Datorteknik VirtualMemory bild 1 Virtual Memory User memory model so far: Separate Instruction and Data memory In reality they share the same memory space User space 0x … 0x7fffffff Instruction memory Data memory

Datorteknik VirtualMemory bild 2 Virtual to Physical Address Mapping 0x … 0x7fffffff Physical memory Virtual address Physical address User space Instruction and Data 2 GB Virtual memory 2 GB, HUGE amountPhysical memory only 16 MB

Datorteknik VirtualMemory bild 3 Address Mapping 32-bit Virtual Address CP0 MIPS PIPELINE 32 InstrData Physical memory 16 Mb 24-bit Physical Address Arbiter

Datorteknik VirtualMemory bild 4 Virtual Address User 1 2 GB Page 0 Page 1 … Selects Page # xOffset within page #x Virtual Address 32-bit Page x 1024 Bytes Page n 2 Pages 22 2 Addresses 10

Datorteknik VirtualMemory bild 5 Virtual Memory …. Page x Page n Physical memory 16 MB Page x Page 1 Page 0 Not Allocated Yet Secondary Storage Primary Memory Hard Disk 2 GB Page 0 Virtual Address 2 GB

Datorteknik VirtualMemory bild 6 Memory Resident Pages Only very few pages are RESIDENT in physical Selects Page # xVirtual 32-bit AddressOffset Address Translation of page #x Physical 24-bit Address

Datorteknik VirtualMemory bild 7 Page Fault What about a NON RESIDENT page? We know the Virtual Address, but: No Physical Address, since the page is on Hard Disk (SWAPPED) What about a not allocated page We know the Virtual Address, but: We try to access a Virtual Address that we have not (yet) access to, that is an ERROR In both cases we get a PAGE FAULT

Datorteknik VirtualMemory bild 8 Page Table …. Page x Hard Disk Physical Memory Page y Virtual Address 2 GB Page y Page Resident Y Physical Addr [23:10] N Place on Hard Disk For Non Resident Pages we get a PAGE FAULT

Datorteknik VirtualMemory bild 9 Swapping 2 Gb …. Secondary Storage Physical Memory Page y Virtual Address Page y Resident Y Page y The OS copies Page y to physical memory and restarts the failing user instruction Physical Addr [23:10] Place on Hard Disk

Datorteknik VirtualMemory bild 10 Page Fault and the OS A Page Fault is handled by the Kernel (OS) 1) If physical memory not full –Copy the page from hard disk to a empty page X in physical memory –Update the Page Table, Resident = YES, Physical Addr [23:10]=X –Restart the failing instruction in the user program 2) If physical memory full –Choose one page X from physical memory, store it on hard disk at XX –Update the Page Table (X), Resident = NO, place on HD = XX –Proceed with 1) What if page X is unchanged (only read operations), skip storing to hard disk, just set Resident = NO

Datorteknik VirtualMemory bild 11 Multiple User Processes User 1 0x … 0x7fffffff 0x … 0x7fffffff 0x … 0x7fffffff Virtual address Physical address User 2 User n …. Virtual memory n * 2 Gb User n User 1 User nUser 1 User 2 Page Table 1 Page Table 2 Page Table n HD address 16 Mbyte

Datorteknik VirtualMemory bild 12 Where do we store the Page Tables? 16 Mbyte Page Table 1 Page Table 2 Page Table n User Memory Kernel Memory We store the Page Tables in Kernel memory Protected from User access! DirtyResident Physical Addr [23:10] Place on Hard Disk 2 entries huge array! Store only allocated pages 22 DR

Datorteknik VirtualMemory bild 13 Address Mapping 32-bit Virtual Address CP0 MIPS PIPELINE 32 InstrData 24-bit Physical Address Page Table 1 Page Table 2 Page Table n User Memory Kernel Memory User process 2 running Here we need page table 2 for address mapping

Datorteknik VirtualMemory bild 14 Translation Lookaside Buffer (TLB) CP0 MIPS PIPELINE Virtual Address DRPhysical Addr [23:10] Page Table 1 Page Table 2 Page Table n User Memory Kernel Memory On TLB hit, the 32-bit virtual address is translated into a 24-bit physical address by hardware We never call the Kernel! 32

Datorteknik VirtualMemory bild 15 Memory Hierarchy 24 D Valid bit Physical Addr [23:10] User Memory Kernel Memory 16 Mb = 2 pages 14 Hardware is FAST but EXPENSIVE No need to use more than 2 entries STILL TO BIG! Make is smaller. Select a subset of the Page Table and store it in the TLB 14 Page Table 2 V22-bit Page #

Datorteknik VirtualMemory bild 16 Address Translation A TLB hit, we get a physical address –The Page # found in TLB and Valid entry (V-bit) –If a Write operation, set Dirty (D-bit) A TLB miss, causes a TLB miss exception –If Page NOT Resident in Physical memory, Page Fault and the OS slide if page X is swapped to hard disk and X in TLB, clear V bit –If Page Resident in Physical memory Find a free TLB entry and update it 1) 22-bit Page #, set V-bit, clear D-bit, 14-bit physical address –If TLB full, chose a TLB entry if D-bit, update Page Table Dirty bit, proceed with 1)

Datorteknik VirtualMemory bild 17 TLB control CP0 MIPS PIPELINE 32 Virtual Addr Data bus Control Signals TLB MISS R/W Page Table