Re: 32 bit processors / 64 bit processors

[vkm <viral.vkm@gmail.com>]

hmthalib wrote:
> nidhi mittal hada wrote:
>> Can someone come and please clarify -- it finally
>> above chain of mail raises more confusion as conflict stays till end.
>>
>> 1)processor 32/64 bit --
>>     a)data it can process at one instance --- register size --- data 
>> bus size --
>> OR
>>     b)internal address bus size of processor ---  if that is true 
>> then what abt case of pentium 36 bit address lines ---
>>
>
> A processor is called 32 bit or 64 bit depending on the width of the 
> data bus. not on the register size or data it can process at one instance
>
> for example a 8 bit micro controller can have 16 bit registers. 
> similarly all modern processor has architecture to process more data 
> to increase the throughput.

What would you call pentium pro.... Its address bus is 36 bits wide, and 
the external data bus is 64 bits wide
>
>
>>
>> 2)OS 32 /64 bit
>>     a)its virtual address space -- what os provides --- to each 
>> process -- which is mapped to physical address space that prcessor 
>> provides
>>                                      OR
>>      b)processor specifies virtual addr space ??
>>
>>
>
> dont confuse with virtual addr space with physical addr space . MMU 
> will assist the OS in this issue.
>
> the moment the bit to enable the 64 bit mode is enabled the MMU and 
> other thing are extend to address 64 bit physical addr space.
>
>>
>>
>> 3)extra ques -- why in case of 32 bit arch -- physical memory RAM 
>> limitation comes of 4GB ?
>>
>>
> because the 32 bit can only address 2^32 address location which is 4GB 
> of RAM max. where 2^64 = 16 EB (exa bytes)
>
> finally download some data sheet of different processor and read to 
> get how thing work
>
>>
>> On Fri, Oct 23, 2009 at 4:54 PM, <microbit@virginbroadband.com.au 
>> <mailto:microbit@virginbroadband.com.au>> wrote:
>>
>>     Hi,
>>
>>     On Thu, 22 Oct 2009 23:58:20 -0700, C <a.la.kaarta@gmail.com
>>     <mailto:a.la.kaarta@gmail.com>> wrote:
>>      >> Well, IMHO the processor does not decide or even know the 
>> size of
>>      >> virtual address space.
>>      > Ofcourse it does. How else do you think it translates a virtual
>>      > address to a physical address? Virtual addresses are simply 
>> what the
>>      > software 'sees', the processor takes these, translates them into
>>      > physical addresses before making any reads / writes to main
>>     memory. No
>>      > software can use a virtual address space larger than what the
>>      > processor specifies.
>>      >
>>      >> 1) User may run a firmware on the processor that gives a 1-1 
>> mapping
>>      >> from virtual to physical (thus making virtual address space 
>> equal to
>>      >> physical address space).
>>      >>
>>      > Well, I don't know much about other architectures, so I'll just
>>     comment
>>     on
>>      > x86.
>>      > When you switch to 64bit mode, you compulsorily need to have a
>>     4-level
>>      > paging table, which translates 64-bit linear addresses 
>> (actually a
>>      > 48-bit linear address, since the address is subject to the 
>> canonical
>>      > address requirement) to (upto) 52-bit physical addresses. So
>>      > irrespective of what firmware you're running, linear addresses 
>> are
>>      > actually 64-bit, but physical addresses are not.
>>      >
>>      > (In fact, physical address space might even be larger than 
>> virtual
>>      > address space, when we take modes like PAE / PSE into account)
>>      >
>>      >> 2) Users may be running different OS(s) that give different
>>     amount of
>>      >> virtual address space to use.
>>      > Irrespective of what OS you're using (and whether it switches 
>> to the
>>      > processor mode that would utilize the 64-bit virtual address 
>> space
>>      > that the processor provides), the 'internal address bus' (virtual
>>      > address space) of the processor is what decides the maximum 
>> virtual
>>      > address space of any programs that run on it (OS or otherwise).
>>      >
>>      > C
>>      >
>>      > On Thu, Oct 22, 2009 at 11:23 PM, Rajat Jain
>>     <Rajat.Jain@infogain.com <mailto:Rajat.Jain@infogain.com>>
>>      > wrote:
>>      >>
>>      >> Hi,
>>      >>
>>      >> ----Original Message----
