1. Lecture 30 Syed Mansoor Sarwar
Operating Systems
Lecture 30
Syed Mansoor Sarwar

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Agenda for Today
Review of previous lecture
What is memory management
Source code to execution
Address binding
Logical and physical address spaces
Dynamic loading, dynamic linking, and overlays
Recap of lecture
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Review of Lecture 29
Deadlock detection
Deadlock recovery
Process termination
Resource preemption
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Memory Hierarchy
Very small, extremely fast, extremely expensive, and volatile CPU registers
Small, very fast, expensive, and volatile cache
Hundreds of megabytes of medium-speed, medium-price, volatile main memory
Hundreds of gigabytes of slow, cheap, and non-volatile secondary storage
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5. Purpose of Memory Management
To ensure fair, secure, orderly, and efficient use of memory
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Memory Management
Keeping track of used and free memory space
When, where, and how much memory to allocate and deallocate
Swapping processes in and out of main memory
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Source to Execution
Compile/Assemble ↓
Link
Load
Execute
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Address Binding
Binding instructions and data to memory addresses
Compile time
Load time
Execution time
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Address Binding
Compile time: If you know at compile time where the process will reside in memory, the absolute code can be generated. Process must reside in the same memory region for it to execute correctly.
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Address Binding
Load time: If the location of a process in memory is not known at compile time, then the compiler must generate re-locatable code. In this case the final binding is delayed until load time. Process can be loaded in different memory regions.
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Address Binding
Execution time: If the process can be moved during its execution from one memory region to another, then binding must be delayed until run time. Special hardware must be available for this to work.
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13. Logical and Physical Addresses
Logical address: An address generated by the process/CPU; refers to an instruction or data in the process
Physical address: An address for a main memory location where instruction or data resides
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14. Logical and Physical Address Spaces
The set of all logical addresses generated by a process comprises its logical address space.
The set of physical addresses corresponding to these logical addresses comprises the physical address space for the process.
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15. Logical and Physical Address Spaces
The run-time mapping from logical to physical addresses is done by a piece of the CPU hardware, called the memory management unit (MMU).
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Example
The base register is called the relocation register.
The value in the relocation register is added to every address generated by a user process at the time it is sent to memory.
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Example
14000
Process
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Example
In i8086, the logical address of the next instruction is specified by the value of instruction pointer (IP). The physical address for the instruction is computed by shifting the code segment register (CS) left by four bits and adding IP to it.
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Example
CS * 24
Physical address
Logical address +
CPU
MMU
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Example
Logical address (16-bit)
IP = 0B10h
CS = D000h
Physical address (20-bit)
CS * 24 + IP = D0B10h
Sizes of logical and physical address spaces?
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Dynamic Loading
With dynamic loading, a routine is not loaded into the main memory until it is called.
All routines are kept on the disk in a re-locatable format.
The main program is loaded into memory and is executed
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Dynamic Loading
Advantages
Potentially less time needed to load a program
Potentially less memory space needed
Disadvantage
Run-time activity
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Dynamic Linking
In static linking, system language libraries are linked at compile time and, like any other object module, are combined by the loader into the binary image
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Dynamic Linking
In dynamic linking, linking is postponed until run-time.
A library call is replaced by a piece of code, called stub, which is used to locate memory-resident library routine
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Dynamic Linking
During execution of a process, stub is replaced by the address of the relevant library code and the code is executed
If library code is not in memory, it is loaded at this time
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Dynamic Linking
Advantages
Potentially less time needed to load a program
Potentially less memory space needed
Less disk space needed to store binaries
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Dynamic Linking
Disadvantages
Time-consuming run-time activity, resulting in slower program execution
gcc compiler
Dynamic linking by default
-static option allows static linking
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Recap of Lecture
What is memory management
From source code to execution
Address binding
Logical and physical address spaces
Dynamic loading
Dynamic linking
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Operating Systems
Lecture 30
Syed Mansoor Sarwar
10 December 2018
© Copyright Virtual University of Pakistan