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Operating Systems: Virtualization, Concurrency & Persistence

TLB Basic Algorithm

Explore how hardware manages virtual memory using the basic TLB algorithm. Understand the process of TLB hits and misses, how the system accesses page tables on misses, and why minimizing TLB misses is crucial for performance.

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The code snippet below shows a rough sketch of how hardware might handle a virtual address translation, assuming a simple linear page table (i.e., the page table is an array) and a hardware-managed TLB (i.e., the hardware handles much of the responsibility of page table accesses; we’ll explain more about this below).

C
VPN = (VirtualAddress & VPN_MASK) >> SHIFT
(Success, TlbEntry) = TLB_Lookup(VPN)
if (Success == True)   // TLB Hit
    if (CanAccess(TlbEntry.ProtectBits) == True)
        Offset   = VirtualAddress & OFFSET_MASK
        PhysAddr = (TlbEntry.PFN << SHIFT) | Offset
        Register = AccessMemory(PhysAddr)
    else
        RaiseException(PROTECTION_FAULT)
else                  // TLB Miss
    PTEAddr = PTBR + (VPN * sizeof(PTE))
    PTE = AccessMemory(PTEAddr)
    if (PTE.Valid == False)
        RaiseException(SEGMENTATION_FAULT)
    else if (CanAccess(PTE.ProtectBits) == False)
        RaiseException(PROTECTION_FAULT)
    else
        TLB_Insert(VPN, PTE.PFN, PTE.ProtectBits)
        RetryInstruction()

In case of a TLB hit

The algorithm the hardware follows works like this: first, extract the virtual ...