>> , CPU modes (real,protected)
>> and address translation (physical to logical etc.) and how all of them work
>> together.
>
> maybe something like this?
>
> http://tldp.org/HOWTO/KernelAnalysis-HOWTO.html

This should nail it :)

http://duartes.org/gustavo/blog/post/how-the-kernel-manages-your-memory
http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory

Thanks,
Joel

Need help understanding memory models, cpu modes and address translation

[Kaustubh Ashtekar]

I think you should first read the book
Operating System Concepts by  Silberschatz Galvin.

It is a general book on OS concepts and will help you gain understanding of
these concepts.

-Kaustubh

On Thu, Jul 14, 2011 at 1:17 AM, Vaibhav Jain <vjoss197@gmail.com> wrote:

> Hi,
>
> Thanks a lot for the resources! I just had a cursory look at them and could
> see that they explain virtual memory and
> address translation in depth. Just wanted to ask if they explain the
> concepts Memory models (flat, segmented ) and cpu modes
> (real,protected) equally well. I am more confused about
> these concepts.
>
> Thanks
> Vaibhav Jain
>
> On Wed, Jul 13, 2011 at 10:40 AM, Anuz Pratap Singh Tomar <
> chambilkethakur at gmail.com> wrote:
>
>>
>>
>>   On Wed, Jul 13, 2011 at 6:29 PM, Jeff Haran <jharan@bytemobile.com>wrote:
>>
>>>  ** **
>>>
>>> ** **
>>>
>>> *From:* kernelnewbies-bounces at kernelnewbies.org [mailto:
>>> kernelnewbies-bounces at kernelnewbies.org] *On Behalf Of *Vaibhav Jain
>>> *Sent:* Wednesday, July 13, 2011 1:38 AM
>>> *To:* Daniel Baluta
>>> *Cc:* kernelnewbies at kernelnewbies.org
>>> *Subject:* Re: Need help understanding memory models,cpu modes and
>>> address translation****
>>>
>>> ** **
>>>
>>> Hi,
>>>
>>> Thanks for the link.I really appreciate but I need something more basic
>>> and something that
>>> explains these concepts from a broader perspective and not in the context
>>> of a
>>> particular cpu architecture.Please send me more such links if you come
>>> across any.
>>>
>>> Thanks
>>> Vaibhav Jain****
>>>
>>> On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>
>>> wrote:****
>>>
>>> Hi,
>>>
>>> > I am eager to understand the basics of Memory models  (flat, segmented
>>> etc)
>>> > , CPU modes (real,protected)
>>> > and address translation (physical to logical etc.) and how all of them
>>> work
>>> > together. I am very confused
>>> > about this and would really appreciate if someone could provide good
>>> > references to these topics.****
>>>
>>> You may find useful information inside i366 Programmers Manual.
>>> Anyhow, reading materials is the first step in understanding these
>>> concepts. You will have to actually  read/write/debug pieces of code
>>> related to them.
>>>
>>> thanks,
>>> Daniel.
>>>
>>> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf****
>>>
>>> ** **
>>>
>>> You might want to try ?Understanding the Linux Virtual Memory Manager? by
>>> Mel Gorman. It?s freely available on the web.****
>>>
>>> ** **
>>>
>>> It?s Linux specific, but since Linux needs to run on a multitude of
>>> different processors with different memory management hardware, the approach
>>> it takes to doing so in inherently broad.****
>>>
>>> ** **
>>>
>>>
>>>
>>>
>>> A lot of basic concepts can be learnt from JIm Turley's book on 8086
>> Other book is modern operating systems by Tanenbaum which explains
>> everything in much detail with  code snippets.
>>
>>
>>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>
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Need help understanding memory models, cpu modes and address translation

[Vaibhav Jain]

Hi,

Actually I have read that book. But when i started reading other books such
as those on assembly
they had these concpets of Real Mode , Protected Mode , Flat Memory model,
Segmented Memory model which are specific to intel 32-bit architecture
and which has got me highly confused. So I am looking for references that
explain these concepts in depth.

