Re: 32 bit processors / 64 bit processors

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

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 ---
Of course, register size
>
>
> 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 ??
>
>
>
>
> 3)extra ques -- why in case of 32 bit arch -- physical memory RAM 
> limitation comes of 4GB ?
2^32 = 4GB simple...
>
>
>
> 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
>
>
>     --
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>
>
>
> -- 
> Thanks & Regards
> Nidhi Mittal Hada
>
>
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