CS202: HW 7: virtual memory

Virtual memory fundamentals, I

Consider a (small) machine with a 11-bit virtual addresses, in which 8 bits are used for the VPN. (Assume that the machine is byte-addressable, as with all of the examples we have seen.)

What is the size of the virtual address space, in bytes?

Virtual memory fundamentals, II

Consider a (huge) machine that has 60-bit virtual addresses, a page size of 1 terabyte (1 TB, or 2⁴⁰ bytes), and 52-bit physical addresses.

How many bits is the VPN (virtual page number)?
How many bits is the PPN (physical page number)?
How many bits is the offset?

Page faults and permissions

For the statements below, please state whether they are true Always, Sometimes, or Never. Justify each answer.

"On the x86, if a given memory reference (load or store) causes a page fault exception, then that memory reference also causes a TLB miss."
"On the x86, if a given memory reference from user mode results in a TLB miss, then the memory reference also causes a page fault."
"On the x86, if a given memory reference from kernel mode results in a TLB miss, then the memory reference also causes a page fault."
"On the x86, if a page table entry's PTE_P and PTE_U bits are set, then it is permissible for the process to load from a virtual address in the corresponding virtual page."
"On the x86, if a page table entry's PTE_P and PTE_U bits are set, then it is permissible for the process to store to a virtual address in the corresponding virtual page."

TLB, page faults

Assume that the assembly code below is executed after a context switch. Make the following additional assumptions:

The TLB is flushed (emptied) after context switch.
Suppose all data pages (in the example below: 0x200000, 0x300000) are stored on disk when instruction 0x500 is executing.
There is no prefetching.

[context switch]
0x500     movq 0x200000, %rax    # move data in address 0x200000 to register %rax
0x504     incq %rax, 1           # add one to %rax
0x508     movq %rax, 0x300000    # move register %rax to memory at address 0x300000

Answer the following questions:

How many TLB misses will happen, and for which pages?
How many page faults will happen, and for which pages?

Handing in the homework

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