15 213 Memory System Case Studies Oct 31 2007 Topics class17 ppt P6 address translation x86 64 extensions Linux memory management Linux page fault handling Memory mapping 15 213 F 07 Intel P6 Bob Colwell s Chip CMU Alumni Internal designation for successor to Pentium Which had internal designation P5 Fundamentally different from Pentium Out of order superscalar operation Resulting processors Pentium Pro 1996 Pentium II 1997 z L2 cache on same chip Pentium III 1999 z The freshwater fish machines Saltwater fish machines Pentium 4 Different operation but similar memory system Abandoned by Intel in 2005 for P6 based Core 2 Duo 2 15 213 F 07 P6 Memory System 32 bit address space 4 KB page size L1 L2 and TLBs DRAM external system bus e g PCI Inst TLB L2 cache instruction fetch unit processor package L1 i cache 32 entries 8 sets Data TLB cache bus bus interface unit 4 way set associative inst TLB data TLB L1 d cache 64 entries 16 sets L1 i cache and d cache 16 KB 32 B line size 128 sets L2 cache unified 128 KB 2 MB 3 15 213 F 07 Review of Abbreviations Symbols Components of the virtual address VA z TLBI TLB index z TLBT TLB tag z VPO virtual page offset z VPN virtual page number Components of the physical address PA z PPO physical page offset same as VPO z PPN physical page number z CO byte offset within cache line z CI cache index z CT cache tag 4 15 213 F 07 Overview of P6 Address Translation 32 result CPU 20 VPN 12 virtual address VA VPO L1 128 sets 4 lines set TLB hit TLB 16 sets 4 entries set 10 10 VPN1 VPN2 20 PPN PDE PDBR 5 L1 miss L1 hit 16 4 TLBT TLBI TLB miss L2 and DRAM PTE 20 CT 12 PPO physical address PA Page tables 15 213 F 07 7 5 CI CO P6 2 level Page Table Structure Up to 1024 page tables Page directory 1024 4 byte page directory entries PDEs that point to page tables One page directory per process Page directory must be in memory when its process is running Always pointed to by PDBR Page tables 1024 4 byte page table entries PTEs that point to pages Page tables can be paged in and out 1024 PTEs page directory 1024 PDEs 1024 PTEs 1024 PTEs 6 15 213 F 07 P6 Page Directory Entry PDE 31 12 11 Page table physical base addr 9 Avail 8 7 G PS 6 5 A 4 3 2 1 0 CD WT U S R W P 1 Page table physical base address 20 most significant bits of physical page table address forces page tables to be 4KB aligned Avail These bits available for system programmers G global page don t evict from TLB on task switch PS page size 4K 0 or 4M 1 A accessed set by MMU on reads and writes cleared by software CD cache disabled 1 or enabled 0 WT write through or write back cache policy for this page table U S user or supervisor mode access R W read only or read write access P page table is present in memory 1 or not 0 31 1 Available for OS page table location in secondary storage 0 P 0 7 15 213 F 07 P6 Page Table Entry PTE 31 12 11 Page physical base address 9 Avail 8 7 6 5 G 0 D A 4 3 2 1 0 CD WT U S R W P 1 Page base address 20 most significant bits of physical page address forces pages to be 4 KB aligned Avail available for system programmers G global page don t evict from TLB on task switch D dirty set by MMU on writes A accessed set by MMU on reads and writes CD cache disabled or enabled WT write through or write back cache policy for this page U S user supervisor R W read write P page is present in physical memory 1 or not 0 31 1 Available for OS page location in secondary storage 0 P 0 8 15 213 F 07 How P6 Page Tables Map Virtual Addresses to Physical Ones 10 VPN1 10 VPN2 word offset into page directory 12 VPO page table PTE PDE PDBR physical address of page directory word offset into physical and virtual page word offset into page table page directory Virtual address physical address of page table base if P 1 20 PPN physical address of page base if P 1 12 PPO Physical address 9 15 213 F 07 Representation of VM Address Space PT 3 Page Directory P 1 M 1 P 1 M 1 P 0 M 0 P 0 M 1 PT 2 PT 0 P 1 M 1 P 0 M 0 P 1 M 1 P 0 M 1 P 1 M 1 P 0 M 0 P 1 M 1 P 0 M 1 P 0 M 1 P 0 M 1 P 0 M 0 P 0 M 0 Simplified Example 16 page virtual address space Page 15 Page 14 Page 13 Page 12 Page 11 Page 10 Page 9 Page 8 Page 7 Page 6 Page 5 Page 4 Mem Addr Page 3 Disk Addr Page 2 Flags 10 P Is entry in physical memory M Has this part of VA space been mapped In Mem Page 1 Page 0 On Disk Unmapped 15 213 F 07 P6 TLB Translation 32 result CPU 20 VPN 12 virtual address VA VPO L1 128 sets 4 lines set TLB hit TLB 16 sets 4 entries set 10 10 VPN1 VPN2 20 PPN PDE PDBR 11 L1 miss L1 hit 16 4 TLBT TLBI TLB miss L2 and DRAM PTE 20 CT 12 PPO physical address PA Page tables 15 213 F 07 7 5 CI CO P6 TLB TLB entry not all documented so this is speculative 32 16 1 1 PDE PTE Tag PD V V indicates a valid 1 or invalid 0 TLB entry PD is this entry a PDE 1 or a PTE 0 tag disambiguates entries cached in the same set PDE PTE page directory or page table entry z Structure of the data TLB 16 sets 4 entries set entry entry entry entry entry entry entry entry entry entry entry entry entry entry set 0 set 1 set 2 entry entry set 15 12 15 213 F 07 Translating with the P6 TLB 1 Partition VPN into TLBT and TLBI CPU 20 VPN 16 4 TLBT TLBI TLB miss 2 Is the PTE for VPN cached in set TLBI 12 virtual address VPO 1 2 PDE page table translation TLB PTE hit 3 20 PPN 12 PPO physical address 4 3 Yes then build physical address 4 No then read PTE and PDE if not cached from memory and build physical address 13 15 213 F 07 P6 Page Table Translation 32 result CPU 20 VPN 12 virtual address VA VPO L1 128 sets 4 lines set TLB hit TLB 16 sets 4 entries set …
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