memory hierarchy + virtual memory

virtual memory

increases available address space by illusion of a large and private main memory

• run programs larger than memory

• must translate each memory reference which is slow

segmentation

process data is allocated to contiguous segment of memory

• base + bound to map virtual to physical address

  • base: physical memory location of segment

  • bound: segment size

  • bound - base = size of virtual address space

how to translate virtual to physical address

1. check if requested address <= bound for that process (bound register)

2. if not, stop under bound violation

3. else add base register value to virtual address

external fragmentation

small portions of memory are free but not contiguous

defragment

move memory around so free memory is in a contiguous block

• computationally expensive and complex

paging

main and virtual memory divided into equal sized segments

• no page shared between multiple processes

• don’t have to be contiguous

how are page addresses stored

virtual page number (virtual page location of address) + offset (location of address within page)

• only page field changes, determined by page table for each process

how does the page table work

uses virtual page field as index to find corresponding page field

• also stored valid bit (0 if corresponding page in memory, 1 if not)

page fault

valid bit is 0 for requested process so page is fetched from disk

internal fragmentation

not entire allocated page is being used so memory is being wasted

what is a good balance between a paging and segementation system

• divide virtual addresses into segments of variable length

• segments and main memory are divided into fixed size pages

• each segment has its own page table, so every process has multiple page table

TLB

translation look aside buffer

• special cache to store recent page look ups

• reduces access to main memory as to access a main memory address you need to access it twice, once for page address then for physical address