16.317: Microprocessor System Design I

Instructor: Dr. Michael Geiger

Spring 2012

Lecture 22: Virtual memory

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

04/18/23 Microprocessors I: Lecture 22 2

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

04/18/23 Microprocessors I: Lecture 22 3

Problems with memory

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)

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Solutions 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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Microprocessors I: Lecture 22 6

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 246 bytes (64

Terabytes) 2 bits used for privilege level in selector

04/18/23

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

04/18/23 Microprocessors I: Lecture 22 7

Microprocessors I: Lecture 22 8

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

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Microprocessors I: Lecture 22 9

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

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Microprocessors I: Lecture 22 10

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

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Microprocessors I: Lecture 22 11

Segmentation Virtual to Physical Address Translation

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

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Microprocessors I: Lecture 22 12

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

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Microprocessors I: Lecture 22 13

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

04/18/23

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

Virtual page number Page offset01131

04/18/23 14Microprocessors I: Lecture 22

Virtual address spaces

A machine usually supports

pages of a few sizes

(MIPS R4000):

PhysicalMemory Space

A valid page table entry codes physical memory “frame” address for the page

A virtual address spaceis divided into blocks

of memory called pagesframe

frame

frame

frame

A page table is indexed by a virtual address

virtual address

Page Table

OS manages the page table for each ASID04/18/23 15Microprocessors I: Lecture 22

PhysicalMemory Space

Page table maps virtual page numbers to physical frames (“PTE” = Page Table Entry)

Virtual memory => treat memory cache for disk

Details of Page Table

Virtual Address

Page Table

indexintopagetable

Page TableBase Reg

V AccessRights PA

V page no. offset12

table locatedin physicalmemory

P page no. offset12

Physical Address

frame

frame

frame

frame

virtual address

Page Table

04/18/23 16Microprocessors I: Lecture 22

Microprocessors I: Lecture 22 17

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

04/18/23

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!)

04/18/23 18Microprocessors I: Lecture 22

Next time

Virtual memory examples

04/18/23 Microprocessors I: Lecture 22 19