Thanks
Vaibhav Jain


On Thu, Jul 14, 2011 at 9:26 PM, Kaustubh Ashtekar <ksashtekar@gmail.com>wrote:

> I think you should first read the book
> Operating System Concepts by  Silberschatz Galvin.
>
> It is a general book on OS concepts and will help you gain understanding of
> these concepts.
>
> -Kaustubh
>
> On Thu, Jul 14, 2011 at 1:17 AM, Vaibhav Jain <vjoss197@gmail.com> wrote:
>
>> Hi,
>>
>> Thanks a lot for the resources! I just had a cursory look at them and
>> could see that they explain virtual memory and
>> address translation in depth. Just wanted to ask if they explain the
>> concepts Memory models (flat, segmented ) and cpu modes
>> (real,protected) equally well. I am more confused about
>> these concepts.
>>
>> Thanks
>> Vaibhav Jain
>>
>> On Wed, Jul 13, 2011 at 10:40 AM, Anuz Pratap Singh Tomar <
>> chambilkethakur at gmail.com> wrote:
>>
>>>
>>>
>>>   On Wed, Jul 13, 2011 at 6:29 PM, Jeff Haran <jharan@bytemobile.com>wrote:
>>>
>>>>  ** **
>>>>
>>>> ** **
>>>>
>>>> *From:* kernelnewbies-bounces at kernelnewbies.org [mailto:
>>>> kernelnewbies-bounces at kernelnewbies.org] *On Behalf Of *Vaibhav Jain
>>>> *Sent:* Wednesday, July 13, 2011 1:38 AM
>>>> *To:* Daniel Baluta
>>>> *Cc:* kernelnewbies at kernelnewbies.org
>>>> *Subject:* Re: Need help understanding memory models,cpu modes and
>>>> address translation****
>>>>
>>>> ** **
>>>>
>>>> Hi,
>>>>
>>>> Thanks for the link.I really appreciate but I need something more basic
>>>> and something that
>>>> explains these concepts from a broader perspective and not in the
>>>> context of a
>>>> particular cpu architecture.Please send me more such links if you come
>>>> across any.
>>>>
>>>> Thanks
>>>> Vaibhav Jain****
>>>>
>>>> On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com>
>>>> wrote:****
>>>>
>>>> Hi,
>>>>
>>>> > I am eager to understand the basics of Memory models  (flat, segmented
>>>> etc)
>>>> > , CPU modes (real,protected)
>>>> > and address translation (physical to logical etc.) and how all of them
>>>> work
>>>> > together. I am very confused
>>>> > about this and would really appreciate if someone could provide good
>>>> > references to these topics.****
>>>>
>>>> You may find useful information inside i366 Programmers Manual.
>>>> Anyhow, reading materials is the first step in understanding these
>>>> concepts. You will have to actually  read/write/debug pieces of code
>>>> related to them.
>>>>
>>>> thanks,
>>>> Daniel.
>>>>
>>>> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf****
>>>>
>>>> ** **
>>>>
>>>> You might want to try ?Understanding the Linux Virtual Memory Manager?
>>>> by Mel Gorman. It?s freely available on the web.****
>>>>
>>>> ** **
>>>>
>>>> It?s Linux specific, but since Linux needs to run on a multitude of
>>>> different processors with different memory management hardware, the approach
>>>> it takes to doing so in inherently broad.****
>>>>
>>>> ** **
>>>>
>>>>
>>>>
>>>>
>>>> A lot of basic concepts can be learnt from JIm Turley's book on 8086
>>> Other book is modern operating systems by Tanenbaum which explains
>>> everything in much detail with  code snippets.
>>>
>>>
>>>
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>>
>
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Need help understanding memory models, cpu modes and address translation

[Vaibhav Jain]

Hi,
Thanks a lot for all the links!
I read the two articles below by Gustavo but I am still very confused about
segmentation. One of the articles below says that
the segments in virtual address space are different from 'intel-style
segments' whereas I used to think of them as being the same.
Could somebody please state the difference clearly for me and explain how
these two work together. I would
really appreciate if someone could explain the whole chain from generation
of addresses by compiler and then translation of
those addresses in case of  Segmentation working along with Paging. I am
unable to comprehend how a compiler
generates address for a segmented memory model. Explaining the whole chain,
I think , can serve the purpose.

Thanks!
Vaibhav Jain


On Wed, Jul 13, 2011 at 7:46 PM, Joel A Fernandes <agnel.joel@gmail.com>wrote:

> On Wed, Jul 13, 2011 at 9:41 PM, Mulyadi Santosa
> <mulyadi.santosa@gmail.com> wrote:
> > On Wed, Jul 13, 2011 at 14:33, Vaibhav Jain <vjoss197@gmail.com> wrote:
> >> Hi,
> >>
> >> I am eager to understand the basics of Memory models  (flat, segmented
> etc)
> >> , CPU modes (real,protected)
> >> and address translation (physical to logical etc.) and how all of them
> work
> >> together.
> >
> > maybe something like this?
> >
> > http://tldp.org/HOWTO/KernelAnalysis-HOWTO.html
>
> This should nail it :)
>
> http://duartes.org/gustavo/blog/post/how-the-kernel-manages-your-memory
> http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory
>
> Thanks,
> Joel
>
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Need help understanding memory models, cpu modes and address translation

[Mulyadi Santosa]

Hello Vaibhav...

