Lecture 014

Address Space (1)

Potential Solutions:

• Not at the same time: run one program at a time. (unload other to swap)

• use linker again when launching a program.

Contiguous Allocation:

• segmentation fault when access address outside of segment

• Growing: growing require address space moving, which is expensive

• Fragmentation: new processes may not fit into holes (external) created by exiting processes.

• Partial Memory Residence: entire program must be in memory to run

• Swapping: entire program move to disk (and not runnable, blocked by implicite I/O), can take seconds

Paging: any page can map to any frame

• Page: small area of virtual memory (1/2K, 4K, 8K)

• Frame: small area of physical memory (same size as page)

• benifits

  • Growing: can grow
  • Fragmentation: no need contiguous
  • Swapping: only need swap small portion
  • Partial Residence: program can have pages unallocated but still run fine

One-layer page table:

• 1/2 average internal fragmentation

• needs:

  • One page table for each address space for convertion.
  • One global frame table for marking ownership.
  • Two memory access per memory access (one for page table, one for data)

• Page Table Entry

  • specify frame number (if valid)
  • valid bit: if 0, page is not allocated.
  • protection bit: read/write/execute permission
  • reference bit: if 1, page is accessed (used for page replacement algorithm)
  • dirty bit: if 1, page is modified and need to be written back to
  • (these PTE flags are specified by OS, inspected and updated by hardware)

Since memory is "sparse list of dense list", giving entire address space a page table entry is wasteful. So we need multi-layer page table.

So you specify segment number and offset (implicitly), then it got translate to linear address space, then page table map linear address space to physical memory by translate page to frame address.

ASLR is not that secure and hurt performance, it is over.