Relevant sections in the dinasour book: Section 8.6 and 8.7.


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Project 2: CPU Scheduling

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Shared Memory Segments and Mapped files in Modern OS:

Shared Memory Segments: 
	shmget and shmat

	a shared memory segment can be created and then mapped into the
	address spaces of different processes;
	Example usage: database buffers, or as a way for communication
		among processes;


Memory mapped files:
	
	similarly, a file can be mapped into the address space of a process
	using mmap, and memory read/write in that region means reading or
	writing the file.



Adding these facilities lead to a view of the address space as a collection
of "addressable ranges", or in other words, segments.

Each segment is a range of virtual addresses that are valid for the program
to address.  Typically each segment has a segment descriptor, stating the
start address of the segment, and the length of the segment.  

Modern kernels has a data structure for each process, which is a linked
list of all the descriptors of the segments of the process' address space.
Based on this data structure, the kernel can decide whether a memory address
is legal or illegal.

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In some of the older architectures, there is no paging support.  However,
there is "segmentation" support.  That is, a whole segment may be loaded from
disk into the main memory, and the virtual->physical address translation
simply means adding the segment offset to the beginning physical address of
the segment.  

* Check Section 8.6 for more detail. *

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Later on, segmentation is combined with paging, and the architecture is
typically called "segmentation with paging".

* Check Section 8.7 for more detail. *

Example: 

	- Windows 95

i386: Segmentation with Paging:

. segments:

	i386: 16K segments, half for private use, half for common use;

	each segment is 32 bits;

	local descriptor table, global descriptor table:
	table entry: 8 bytes, including base location, and length;

        every virtual address is a pair: <13-bit s, 1-bit g, 2-bit p> + 32-bit offset;

	six segment registers;

	segment tables ---> segment descriptor --> another virtual address;

. two-level page tables;


	Figure in page 283 of the dinasour book;


Windows 95:

	three memory models: 
		. Win 3.1 protected mode segmented memory model; (16-bit windows program)
			. use selector + offset to reference a physical page;
			. no paging;
			. segments up to 64KB each;
			

		. WinNT flat memory model; 	(32-bit windows program)
			. paging; 
			. two-level page table;

		. Virtual-86 model;
			. only 20-bit addresses;
			. 1MB of addressable space;
			. segment + offset used to generate the 20-bit address;