Understanding Virtual Memory in 80386 Microprocessors

1. 16.317: Microprocessor System Design I Instructor: Dr. Michael Geiger Spring 2012 Lecture 22: Virtual memory

2. 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

3. 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 Microprocessors I: Lecture 22

4. 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) Microprocessors I: Lecture 22

5. 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! 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 Microprocessors I: Lecture 22

7. 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 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 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 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 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 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 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 Microprocessors I: Lecture 22

14. 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 offset 31 11 0 Microprocessors I: Lecture 22

15. Physical Memory Space Page Table frame frame frame A machine usually supports pages of a few sizes (MIPS R4000): frame virtual address A page table is indexed by a virtual address OS manages the page table for each ASID A valid page table entry codes physical memory “frame” address for the page Virtual address spaces A virtual address space is divided into blocks of memory called pages Microprocessors I: Lecture 22

16. Physical Memory Space Virtual Address 12 V page no. offset Page Table Page Table Base Reg Access Rights V PA index into page table table located in physical memory 12 P page no. offset Physical Address Details of Page Table Page Table • Page table maps virtual page numbers to physical frames (“PTE” = Page Table Entry) • Virtual memory => treat memory  cache for disk frame frame frame frame virtual address 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 Microprocessors I: Lecture 22

18. 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!) Microprocessors I: Lecture 22

19. Next time • Virtual memory examples Microprocessors I: Lecture 22