Operating Systems Lecture 35 Segmentation Read Ch. 9.5 - 9.6

Silberschatz, Galvin and Gagne 2002Modified for CSCI 399, Royden, 2005

7.1Operating System Concepts

Operating Systems

Lecture 35Segmentation

Read Ch. 9.5 - 9.6



Operator Overrides

To use some of the list functions, such as display( ), search( ) and valueInsert( ) or valueRemove( ), you need to override the output and comparison operators for your Process struct. Here's how:

ostream& operator << (ostream& outs, const Process &myProcess) { outs << "Process ID: " << myProcess.pid << " Number of bursts: " << myProcess.numBursts << endl; return outs;}

bool operator < (const Process proc1, const Process proc2) { return proc1.pid < proc2.pid; //This sorts by pid. You could choose } //something else, e.g. burstTime

bool operator == (const Process proc1, const Process proc2) { return proc1.pid == proc2.pid;}

bool operator != (const Process proc1, const Process proc2) { return proc1.pid != proc2.pid;}



User's view of physical memory

Physical memory is a linear array of memory addresses. With paging, the user's view is a contiguous storage space that is

linear. The process is not actually stored contiguously in memory. Users don't usually think of memory in terms of a linear array. Users generally think in terms of a collection of items (segments) that

are stored in memory with no particular order among segments. Example: A C++ program may consist of:

A main function A set of functions or classes data structures etc.

We refer to each of these by name. We don't care where each piece is stored relative to another.



User’s View of a Program



Segmentation

Segmentation is a memory-management scheme that supports user view of memory.

The logical address space is a collection of segments. A segment is a logical unit such as:

main program,function,method,object,local variables, global variables,stack,symbol table, arrays

Each segment has a name and a length. For simplicity, the segments are given numbers.

The logical address specifies the segment number and the offset: <segment-number, offset>



Logical View of Segmentation

1

3

2

4

1

4

2

3

user space physical memory space



Segmentation Architecture

Logical address consists of two numbers:

<segment-number, offset>, Segment table – maps two-dimensional physical addresses; each

table entry has: base – contains the starting physical address where the segments reside

in memory. limit – specifies the length of the segment.

The limit register is used to check for valid memory references. Segment-table base register (STBR) points to the segment table’s

location in memory. Segment-table length register (STLR) indicates number of

segments used by a program;

segment number s is legal if s < STLR.



Segmentation Hardware

Diagram of segmentation hardware: We will draw this in class.



Segmentation Hardware



Example

Suppose a program is divided into 5 segments: Segment 0: Subroutine Segment 1: sqrt function Segment 2: main program Segment 3: stack Segment 4: symbol table

The segment table is as follows:

Segment Limit Base

0 1000 1400

1 400 6300

2 400 4300

3 1100 3200

4 1000 4700



Example of Segmentation

We will draw a diagram in class.

Question: Where does segment 1, offset 43 map in physical memory? Where does segment 2, offset 567 map?



Example of Segmentation



Protection with Segmentation

Protection. With each entry in segment table associate: validation bit = 0 illegal segment read/write/execute privileges

Protection bits associated with segments. This makes more sense than with paging, because the

entire segment is likely to have the same read/write/execute privileges.

For example, instructions for non-self-modifying code may be read-only or execute only. These will belong to a single segment and will not be grouped with other types of data.

In the paging system, a page may be half filled with instructions and half filled with data.



Sharing of Segments

Each process has a segment table associated with it. Segments can be shared when entries in the segment

tables of two different processes point to the same physical location.

Sharing occurs at the segment level. Any information can be shared if it is defined as a segment.

Full programs can be shared (even with multiple segments). For example, text editors.

Parts of programs can be shared (e.g. functions from function libraries).



Diagram of Segment Sharing

Diagram of a text editor shared between 2 processes.We will draw this in class.



Sharing of Segments



Self Reference in Shared Code

What happens if a shared segment refers to itself? If it refers to itself by segment number, then

all processes using it must have the same segment number in their segment tables.

If it refers to itself indirectly, e.g. specifying an offset from the current program counter, then the processes can have different segment numbers for it in their segment tables.



Fragmentation

Segments are of variable length. Memory allocation is a dynamic process that uses a best

fit or first fit algorithm. Is there internal fragmentation in a segmentation system?

Is there external fragmentation?



Segmentation vs. Paging

Paging and Segmentation each have different advantages and disadvantages.

What are advantages and disadvantages of paging? No external fragmentation (advantage) There is internal fragmentation (disadvantage) Units of code and data are broken up into separate pages

(disadvantage) What are advantages and disadvantages of segmentation?

No internal fragmentation (advantage) There is external fragmentation (disadvantage) Keeps blocks of code or data as single units (advantage.

Can get advantages of both systems by combining them.



Segmentation with Paging – MULTICS

The MULTICS system solved problems of external fragmentation and lengthy search times by paging the segments.

Solution differs from pure segmentation in that the segment-table entry contains not the base address of the segment, but rather the base address of a page table for this segment.

The offset for the segment is translated to a page number and an offset for that page.



MULTICS Address Translation Scheme

We will draw this diagram in class.



MULTICS Address Translation Scheme

Documents

Operating Systems Lecture 35 Segmentation Read Ch. 9.5 - 9.6