>>      >> From: C [mailto:a.la.kaarta@gmail.com
>>     <mailto:a.la.kaarta@gmail.com>]
>>      >> Sent: Friday, October 23, 2009 10:51 AM
>>      >> To: Rajat Jain
>>      >> Cc: simonyanix@gmail.com <mailto:simonyanix@gmail.com>; Siddu;
>>     Rick Brown; kernelnewbies;
>>      >> linux-newbie@vger.kernel.org 
>> <mailto:linux-newbie@vger.kernel.org>
>>      >> Subject: Re: 32 bit processors / 64 bit processors
>>      >>
>>      >>> PAE (Physical Address Extension) expands the _physical_ address
>>     space
>>      >>> to > 32 bits, but the _virtual_ address space stays the same at
>>      >>> 32-bits, and the virtual address size is what I mentioned as
>>     qualifies
>>      >>> the processor as 32-bit or 64-bit.
>>      >>
>>      >> Well, IMHO the processor does not decide or even know the 
>> size of
>>      >> virtual address space.
>>      > Ofcourse it does. How else do you think it translates a virtual
>>      > address to a physical address?
>>      >
>>      > It purely depends on the software (OS in this
>>      >> case) that runs on it. Consider all of the following is possible
>>     on the
>>      >> same 32 bit processor:
>>      >>
>>      >> 1) User may run a firmware on the processor that gives a 1-1 
>> mapping
>>      >> from virtual to physical (thus making virtual address space 
>> equal to
>>      >> physical address space).
>>      >>
>>      >> 2) Users may be running different OS(s) that give different
>>     amount of
>>      >> virtual address space to use.
>>      >>
>>      >> What am I missing?
>>      >>
>>      >> Thanks,
>>      >>
>>      >> Rajat
>>      >>
>>      >>
>>      >>>
>>      >>> 1. Addressable physical memory / physical address size does not
>>     decide
>>      >>> whether a processor is 32-bit / 64-bit, there is no processor
>>     (AFAIK)
>>      >>> which can address 64 bits of physical memory. I suppose
>>     sizeof(void*)
>>      >>> gives you the size of the _virtual_ address, so yes, I 
>> suppose that
>>      >>> should be 64 bits on a 64-bit processor (and using a 64-bit
>>     compiler)
>>      >>> 2. Register size does not decide whether a processor is 
>> 32-bit /
>>      >>> 64-bit.
>>      >>>
>>      >>> C
>>      >>>
>>      >>> On Thu, Oct 22, 2009 at 10:05 PM, Rajat Jain
>>      >>> <Rajat.Jain@infogain.com <mailto:Rajat.Jain@infogain.com>> 
>> wrote:
>>      >>>>
>>      >>>> Hi,
>>      >>>>
>>      >>>>> 1. The size of the processor's internal address bus (virtual
>>     address
>>      >>>>> space) is what qualifies it as a 32-bit / 64-bit processor.
>>      >>>>
>>      >>>> Well, in that sense, isn't Pentium a "36-bit" processor 
>> (since it
>>      >>>> gives the option of PAE to use 64 GB of memory - it must be 
>> having
>>      >>>> atleast 36 address lines)?
>>      >>>>
>>      >>>> On this topic and in this thread, we have had following
>>     responses to
>>      >>>> the question on what is called a 32-bit or 64-bit processor:
>>      >>>>
>>      >>>> 1) Addressable Physical memory (=sizeof(void*))
>>      >>>> 2) Register Size (=instruction size)
>>      >>>>
>>      >>>> Are the above two independent of each other? If yes, then how
>>     do we
>>      >>>> deine a processor as 32-bit / 64-bit?
>>      >>>>
>>      >>>> Thanks,
>>      >>>>
>>      >>>> Rajat
>>      >>
>>      >
>>      > --
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>>
>>
>>
>>      >> Well, IMHO the processor does not decide or even know the 
>> size of
>>      >> virtual address space.
>>
>>      > Ofcourse it does. How else do you think it translates a virtual
>>
>>     No it doesn't... the previous poster is right. (unless we
>>     excessively get
>>     into semantics....)
>>     That is up to the MMU, it has absolutely nothing to do with the CPU.
>>     And FWIW, x86 is hardly a reference... x86, along with 8051 would
>>     have to
>>     be the biggest abonimation
>>     to the concept of elegant processing......
>>
>>     -- Kris
>>
>>
>>     --
>>     To unsubscribe from this list: send an email with
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>>     Please read the FAQ at http://kernelnewbies.org/FAQ
>>
>>
>>
>>
>> -- 
>> Thanks & Regards
>> Nidhi Mittal Hada
>
>
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