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School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

Address Spaces

Traditional OS use a separate address space for each process.

Multiple address spaces:

• Each address space has own virtualphysical mapping.
• Advantages:
  • Maximises available address space
  • Isolates processes (provide protection)
• Drawbacks:
  • Meaning of virtual address depends on process context
  • Isolation inhibits sharing

How do processes share data?

• Via files:
  • One process writes data to a file, another reads file
  • Similarly pipes, sockets, ...
• Via message passing (IPC):
  • One process sends message, another receives
• Via shared memory:
  • both establish shared memory arena (mmap())
  • shared buffers are mapped to the same physical memory locations
  • both can access the same data directly

All require OS intervention.

Sharing between address spaces

Problems with sharing: Pointers!

• pointers are bound to an address space
• they are meaningless outside

Sharing across address spaces

... requires copying and conversions

• implies loss of typing
• increases code complexity (order of 30% of app code!)
• increases run-time overhead

Other problems with address spaces

	memory data:		file data:
	item_t a, *x;		item_t a;
			int    x;
			FILE   *f;
	...		...
	a = *x;		f = fopen("f","r");
			fseek (f, x, SEEK_SET);
			fread (*a, sizeof(item_t), 1, f);
	address is *x		address is ("f",*x)

Inconsistent naming of persistent and volatile data

Why do we have problems with sharing?

 $.$

The problems are with pointers

• pointer problems result from per-address-space mappings
• result from the desire to maximise the available address space
• results from limitations on address bits

 $.$

But we have 64-bit architectures now!

 $.$

Why not abolish private mappings????

• all address spaces are merged into one
• each process has same virtualphysical mapping
• all memory objects (text, data, stack, libraries) are allocated at unique addresses
• is big enough to include ``files'' as memory objects

==>

single-address-space system