What you asked, is something really difficult to summarize in
short...but let me try...

On Sat, Jul 16, 2011 at 10:14, Vaibhav Jain <vjoss197@gmail.com> wrote:
> Hi,
> Thanks a lot for all the links!
> I read the two articles below by Gustavo but I am still very confused about
> segmentation. One of the articles below says that
> the segments in virtual address space are different from 'intel-style
> segments' whereas I used to think of them as being the same.

I am not really sure what segments in virtual address space really
means. What we have in virtual address space is AFAIK kernel space and
user space.

User space, is a space where OS put user and data from your casual
programs (e.g your games, your word processors etc). Assuming it's in
x86 32 bit, the range in virtual address is between address 0 and
slightly under 3 GiB.

Whereas kernel space, is a space where OS put its own data and code
(i.e interrupt handler, system call handler, page tables). Again in
x86 32 bit, it lies between 3 GiB up to 4 GiB.

> Could somebody please state the difference clearly for me and explain how
> these two work together. I would
> really appreciate if someone could explain the whole chain from generation
> of addresses by compiler and then translation of
> those addresses in case of? Segmentation working along with Paging.

when you generate object code from your source code (let's say in C)
using gcc, first your code and variables (data) are turn into Position
Independent Code. It means, it is just an offset. If there is an
offset, surely we need base address, right? But not at this object
(resulting in .o) stage.

Then in reach producing final ELF binary (executable). Using known
standart ELF rule, those offset are turn into final final address. So
let's, code are placed starting at 0x080499f0 and so on.

When that binary is loaded into memory, loader (ld.so) take that
information and use it as a clue on where to put the code and data.
Using standart mmap() syscall, memory area is reserved and data/code
is loaded there. The exception is stack, where it is allocated
dynamically (and grows down, for Intel arch) starting at the upper
limit of user space (near 3 GiB).

The new feature, called PIE (position independent code) allow loader
to do further address reorganization. So instead of let's say taking
0x080499f0 for start of code segment for granted, it can relocate it
to another address (but still in user space). However, in order to do
that, every addresing done by the code must be compiled differently,
so instead using constant address, now it uses combination of base
register and offset AFAIK.

Code runs, data addressed, they are all under virtual address. But
memory is pointed using real address. Here comes the work of MMU
(Memory Management Unit). With the help of tables of address
translation called page tables, virtual address is translated into
real address. The detail is left for you for further research. But
enough to say it works like lookup table, e.g "12" in virtual address
could means real address 10012 and so on.

Please notice that continous address in virtual address doesn't
neccessarily means continous address in real address e.g 12-20 in
virtual address could means 10012 up to 10016 then 40017 up to 40020.


Getting clearer now? :D

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

Need help understanding memory models, cpu modes and address translation

[Joel A Fernandes]

Hi Mulyadi,

Good to read your posts. It has certainly been a long time and it
feels good to be back on this list!

> On Sat, Jul 16, 2011 at 10:14, Vaibhav Jain <vjoss197@gmail.com> wrote:
....
>
>> Could somebody please state the difference clearly for me and explain how
>> these two work together. I would
>> really appreciate if someone could explain the whole chain from generation
>> of addresses by compiler and then translation of
>> those addresses in case of? Segmentation working along with Paging.
>
> when you generate object code from your source code (let's say in C)
> using gcc, first your code and variables (data) are turn into Position
> Independent Code. It means, it is just an offset. If there is an
> offset, surely we need base address, right? But not at this object
> (resulting in .o) stage.
>
> Then in reach producing final ELF binary (executable). Using known
> standart ELF rule, those offset are turn into final final address. So
> let's, code are placed starting at 0x080499f0 and so on.

Very nice explanation!

>
> When that binary is loaded into memory, loader (ld.so) take that
> information and use it as a clue on where to put the code and data.
> Using standart mmap() syscall, memory area is reserved and data/code
> is loaded there. The exception is stack, where it is allocated
> dynamically (and grows down, for Intel arch) starting at the upper
> limit of user space (near 3 GiB).
>

I'm just a little troubled by this bit.

AFAIK, the kernel takes ELF executables and loads them into the
appropriate sections after parsing the ELF tables and headers. Correct
me if I'm wrong? Ofcourse, ld.so takes care of dynamic linking/loading
shared libraries into the address space.

