Virtual Memory
How to run different programs if they assume they are the only program running?
- All the
.ofiles share address space, and the linker would merge them together into a continuous chunk
Non-Solution
- Programs take turns to run
- BAD: this could be slow
- Linker relocate one last time
Better Non-Solution
- Continuously map programs to different places (process id => base + offset)
Problems:
- Gowing of processes are hard
- Entire program in memory wasteful (only partial residence is needed)
- External fragmentation
Solution - Paging
- Use more fine grained, fix size “Page”
Benefits:
- Can grow dynamically
- no more external fragmentation
- partial memory resident possible
How to do page to frame translation?
- Linear address part of it being the Page #
- Use a Page Table with Page # as index, and entry as Frame #
- Use a Page Tabel Base Register for different processes
Double access of memory?
For example:
movl (%esi), %eax
- Get PTE of %esi
- Get the %esi data
What happen if you have large memory address space, like 4GB?
- You will have MBs of page table , EACH PROCESS
**Solution 1: Page Table Length Register **
- Restrict some of the pages only
Solution 2: Multi-level Paging
Why this:
- Page tables mappings are a sparse list of dense lists:
- many holes => sparse
- sequential access so that pages are together (spatial locality) => dense
How is it space saving
- Some of the PDEs can just be NULL
