CPSC 3220 - DAY 14
OCTOBER 31, 2017

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Address Translation Concept
	-How do we convert a virtual address to a physical address?

Flexible Address Translation
	-Base and Bound
	-Segmentation
	-Paging
	-Multilevel translation

Efficient Address Translation
	-Translation Lookaside Buffers
	-Virtually and physically addressed caches

+-------------------+ Virt.   +------------------+
| Processor	    | ------> | Translation      |		 Raise
|		    | Address |			 | Invalid? ---> Exception
+-------------------+	      +------------------+
	^			   Valid?
	|				|
	|				|	+-----------------+
	|				\----->	| Physical	  |
	|			       Physical	|	Memory	  |
	^				Memory	+-----------------+
	DATA						|
	|						V
	|					      Data
	|						|
	|						|
	\-----------------------------------------------/


ADDRESS TRANSLATION GOALS
-------------------------

	-Memory protection
	-Memory sharing
		-shared libraries, interprocess communication
	-Sparce addresses
		-multiple regions of dynamic allocation (heaps/stacks)
	-Efficiency
		-memory placement
		-runtime lookup
		-compact translation tables
	-Portability

Virtually Addressed Base and Bounds
-----------------------------------

	Segment is a contiguous region of virtual memory

	Each process has a segment table (in hardware)
		-Entry in table = segment
	
	Segment can be located anywhere in physical memory
		-Each segment has start, length, access permission

	Process can share segments
		-Same start, length, same/different access permissions

Zero on Reference
-----------------

How much physical memory is needed for the stack or heap?
	-Only what is currently in use

When program uses memory beyond end of stack
	-Segmentation fault in OS kernel
	-Kernel allocates some memory
		-How much?
	-Zeros the memory
		-Avoid accidentally leaking information
	-Modify segment table
	-Resume process