B.4: Virtual Memory

• With virtual memory, main memory is used as a cache for disk.
• The traditional reason for using a cache is to reduce latency, so a smaller, faster memory is used for the cache.
• Another way of looking at it is to think of the cache as being too small, and using a slower, larger memory to extend capacity.
• This is the way you should think of virtual memory: extending the size of main memory.
• Virtual memory also provides other services such as protection, but these are discussed mainly in OS.
• Why is virtual memory different from other forms of cache we have looked at:
  • The size of the faster memory is orders of magnitude larger (1000 times) than typical cache sizes
  • The size of the slower memory is orders of magnitude larger than (1000 times) than the faster memory
  • For typical caches, the faster memory is 3-100 times faster than the slower memory.
  • For virtual memory, the faster memory (RAM) is at least 100,000 times faster than the slower memory (disk).
  • Because of this difference in speed, the hit rate needs to be very close to 1.
    
    • Suppose the main memory access time is 50 ns.
    • Typical disk access times are 5 ms = 5,000,000 ns.
    • If the hit rate is 99.99%, the miss rate is .0001
    • The average access time is 50 ns + .0001 × 5,000,000 ns = 550 ns.
    • This is not good enough.
  • Direct mapping or set associative mapping are not good enough.
  • Main memory is not designed to support parallel lookup of tags.
  • Block sizes need to be relatively large (4-16KB).
  • Cannot use bit selection.
  • Since access times are very large, we can manage misses with software.

Paged Virtual Memory

• blocks of physical memory are called frames
• blocks on disk are called pages
• a memory address generated by the CPU is called a virtual (or logical) address
• a memory address used by the memory system is called a physical address
• tags are called page numbers
• the conversion of a logical address to a physical address is called address translation