Thanks,
Joel

Need help understanding memory models, cpu modes and address translation

[Joel A Fernandes]

On Fri, Jul 15, 2011 at 10:04 PM, Vaibhav Jain <vjoss197@gmail.com> wrote:
> Hi,
>
> Actually I have read that book. But when i started reading other books such
> as those on assembly
> they had these concpets of Real Mode , Protected Mode , Flat Memory model,
> Segmented Memory model which are specific to intel 32-bit architecture
> and which has got me highly confused. So I am looking for references that
> explain these concepts in depth.
>

Then you should read the book "Understanding The Linux Kernel", and
Intel's reference manuals which document these features.

Also this is the book we read in Undergrad for x86 internals :) It was
considered to be the bible for the topic (atleast at the time)
http://www.amazon.com/Intel-Microprocessors-80186-80286-80386/dp/0132606704/ref=sr_1_1?s=books&ie=UTF8&qid=1310792692&sr=1-1

Regards,
Joel

Need help understanding memory models, cpu modes and address translation

[Kaustubh Ashtekar]

On Sat, Jul 16, 2011 at 8:34 AM, Vaibhav Jain <vjoss197@gmail.com> wrote:
>
> Hi,
>
> Actually I have read that book. But when i started reading other books such as those on assembly
> they had these concpets of Real Mode , Protected Mode , Flat Memory model, Segmented Memory model which are specific to intel 32-bit architecture
> and which has got me highly confused. So I am looking for references that explain these concepts in depth.


What I suggest is. You should download a datasheet for Intel 8086,
designed by Intel in late 1970's.  Read espacially the memory
organization part. This will give you some basic understanding of read
mode and segmentation stuff and how it all started. The current
processor manuals are too complex to start with.

Link:
http://datasheets.chipdb.org/Intel/x86/808x/datashts/8086/




>
> Thanks
> Vaibhav Jain
>
>
> On Thu, Jul 14, 2011 at 9:26 PM, Kaustubh Ashtekar <ksashtekar@gmail.com> wrote:
>>
>> I think you should first read the book
>> Operating System Concepts by ?Silberschatz Galvin.
>> It is a general book on OS concepts and will help you gain understanding of these concepts.
>> -Kaustubh
>> On Thu, Jul 14, 2011 at 1:17 AM, Vaibhav Jain <vjoss197@gmail.com> wrote:
>>>
>>> Hi,
>>>
>>> Thanks a lot for the resources! I just had a cursory look at them and could see that they explain virtual memory and
>>> address translation in depth. Just wanted to ask if they explain the concepts?Memory models (flat, segmented )?and cpu modes (real,protected)?equally well. I am more confused about
>>> these concepts.
>>>
>>> Thanks
>>> Vaibhav Jain
>>>
>>> On Wed, Jul 13, 2011 at 10:40 AM, Anuz Pratap Singh Tomar <chambilkethakur@gmail.com> wrote:
>>>>
>>>>
>>>> On Wed, Jul 13, 2011 at 6:29 PM, Jeff Haran <jharan@bytemobile.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> From: kernelnewbies-bounces at kernelnewbies.org [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Vaibhav Jain
>>>>> Sent: Wednesday, July 13, 2011 1:38 AM
>>>>> To: Daniel Baluta
>>>>> Cc: kernelnewbies at kernelnewbies.org
>>>>> Subject: Re: Need help understanding memory models,cpu modes and address translation
>>>>>
>>>>>
>>>>>
>>>>> Hi,
>>>>>
>>>>> Thanks for the link.I really appreciate but I need something more basic and something that
>>>>> explains these concepts from a broader perspective and not in the context of a
>>>>> particular cpu architecture.Please send me more such links if you come across any.
>>>>>
>>>>> Thanks
>>>>> Vaibhav Jain
>>>>>
>>>>> On Wed, Jul 13, 2011 at 1:14 AM, Daniel Baluta <daniel.baluta@gmail.com> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> > I am eager to understand the basics of Memory models? (flat, segmented etc)
>>>>> > , CPU modes (real,protected)
>>>>> > and address translation (physical to logical etc.) and how all of them work
>>>>> > together. I am very confused
>>>>> > about this and would really appreciate if someone could provide good
>>>>> > references to these topics.
>>>>>
>>>>> You may find useful information inside i366 Programmers Manual.
>>>>> Anyhow, reading materials is the first step in understanding these
>>>>> concepts. You will have to actually ?read/write/debug pieces of code
>>>>> related to them.
>>>>>
>>>>> thanks,
>>>>> Daniel.
>>>>>
>>>>> [1] pdos.csail.mit.edu/6.828/2010/readings/i386.pdf
>>>>>
>>>>>
>>>>>
>>>>> You might want to try ?Understanding the Linux Virtual Memory Manager? by Mel Gorman. It?s freely available on the web.
>>>>>
>>>>>
>>>>>
>>>>> It?s Linux specific, but since Linux needs to run on a multitude of different processors with different memory management hardware, the approach it takes to doing so in inherently broad.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>> A lot of basic concepts can be learnt from JIm Turley's book on 8086
>>>> Other book is modern operating systems by Tanenbaum which explains everything in much detail with? code snippets.
>>>>
>>>>
>>>
>>>
>>> _______________________________________________
>>> Kernelnewbies mailing list
>>> Kernelnewbies at kernelnewbies.org
>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>>
>>
>

