Recitation 9 Section F 15 213 Sp 06 TA Jernej Barbic Virtual memory Reminders Modified from Quiz today virtual memory Shell Lab due Thursday Kun Gao s and Minglong Shao s recitations Spring 2005 Fall 2004 1 Virtual memory One of the most important concepts in CS Benefits of virtual memory VM Use RAM as a cache for disk Easier memory management Simplifies linking and loading Access protection Share memory efficiently 2 Virtual memory Memory Per Process Virtual Addresses 0 1 Page Table 0 1 Physical Addresses CPU P 1 N 1 Disk Page hit page is in memory Page fault page is on disk 3 Virtual and physical addresses p p 1 n 1 virtual page number VPN page offset 0 virtual address address translation m 1 p p 1 physical page number PPN 0 page offset physical address Virtual Memory up to 2n 1 bytes Physical Memory up to 2m 1 bytes 4 Page Table translates VPN to PPN Separate page table per each process VPN is the index into the page table page table base register VPN acts as table index if valid 0 then page not in memory virtual address n 1 p p 1 virtual page number VPN page offset 0 valid access physical page number PPN m 1 p p 1 physical page number PPN page offset physical address 0 5 Translation Lookaside Buffer TLB Page table is in memory or even on disk slow access TLB keeps things manageable 1 VA CPU MMU Translation 2 VPN 4 PA 3 PTE Cache Memory TLB 5 Data 6 TLB and the L1 Cache n 1 p p 1 0 virtual page number page offset valid virtual address tag physical page number TLB TLB hit physical address tag index valid tag cache hit data byte offset L1 Cache data 7 Worst case most delay TLB miss page fault on page table then page fault on memory read 1 VA CPU 3 PTEA PTBR VPN MMU Translation 2 VPN 5 PA 4 PTE Cache Memory TLB 6 Data Page Fault Page Fault Page Table Mem Read DISK 8 Example 20 bit virtual addresses 18 bit physical addresses Page size is 1024 bytes TLB is 2 way associative with 16 total entries 9 Part 1 A Virtual address TLBT 19 18 17 16 TLBI 15 14 13 12 11 10 9 8 7 6 VPN 5 4 3 2 1 0 3 2 1 0 VPO B Physical address 17 16 15 14 13 PPN 12 11 10 9 8 7 6 5 4 PPO 10 Example TLB and page table 11 Part 2 Virtual address 0x78E6 A 078E6 0000 0111 1000 1110 0110 B Address translation parameter Value Parameter Value VPN 0x01E TLB hit N TLB Index 0x6 Page fault N TLB Tag 0x03 PPN 0x57 C Physical address 01 0101 1100 1110 0110 12 Part 2 Virtual address 0x04AA4 A 04AA4 0000 0100 1010 1010 0100 B Address translation parameter Value Parameter Value VPN 0x012 TLB hit Y TLB Index 0x2 Page fault N TLB Tag 0x02 PPN 0x68 C Physical address 01 1010 0010 1010 0100 13 Further material Section 10 6 4 a concrete example Read carefully and solve practice problem 10 4 14
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