> 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@xxxxxxxxxxxx> wrote: > > Hi, > > ----Original Message---- > From: C [mailto:a.la.kaarta@xxxxxxxxx] > Sent: Friday, October 23, 2009 10:51 AM > To: Rajat Jain > Cc: simonyanix@xxxxxxxxx; Siddu; Rick Brown; kernelnewbies; > linux-newbie@xxxxxxxxxxxxxxx > 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@xxxxxxxxxxxx> 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs
