11/14/05 ELEC Virtual Memory -Neha Jain -Neha Jain

211/14/05ELEC Topics Covered Introduction Virtual Memory Address Translation Thrashing and Protection References

311/14/05ELEC Introduction Memory Hierarchy Functional units Registers Internal cache External Cache Main Memory Secondary Memory (Disk) Memory Content: Cache  Main Mem  Secondary Mem CPU Memory Parameters: Access Time: increase with distance from CPU Cost/Bit: decrease with distance from CPU Capacity: increase with distance from CPU Cache : use SRAM (faster and costly) Main Memory : use DRAM (slower and cheap)

411/14/05ELEC Principle of Locality Temporal Locality : Item tend to be referenced again soon Spatial Locality : Nearby items tend to be referenced again soon Cache Memory Cache Memory -Takes advantage of principle of locality

511/14/05ELEC Virtual Memory A technique that uses main memory as a “cache” for secondary storage Basic Terminology Page : Fixed number of bytes recognized by operating system Page : Fixed number of bytes recognized by operating system Page Fault : Occurs when accessed page is not in main memory Page Fault : Occurs when accessed page is not in main memory Physical Address : Address in main memory Physical Address : Address in main memory Virtual Address: Addresses that application programmers deal with and are translated by MMU to physical address Virtual Address: Addresses that application programmers deal with and are translated by MMU to physical address

611/14/05ELEC Processor Cache MMU: Memory management unit Physical address Main memory Data Virtual Address Physical address Virtual Page NumberPage Offset …………… ………….…….…2 1 0 Physical page numberPage Offset …………………… ………….…….…2 1 0 Translation Virtual Address Physical Address

711/14/05ELEC Segmented Memory S1 S2 S3 S4 S1 S3 free Memory Management Unit (MMU) Segment Table x x free Physical Memory

811/14/05ELEC Mapping Segmented Address + Segment #Displacement Virtual Address V prot limit base Segment Table Physical Memory datum Physical address Protection Compare segment Limit to displacement Segment# : specifies which segment to use( base address and length) Displacement : offset from base address for actual memory access

911/14/05ELEC Page table for address translation Virtual page number Disk storage Valid

1011/14/05ELEC Page Tables The page table,PC and registers specifies the state or process of a program Swap Space : Space on hard disk for all the pages of a process

1111/14/05ELEC Making Address Translation Fast By Translation-lookaside Buffer

1211/14/05ELEC TLBs and Caches

1311/14/05ELEC Who handles page faults? Operating System (in system software) - Look up page table - Look up page table - Choose a physical page to replace - Choose a physical page to replace - Bring referenced page from disk into - Bring referenced page from disk into chosen physical page chosen physical page

1411/14/05ELEC Thrashing is a disaster and it occurs when program does not follow locality principle and continuously swaps pages from main memory to disk Protection Protection -One of the way is by having different physical addresses for a same virtual address

1511/14/05ELEC References courses/elec5200_6200/ courses/elec5200_6200/ /E5200/course.html /E5200/course.html TML/MemoryArchitecturea3.html