how much memory can be accessed by a 32 bit machine

Question

What is meant by 32bit or 64 bit machine   It   s the processor architecture   a 32 bit machine can read and write 32bit data at a time same way with 64 bit machine      whats the maximum memory that a 32 bit machine can access   It is 2 32 4Gb  4Gigabit   0 5 GigaByte   That means 4Gb ram   If  I consider the same way for a 64 bit machine then I can have a ram of 16ExbiBytes   is that possible   Are my concepts right

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Yes  a 32-bit architecture is limited to addressing a maximum of 4 gigabytes of memory  Depending on the operating system  this number can be cut down even further due to reserved address space   This limitation can be removed on certain 32-bit architectures via the use of PAE  Physical Address Extension   but it must be supported by the processor  PAE eanbles the processor to access more than 4 GB of memory  but it does not change the amount of virtual address space available to a single process   each process would still be limited to a maximum of 4 GB of address space   And yes  theoretically a 64-bit architecture can address 16 8 million terabytes of memory  or 2 64 bytes  But I don t believe the current popular implementations fully support this  for example  the AMD64 architecture can only address up to 1 terabyte of memory  Additionally  your operating system will also place limitations on the amount of supported  addressable memory  Many versions of Windows  particularly versions designed for home or other non-server use  are arbitrarily limited

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Yes  on a 32bit machine the maximum amount of memory usable is around 4GB  Actually  depending on the OS it might be less due to parts of the address space being reserved  On Windows you can only use 3 5GB for example   On 64bit you can indeed address 2 64 bytes of memory  Not that you ll ever have those - but then again  a long time ago the same thing was said about ever needing more than 640kb of memory

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Basically  the term  x-bit machine  does not depend on your machine  That is why we do not need to change our processors or other hardware in order to migrate from a 32bit system to a 64bit one  or vice versa    32bit and 64bit stands for the addressing capability of the OS running on your machine   However  it still does not mean that a x-bit operating system is capable to address 2 x GB memory  Because the  B  in  GB  means  byte  and not  bit   1 byte equals 8 bits   Actually a 32bit system can not even address 2 32 8   2 29 GB memory space while there should some memory be reserved to the OS   It is something just below 3 GB

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Going back to a really basic idea  we have 32 bits for our memory addresses  That works out to 2 32 unique combinations of addresses  By convention  each address points to 1 byte of data  Therefore  we can access up to a total 2 32 bytes of data    In a 32 bit OS  each register stores 32 bits or 4 bytes  32 bits  1 word  of information are processed per clock cycle  If you want to access a particular 1 byte  conceptually  we can  extract  the individual bytes  e g  byte 0  byte 1  byte 2  byte 3 etc   by doing bitwise logical operations   E g  to get  dddddddd   take   aaaaaaaabbbbbbbbccccccccdddddddd  and logical AND with   00000000000000000000000011111111

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What s typically meant by 32-bit or 64-bit machine is the size of the externally visible   architected   general-purpose integer registers   This has very little to do with how the hardware is built though  For example  let s consider the  long obsolete  Intel Pentium Pro  It s normally considered a  32-bit  processor  even though it supports up to 36-bit physical addresses  has a 64-bit wide data bus  and internally computations on all supported operand types are carried out in a single set of registers  which are therefore 80 bits wide  to support the largest floating point type    At least in the case of Intel processors  even though larger physical addressing has been available for a long time  the largest amount of memory directly visible within the address space of any one process on a 32-bit processor is also limited to 4 gigabytes  32-bit addressing   The 36-bit physical addressing allows addressing up to 64 gigabytes of RAM  but only 4 gigabytes of that can be directly visible at any given time   The change to 64-bit machines mostly involved changing what was made visible to the user  or to code at the assembly language level   Again  what you see is rarely identical to what s real  For example  most 64-bit code sees pointers addresses as being 64 bits  but actual processors don t support that large of addresses  Current CPUs support 48-bit virtual addresses  and  at least as far as I ve noticed  a maximum of 40 bits of physical addressing  On the other hand  they re designed so in the future  when larger memory becomes practical  they can extend the physical addressing out to 48 bits without affecting software at all  Even when they increase the 48-bit virtual addressing  in a typical case it ll only affect a small amount of the operating system kernel  normal code is unaffected  because it already assumed addresses are 64 bits    So  no  a 64-bit machine does not really support up to 64 bits of physical addressing  but most typical 64-bit software should remain compatible with a future processor that did support directly addressing that much RAM

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No your concepts are not right  And to set it right you need the answer to the question that you incorrectly answered   What is meant by 32bit or 64 bit machine                  The answer to the question is  something significant in the CPU is 32bit or 64 bit   So the question is what is that something significant  Lot of people say the width of data bus that determine whether the machine is 32bit or 64 bit  But none of the latest 32 bit processors have 32 bit or 64 bit wide data buses  most 32 bit systems will have 36 bit at least to support more RAM  Most 64 bit processors have no more than 48bit wide data bus because that is hell lot of memory already            So according to me a 32 bit or 64 bit machine is determined by the size of its general purpose registers used in computation or  the natural word size  used by the computer   Note that a 32 bit OS is a different thing  You can have a 32 bit OS running on 64 bit computer  Additionally  you can have 32 bit application running on 64 bit OS  If you do not understand the difference  post another question   So the maximum amount of RAM a processor can address is 2  width of data bus in bits   given that the proper addressing mode is switched on in the processor   Further note  there is nothing stopping someone to introduce a multiplex between data Bus and memory banks  that will select a bank and then address the RAM  in two steps   This way you can address even more RAM  But that is impractical  and highly inefficient

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basically 32bit architecture can address 4GB as you expected  There are some techniques which allows processor to address more data like AWE or PAE

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