Need help understanding memory models, cpu modes and address translation

[Joel A Fernandes]

On Sat, Jul 16, 2011 at 12:06 AM, Joel A Fernandes <agnel.joel@gmail.com> wrote:
> On Fri, Jul 15, 2011 at 10:04 PM, Vaibhav Jain <vjoss197@gmail.com> wrote:
>> Hi,
>>
>> Actually I have read that book. But when i started reading other books such
>> as those on assembly
>> they had these concpets of Real Mode , Protected Mode , Flat Memory model,
>> Segmented Memory model which are specific to intel 32-bit architecture
>> and which has got me highly confused. So I am looking for references that
>> explain these concepts in depth.
>>
>
> Then you should read the book "Understanding The Linux Kernel", and
> Intel's reference manuals which document these features.
>
> Also this is the book we read in Undergrad for x86 internals :) It was
> considered to be the bible for the topic (atleast at the time)
> http://www.amazon.com/Intel-Microprocessors-80186-80286-80386/dp/0132606704/ref=sr_1_1?s=books&ie=UTF8&qid=1310792692&sr=1-1
>

I'd like to also add that the above mentioned book covers a lot of
fundamentals and is a good starting point.

Thanks
Joel

Need help understanding memory models, cpu modes and address translation

[santhosh kumars]

On Sat, Jul 16, 2011 at 9:45 AM, Mulyadi Santosa
<mulyadi.santosa@gmail.com> wrote:
> Hello Vaibhav...
>
> What you asked, is something really difficult to summarize in
> short...but let me try...
>
> On Sat, Jul 16, 2011 at 10:14, Vaibhav Jain <vjoss197@gmail.com> wrote:
>> Hi,
>> Thanks a lot for all the links!
>> I read the two articles below by Gustavo but I am still very confused about
>> segmentation. One of the articles below says that
>> the segments in virtual address space are different from 'intel-style
>> segments' whereas I used to think of them as being the same.
>
> I am not really sure what segments in virtual address space really
> means. What we have in virtual address space is AFAIK kernel space and
> user space.
>
I think in linux all
segments(__USER_CS,__USER_DS,__KERNEL_CS,__KERNEL_DS) starts at
0x00000000.So offset fileld of logical address always coincide with
linear address.So in case of Linux, segmentation is some dumb kind of
thing.can
any one correct me if i am wrong.
> User space, is a space where OS put user and data from your casual
> programs (e.g your games, your word processors etc). Assuming it's in
> x86 32 bit, the range in virtual address is between address 0 and
> slightly under 3 GiB.
>
> Whereas kernel space, is a space where OS put its own data and code
> (i.e interrupt handler, system call handler, page tables). Again in
> x86 32 bit, it lies between 3 GiB up to 4 GiB.
>
>> Could somebody please state the difference clearly for me and explain how
>> these two work together. I would
>> really appreciate if someone could explain the whole chain from generation
>> of addresses by compiler and then translation of
>> those addresses in case of? Segmentation working along with Paging.
>
> when you generate object code from your source code (let's say in C)
> using gcc, first your code and variables (data) are turn into Position
> Independent Code. It means, it is just an offset. If there is an
> offset, surely we need base address, right? But not at this object
> (resulting in .o) stage.
>
> Then in reach producing final ELF binary (executable). Using known
> standart ELF rule, those offset are turn into final final address. So
> let's, code are placed starting at 0x080499f0 and so on.
>
> When that binary is loaded into memory, loader (ld.so) take that
> information and use it as a clue on where to put the code and data.
> Using standart mmap() syscall, memory area is reserved and data/code
> is loaded there. The exception is stack, where it is allocated
> dynamically (and grows down, for Intel arch) starting at the upper
> limit of user space (near 3 GiB).
>
> The new feature, called PIE (position independent code) allow loader
> to do further address reorganization. So instead of let's say taking
> 0x080499f0 for start of code segment for granted, it can relocate it
> to another address (but still in user space). However, in order to do
> that, every addresing done by the code must be compiled differently,
> so instead using constant address, now it uses combination of base
> register and offset AFAIK.
>
> Code runs, data addressed, they are all under virtual address. But
> memory is pointed using real address. Here comes the work of MMU
> (Memory Management Unit). With the help of tables of address
> translation called page tables, virtual address is translated into
> real address. The detail is left for you for further research. But
> enough to say it works like lookup table, e.g "12" in virtual address
> could means real address 10012 and so on.
>
> Please notice that continous address in virtual address doesn't
> neccessarily means continous address in real address e.g 12-20 in
> virtual address could means 10012 up to 10016 then 40017 up to 40020.
>
>
> Getting clearer now? :D
>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>

Need help understanding memory models, cpu modes and address translation

[Rajesh S R]

Hi Mulyadi,
Thanks for a nice explanation. A few nitpicks. Correct me if am wrong.

On Sat, Jul 16, 2011 at 9:45 AM, Mulyadi Santosa
<mulyadi.santosa@gmail.com>wrote:

> Hello Vaibhav...
>
> What you asked, is something really difficult to summarize in
> short...but let me try...
>
> On Sat, Jul 16, 2011 at 10:14, Vaibhav Jain <vjoss197@gmail.com> wrote:
> > Hi,
> > Thanks a lot for all the links!
> > I read the two articles below by Gustavo but I am still very confused
> about
> > segmentation. One of the articles below says that
> > the segments in virtual address space are different from 'intel-style
> > segments' whereas I used to think of them as being the same.
>
> I am not really sure what segments in virtual address space really
> means. What we have in virtual address space is AFAIK kernel space and
> user space.
>
> User space, is a space where OS put user and data from your casual
> programs (e.g your games, your word processors etc). Assuming it's in
> x86 32 bit, the range in virtual address is between address 0 and
> slightly under 3 GiB.
>
> Whereas kernel space, is a space where OS put its own data and code
> (i.e interrupt handler, system call handler, page tables). Again in
> x86 32 bit, it lies between 3 GiB up to 4 GiB.
>
> > Could somebody please state the difference clearly for me and explain how
> > these two work together. I would
> > really appreciate if someone could explain the whole chain from
> generation
> > of addresses by compiler and then translation of
> > those addresses in case of  Segmentation working along with Paging.
>
> when you generate object code from your source code (let's say in C)
> using gcc, first your code and variables (data) are turn into Position
> Independent Code. It means, it is just an offset. If there is an
> offset, surely we need base address, right? But not at this object
> (resulting in .o) stage.
>

PIC is used only for shared libraries (.so files). .o files have relocation
section specifically for that(of course also for undefined symbols linked
later). So, when a symbol's address is required, it wil be filled with 0x0
and a corresponding entry will be created in the Relocation Table. So, when
the final link editor links all .o files, it will readjust the addresses and
generate the final executable with a specific starting virtual address.


> Then in reach producing final ELF binary (executable). Using known
> standart ELF rule, those offset are turn into final final address. So
> let's, code are placed starting at 0x080499f0 and so on.
>
> When that binary is loaded into memory, loader (ld.so) take that
> information and use it as a clue on where to put the code and data.
> Using standart mmap() syscall, memory area is reserved and data/code
> is loaded there. The exception is stack, where it is allocated
> dynamically (and grows down, for Intel arch) starting at the upper
> limit of user space (near 3 GiB).
>
> The new feature, called PIE (position independent code) allow loader
> to do further address reorganization. So instead of let's say taking
> 0x080499f0 for start of code segment for granted, it can relocate it
> to another address (but still in user space). However, in order to do
> that, every addresing done by the code must be compiled differently,
> so instead using constant address, now it uses combination of base
> register and offset AFAIK.
>
> Code runs, data addressed, they are all under virtual address. But
> memory is pointed using real address. Here comes the work of MMU
> (Memory Management Unit). With the help of tables of address
> translation called page tables, virtual address is translated into
> real address. The detail is left for you for further research. But
> enough to say it works like lookup table, e.g "12" in virtual address
> could means real address 10012 and so on.
>
> Please notice that continous address in virtual address doesn't
> neccessarily means continous address in real address e.g 12-20 in
> virtual address could means 10012 up to 10016 then 40017 up to 40020.
>
>
> Getting clearer now? :D
>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
Rajesh S R
http://rajeshsr.co.cc/blogs/
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Need help understanding memory models, cpu modes and address translation

[Vaibhav Jain]

Hi Mulyadi,

Thanks for the explanation.Its really nice! But
what I was referring to the was this article on virtual address layout of
program :

http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory

which mentions : "The distinct bands in the address space correspond to *memory
segments* like the heap, stack, and so on. Keep in mind these segments are
simply a range of memory addresses and *have nothing to do* with Intel-style
segments<http://duartes.org/gustavo/blog/post/memory-translation-and-segmentation>
."

