Fall CS232 2006
Virtual Memory
November 6, 2006
Problem 1: Virtual and Physical addresses
A memory system has the following components: Cache: 4-way set-associative, size = 64 KB, write-back policy, block size = 16 words TLB: 128 entries, 2-way set associative Physical address: 32 bits, byte-addressable Virtual address: 50 bits, byte-addressable Page table: Two-level, Level-1 table has 220 entries Page size: 4 KB (a) Compute the lengths of the following elds: Page Oset = bits bits bits bits Level-1 Virtual Page Number (VPN) = Level-2 Virtual Page Number = Physical Page Number (PPN) =
(b) For the cache, the bits for the index and block-oset elds come from the virtual address, and bits for the tag eld come from the physical address that the virtual address is translated into. Compute the lengths of each of the following elds: Block oset
Index
Tag (Careful! Remember that the tag is used to dierentiate two blocks that have a common index. Since the index comes from the virtual address whereas the tag comes from the physical address, what should the tag eld be?)
(c) For the TLB, the index is
bits long and the tag is
bits long.
1
CS232 Fall 2006
Virtual Memory
November 6, 2006
Problem 2: Average access time
Assume following the specications for components of the the memory system from Problem 1: Memory: Interleaved with 16 banks, word addressable, access time for each bank = 14 cycles Memory bus: Width = 1 word, latency = 1 cycle TLB: Hit time = 2 cycles, miss rate = 1% Page table: Page-fault rate = 0.01% Hard disk: Time to replace a page = 106 cycles Cache: Hit time = 1 cycle, miss rate = 2%, 50% of replaced blocks are dirty (a) On a TLB miss, compute how many cycles it takes to read the PPN from the page table or determine that it does not exist in the page table. In other words, compute the hit time of the page table. Hint: The page table is in memory. How many accesses to memory does it take to recover the PPN? Can these acc...