Without paging only 1 memory reference is required

[prashantgolu]

"suppose there is an instruction to copy one register contents to another...
now,
without paging only 1 memory reference is required to fetch the instruction...but with paging..we need 3 memory reference (as page tables are to be accesed)..."

please explain why 3...?

 

[TylerH]

In x86, to access memory with paging, the CPU must get the address of the PD from cr3, then get the address of the PT from the PD, then get the address of the page from the PT, and then access the referenced data in that page.

 

[prashantgolu]

whats PD,cr3 and PT...
without paging how come only 1 time memory to be referenced...?

 

[rcgldr]

whats PD,cr3 and PT ... without paging how come only 1 time memory to be referenced...

Those are Intel X86 cpu components. How paging and segementation are used depends on CPU and the OS. Some environments would only need 2 memory references instead of 3. For the X86, if the TLB (translation look aside buffer) contains the virtual to physical address in it's memory, then only 2 memory references are needed. If the multi-level paging scheme is used and there isn't an entry in the TLB, then 3 memory references are needed, plus a 4 reference to update the TLB.

http://en.wikipedia.org/wiki/Page_table

http://en.wikipedia.org/wiki/Translation_lookaside_buffer

Complicating matter is that segmentation could also be used:
http://en.wikipedia.org/wiki/X86_memory_segmentation
http://en.wikipedia.org/wiki/Physical_Address_Extension

If a system has a flat real address space (no virtual memory), then all addresses or offsets are physical and can be used without translation.

 

[LudusRex]

Without paging, the instruction to copy one register contents to another would require only one memory reference because the entire physical memory space is directly accessible. However, with paging, the memory space is divided into smaller units called pages, and the mapping between logical and physical addresses is stored in a data structure called the page table.

In order to fetch the instruction, the processor needs to first access the page table to determine the physical address of the instruction. This requires one memory reference. Then, the processor needs to access the actual page containing the instruction, which requires a second memory reference. Finally, the instruction can be fetched from the page, requiring a third memory reference. Therefore, with paging, a total of three memory references are needed to fetch the instruction.

This additional overhead of accessing the page table and multiple memory references is necessary in order to implement paging, which allows for efficient use of physical memory and better memory management.