This gave rise to all the confusion.I used to think that code, stack and
heap segments in the virtual address layout of a program
are the same as the segments which we talk about when referring to hardware
provided Segmentation. But seems like this is not the case.
I also read a little about real and protected mode and came to know that in
32-bit protected mode all the Segment registers point to the same address
This confused me even more. So I need an explanation of how all these work
together. I am Sorry if the question is
not clear or if it sounds confusing.

Thanks
Vaibhav Jain



On Fri, Jul 15, 2011 at 9:15 PM, Mulyadi Santosa
<mulyadi.santosa@gmail.com>wrote:

> Hello Vaibhav...
>
> What you asked, is something really difficult to summarize in
> short...but let me try...
>
> On Sat, Jul 16, 2011 at 10:14, Vaibhav Jain <vjoss197@gmail.com> wrote:
> > Hi,
> > Thanks a lot for all the links!
> > I read the two articles below by Gustavo but I am still very confused
> about
> > segmentation. One of the articles below says that
> > the segments in virtual address space are different from 'intel-style
> > segments' whereas I used to think of them as being the same.
>
> I am not really sure what segments in virtual address space really
> means. What we have in virtual address space is AFAIK kernel space and
> user space.
>
> User space, is a space where OS put user and data from your casual
> programs (e.g your games, your word processors etc). Assuming it's in
> x86 32 bit, the range in virtual address is between address 0 and
> slightly under 3 GiB.
>
> Whereas kernel space, is a space where OS put its own data and code
> (i.e interrupt handler, system call handler, page tables). Again in
> x86 32 bit, it lies between 3 GiB up to 4 GiB.
>
> > Could somebody please state the difference clearly for me and explain how
> > these two work together. I would
> > really appreciate if someone could explain the whole chain from
> generation
> > of addresses by compiler and then translation of
> > those addresses in case of  Segmentation working along with Paging.
>
> when you generate object code from your source code (let's say in C)
> using gcc, first your code and variables (data) are turn into Position
> Independent Code. It means, it is just an offset. If there is an
> offset, surely we need base address, right? But not at this object
> (resulting in .o) stage.
>
> Then in reach producing final ELF binary (executable). Using known
> standart ELF rule, those offset are turn into final final address. So
> let's, code are placed starting at 0x080499f0 and so on.
>
> When that binary is loaded into memory, loader (ld.so) take that
> information and use it as a clue on where to put the code and data.
> Using standart mmap() syscall, memory area is reserved and data/code
> is loaded there. The exception is stack, where it is allocated
> dynamically (and grows down, for Intel arch) starting at the upper
> limit of user space (near 3 GiB).
>
> The new feature, called PIE (position independent code) allow loader
> to do further address reorganization. So instead of let's say taking
> 0x080499f0 for start of code segment for granted, it can relocate it
> to another address (but still in user space). However, in order to do
> that, every addresing done by the code must be compiled differently,
> so instead using constant address, now it uses combination of base
> register and offset AFAIK.
>
> Code runs, data addressed, they are all under virtual address. But
> memory is pointed using real address. Here comes the work of MMU
> (Memory Management Unit). With the help of tables of address
> translation called page tables, virtual address is translated into
> real address. The detail is left for you for further research. But
> enough to say it works like lookup table, e.g "12" in virtual address
> could means real address 10012 and so on.
>
> Please notice that continous address in virtual address doesn't
> neccessarily means continous address in real address e.g 12-20 in
> virtual address could means 10012 up to 10016 then 40017 up to 40020.
>
>
> Getting clearer now? :D
>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com
>
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Need help understanding memory models, cpu modes and address translation

[Joel A Fernandes]

On Sat, Jul 16, 2011 at 3:47 PM, Vaibhav Jain <vjoss197@gmail.com> wrote:
> Hi Mulyadi,
>
> Thanks for the explanation.Its really nice! But
> what I was referring to the was this article on virtual address layout of
> program :
>
> http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory
>
> which mentions : "The distinct bands in the address space correspond to
> memory segments like the heap, stack, and so on. Keep in mind these segments
> are simply a range of memory addresses and have nothing to do with
> Intel-style segments."
>
> This gave rise to all the confusion.I used to think that code, stack and
> heap segments in the virtual address layout of a program
> are the same as the segments which we talk about when referring to hardware
> provided Segmentation. But seems like this is not the case.

Segmentation and the virtual address layout are independent of each
other so you shouldn't confuse the two. You can choose to have
segmentation and then virtual addressing.

