16.317: Microprocessor System Design I Instructor: Dr. Michael Geiger Spring 2012 Lecture 22: Virtual memory

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Lecture outline

  Announcements/reminders   Lab 2 due 3/28 HW 3 due 3/26 Lecture outline  Review  Local memory accesses   Interrupt descriptor table Task switching  Virtual memory  Benefits   VM and 80386 segmentation Paging Microprocessors I: Lecture 22 2

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Review

   Local memory access  Selector indicates access is global (TI == 1)    LDTR points to LDT descriptor in GDT  Actual base, limit of LDT stored in LDTR cache Index field in selector chooses descriptor from LDT  Descriptor addr = (LDT base) + (selector index * 8) Descriptor provides starting address of segment Interrupt descriptors    Provide starting address, length of interrupt service routines Limited to 256 descriptors Stored in IDT; IDTR holds base/limit of IDT Task switching   Task register (TR): selector for current task state segment (TSS) TSS stores all state (register values) for current task  Task switch: jump/call that changes TR; old TSS saved and new one loaded 3 Microprocessors I: Lecture 22

Problems with memory

4/24/2020  DRAM may be too expensive to buy enough to cover whole address space  We need our programs to work even if they require more memory than we have  A program that works with 512 MB RAM should still work with 256 MB RAM  Most systems run multiple programs  Most processors don’t have hardware multitasking support (like the 386) 4 Microprocessors I: Lecture 22

Solutions

4/24/2020    Leave the problem up to the programmer  Assume programmer knows exact memory configuration Overlays  Compiler identifies mutually exclusive regions

Virtual memory

  Use hardware and software to automatically translate references from

virtual address

(what the programmer sees) to

physical address

(index to DRAM or disk) Most virtual addresses not present in physical memory!

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Virtual Address and VA Space

 80386 virtual addresses: 48-bit  Used by Memory Management Unit (MMU)  Consists of  Selector (16bit): can be one of the segment selector register  Offset (32bit): can be EIP or other 32-bit registers   Segment can be as large as 4GB Virtual address space can be 2 46 Terabytes) bytes (64  2 bits used for privilege level in selector 6 Microprocessors I: Lecture 22

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Address translation

 Virtual address  physical address  Need address translation mechanism  May take multiple steps  On 80386, two (main) levels  Virtual address (VA)  linear address (LA)   Uses selectors, descriptors discussed so far Linear address  physical address (PA)  If using segmented memory model, PA == LA  If using paged memory model, translate LA to PA 7 Microprocessors I: Lecture 22

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Segmented Partition of Virtual Address Space

   80386 virtual memory space is divided into global and local memory address space   32 Terabytes global address space 32 Terabytes local address space Up to 8192 segments may exist in either global or local address space   Because maximum size of GDT is 64KBytes, each descriptor is 8bytes, 64KB/8B = 8192 Not all descriptors are normally in use Task has both global and local memory space 8 Microprocessors I: Lecture 22

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Physical Memory Space and Virtual to-Physical Address Translation

 4GB physical memory vs 64TB virtual memory space  Just a small amount of the information in virtual memory can reside in physical memory  Segments not in use is stored on secondary storage device  Address translation: 48bit VA -> 32bit PA   Segment translation Page translation 9 Microprocessors I: Lecture 22

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Memory Swap

  MMU determines whether or not a segment or page resides in physical memory If not present, “swap”  memory management software initiates loading of the segment or page from external storage device to physical memory  A segment or page in physical memory will be swapped out and stored in external storage device 10 Microprocessors I: Lecture 22

Segmentation Virtual to Physical Address Translation

4/24/2020       48-bit virtual address (selector + offset) translated to 32-bit physical address “Selector” used to find segment descriptor in LDT  64-bit segment descriptor cache register in 80386 contains: access rights (12b), base address(32b), limit(20b) Segment descriptor cache defines the location and size of code/data segment  Code/data segments in physical memory Offset is the address of the data to be accessed in the segment Segment base address + offset = 32b linear address 32b linear address is physical address, if paging is disabled Microprocessors I: Lecture 22 11

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Example

 Segment selector = 0100H offset = 00002000H segment base address = 00030000H Q:What is the virtual address? Physical address?

A: VA = 0100:00002000H linear address = base address + offset = 00030000H+00002000H = 00032000H if paging disabled, PA = linear addr = 00032000H Microprocessors I: Lecture 22 12

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Paged partition of Virtual Address Space

    Physical memory is organized in 4KB pages  4GB/4KB = 1,048,496 pages Simplifies the implementation of the memory management software  Fixed 4K pages make space allocation and deallocation easier than segmentation Space in a page might not be fully utilized Linear address is not direct physical address  Undergo a second translation - page translation  Format: 12-b offset, 10-b page, 10-b directory field 13 Microprocessors I: Lecture 22

Managing virtual memory

 Effectively treat main memory as a cache  Blocks are called

pages

 Misses are called

page faults

 Virtual address consists of

virtual page number

and

page offset

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31 Virtual page number 11 Page offset 0

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Virtual address spaces

Page Table Physical Memory Space frame frame frame frame A virtual address space is divided into blocks of memory called pages virtual address A machine usually supports pages of a few sizes (MIPS R4000): OS manages the page table for each ASID A page table is indexed by a virtual address A valid page table entry codes physical memory “frame” address for the page

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Details of Page Table

Page Table Physical Memory Space Virtual Address frame frame frame V page no.

12 offset virtual address

 

frame

Page Table Base Reg index into page table

V

Page Table

Access Rights PA

table located in physical memory

P page no.

offset 12 Physical Address

Page table maps virtual page numbers to physical frames (“PTE” = Page Table Entry) Virtual memory => treat memory  cache for disk 4/24/2020 Microprocessors I: Lecture 22 16

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Paging on 80386

  Two-level page table  First level: “page directory”   Starting address stored in CR3 (page directory base register (PDBR))  Indexed by upper 10 bits of linear address Second level: “page table”  Starting address of each “page table” stored in page directory   Indexed by middle 10 bits of linear address Provides starting address of physical page frame Physical address = (page frame base) + (offset)  Offset = lowest 12 bits of linear address Microprocessors I: Lecture 22 17

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Virtual memory performance

 Address translation accesses memory to get PTE  every memory access twice as long  Solution: store recently used translations  Translation lookaside buffer (TLB) : a cache for page table entries   “Tag” is the virtual page # TLB small  often fully associative  TLB entry also contains valid bit (for that translation); reference & dirty bits (for the page itself!) 18 Microprocessors I: Lecture 22

Next time

 Virtual memory examples 4/24/2020 Microprocessors I: Lecture 22 19