MEMORY MANAGEMENT

MEMORY MANAGEMENT

1. Tecnológico de Costa Rica, Carrera: Ingeniería en Computación, Sede: Cartago, Curso: Principios de sistemas operativos, Profesor: Esteban Arias Méndez, Estudiante: Jason David Espinoza Siles, Carné: 201223941, II semestre, 2016, Libro: Operating System Concepts: capítulo 8. Reporte #8

2. BACKGROUND

   • Program must be brought (from disk) into memory and placed within a process for it to be run -- Main memory and registers are only storage CPU can access directly -- Memory unit only sees a stream of addresses + read requests, or address + data and write requests -- Register access in one CPU clock (or less) -- Main memory can take many cycles, causing a stall -- Cache sits between main memory and CPU registers -- Protection of memory required to ensure correct operation

3. ADDRESS BINDING

   • Programs on disk, ready to be brought into memory to execute form an input queue
   • Inconvenient to have first user process physical address always at 0000
   • BINDING OF INSTRUCTIONS AND DATA MEMORY
     • Address binding of instructions and data to memory addresses can happen at three different stages
       • COMPILE TIME
       • LOAD TIME
       • EXECUTION TIME

4. Multistep Processing of a User Program

5. Logical vs. Physical Address Space

   • LAS DIRECCIONES LÓGICAS SON CREADAS POR EL CPU HACIA MEMORIA VIRTUAL, SI VEMOS LA MOMORIA FÍSICA SERÁ LA DIRECCIÓN FÍSICA
   • UN PROGRAMA PUEDE UTILIZAR MEMORIA VIRUTAL O FÍSICA

6. Memory-Management Unit (MMU)

   • Hardware device that at run time maps virtual to physical address
     • Dynamic Linking
       • Static linking – system libraries and program code combined by the loader into the binary program image  Dynamic linking –linking postponed until execution time  Small piece of code, stub, used to locate the appropriate memory-resident library routine  Stub replaces itself with the address of the routine, and executes the routine  Operating system checks if routine is in processes’ memory address  Dynamic linking is particularly useful for libraries  System also known as shared libraries  Consider applicability to patching system libraries
     • Swapping
       • A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution  Total physical memory space of processes can exceed physical memory  Backing store – fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images  Roll out, roll in – swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed  Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped  System maintains a ready queue of ready-to-run processes which have memory images on disk
   • Dynamic relocation using a relocation register
     • Routine is not loaded until it is called  Better memory-space utilization; unused routine is never loaded  All routines kept on disk in relocatable load format  Useful when large amounts of code are needed to handle infrequently occurring cases

7. CONTIGUOUS ALLOCATION

   • Main memory must support both OS and user processes  Limited resource, must allocate efficiently  Contiguous allocation is one early method ---- Relocation registers used to protect user processes from each other, and from changing operating-system code and data

8. Dynamic Storage-Allocation Problem

   • First-fit: Allocate the first hole that is big enough
   • Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size
   • Worst-fit: Allocate the largest hole; must also search entire list

9. FRAGMENTATION

   • External Fragmentation – total memory space exists to satisfy a request, but it is not contiguous
   • Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used

10. SEGMENTATION

    • ARQUITECTURA
      • SEGMENT TABLE
      • SEGMENT-TABLE REGISTER
      • SEMENT-TABLE LENGTH REGISTER

11. PAGING

    • Physical address space of a process can be noncontiguous; process is allocated physical memory whenever the latter is available

12. Associative Memory

    • Paging Hardware With TLB
    • Effective Access Time
    • Memory Protection
    • Shared Pages
      • Shared code
      • Private code and data

13. Hashed Page Tables

14. Inverted Page Table

    • Rather than each process having a page table and keeping track of all possible logical pages, track all physical pages  One entry for each real page of memory  Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns that page  Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs

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