The way it works is:

logical address (segmented) -> virtual address -> physical address

The logical to virtual conversion is called segmentation, and virtual
to physical is called paging. The Software always uses logical
addresses.

The article explains "heap", "stack" etc are segments created by the
operating system and have nothing to do with traditional Intel-style
"segments". Infact the hardware would not be even aware of the
presence of the heap. The HW only knows virtual addresses.

> I also read a little about real and protected mode and came to know that in
> 32-bit protected mode all the Segment registers point to the same address

It happens so that Linux sets up the processor tables in such a way
that the logical addresses map to the same as virtual addresses. IIRC,
For x86 there's a "Global Descriptor table" that Linux manipulates to
produce this one-to-one mapping.

> This confused me even more. So I need an explanation of how all these work
> together. I am Sorry if the question is
> not clear or if it sounds confusing.

Hope this clears it, do read the introduction chapters of
Understanding the Linux Kernel which touches on a lot of these topics.
Feel free to ask more questions.

Thanks,
Joel

Need help understanding memory models, cpu modes and address translation

[Vaibhav Jain]

Thanks a lot Joel! This is a great explanation.
Just one more question. I used to think that the compiler always
assigns/generates starting from 0 as Mulyadi has also mentioned.
In the case when Segmentation(intel-style) is being used how does the
compiler assigns
addresses?

Thanks
Vaibhav Jain

On Sat, Jul 16, 2011 at 11:13 PM, Joel A Fernandes <agnel.joel@gmail.com>wrote:

> On Sat, Jul 16, 2011 at 3:47 PM, Vaibhav Jain <vjoss197@gmail.com> wrote:
> > Hi Mulyadi,
> >
> > Thanks for the explanation.Its really nice! But
> > what I was referring to the was this article on virtual address layout of
> > program :
> >
> > http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory
> >
> > which mentions : "The distinct bands in the address space correspond to
> > memory segments like the heap, stack, and so on. Keep in mind these
> segments
> > are simply a range of memory addresses and have nothing to do with
> > Intel-style segments."
> >
> > This gave rise to all the confusion.I used to think that code, stack and
> > heap segments in the virtual address layout of a program
> > are the same as the segments which we talk about when referring to
> hardware
> > provided Segmentation. But seems like this is not the case.
>
> Segmentation and the virtual address layout are independent of each
> other so you shouldn't confuse the two. You can choose to have
> segmentation and then virtual addressing.
>
> The way it works is:
>
> logical address (segmented) -> virtual address -> physical address
>
> The logical to virtual conversion is called segmentation, and virtual
> to physical is called paging. The Software always uses logical
> addresses.
>
> The article explains "heap", "stack" etc are segments created by the
> operating system and have nothing to do with traditional Intel-style
> "segments". Infact the hardware would not be even aware of the
> presence of the heap. The HW only knows virtual addresses.
>
> > I also read a little about real and protected mode and came to know that
> in
> > 32-bit protected mode all the Segment registers point to the same address
>
> It happens so that Linux sets up the processor tables in such a way
> that the logical addresses map to the same as virtual addresses. IIRC,
> For x86 there's a "Global Descriptor table" that Linux manipulates to
> produce this one-to-one mapping.
>
> > This confused me even more. So I need an explanation of how all these
> work
> > together. I am Sorry if the question is
> > not clear or if it sounds confusing.
>
> Hope this clears it, do read the introduction chapters of
> Understanding the Linux Kernel which touches on a lot of these topics.
> Feel free to ask more questions.
>
> Thanks,
> Joel
>
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Need help understanding memory models, cpu modes and address translation

[Joel A Fernandes]

On Sun, Jul 17, 2011 at 1:24 AM, Vaibhav Jain <vjoss197@gmail.com> wrote:
> Thanks a lot Joel! This is a great explanation.
> Just one more question. I used to think that the compiler always
> assigns/generates starting from 0 as Mulyadi has also mentioned.
> In the case when Segmentation(intel-style) is being used how does the
> compiler assigns
> addresses?
>

The compiler just generates code as if segmentation is not being
used.I'm not familiar with segmentation in x86 Protected mode. Further
because Linux doesn't uses segmentation (the logical addresses are
one-to-one mapped to virtual addresses) so neither have I really cared
about how it works ;) The compiler just generates code as if
segmentation is not being used. But I'd say some google searches on
"Global Descriptor Table" would give you some pointers.

You shouldn't worry about segmentation too much because virtual
addressing achieves everything it does and is more flexible. I'd say
ignore segmentation and focus on paging. :)

Thanks,
Joel