Module 4.1 Operating Systems

backing store (disk)

A program is permanently kept on the?

True

True or False:

• For a program to be run it must be brought from the backing store into memory and placed within a process

When the main memory can take many cycles it causes

Cache

This sits between the main memory and CPU registers

Input Queue

Programs on disk, ready to be brought into memory to execute, are placed in an

Symbolic

Source code addresses are usually

Relocatable Addresses

Compiled code addresses bind to

linker or loader

binds relocatable addresses to absolute addresses

Absolute Code

In Compile-time: If memory location known a priori, _______ can be generated; must recompile code if starting location changes

relocatable code

In load time: Must generate ______ if the memory location is not known at compile time

Execution time

Binding delayed until run time if the process can be moved during its execution from one memory segment to another

Logical address

generated by the CPU;

Physical address

address seen by the memory unit

Logical address space

the set of all logical addresses generated by a program

Physical address space

set of all physical addresses generated by a program

Relocation Register

generalization of the base-register scheme

True

True or False:

• The value in the relocation register is added to every address generated by a user process at the time it is sent to memory

Execution-time binding

occurs when reference is made to the location in memory

Dynamic Loading

program consist of a main part and a number of routines

Routine

This is not loaded unless it’s called

True

True or False

• All routines kept on disk in relocatable load format

Static linking

system libraries and program code combined by the loader into the binary program image

Dynamic linking

linking postponed until execution time

Stub

A small piece of code tha is used to locate the appropriate memory-resident library routine

Stub

replaces itself with the address of the routine and executes the routine

Relocation registers

used to protect user processes from each other, and from changing operating-system code and data

base register

contains the value of the smallest physical address

limit register

contains a range of logical addresses – each logical address must be less than the limit register

Hole

block of available memory; holes of various size are scattered throughout memory

First-fit

Allocate the first hole that is big enough

Best-fit

Allocate the smallest hole that is big enough; must search the entire list, unless ordered by size

Worst-fit

Allocate the largest hole; must also search entire list

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

Segmentation

• Memory-management scheme that supports user view of memory

Programs

a collection of segments

a logical unit such as:

•   main program

•   procedure

•   function

•   method

•   object

•   local variables, global variables

•   common block

•   stack

•   symbol table

•   arrays

Paging

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

Paging

• Avoids external fragmentation

• Avoids the problem of varying sized memory chunks

frames

Divides physical memory into fixed-sized blocks

Page Table

translate logical to physical addresses

True

True or False?

• The page table is kept in main memory

Page-table base register (PTBR)

points to the page table

Page-table length register (PTLR)

indicates the size of the page table

translation look-aside buffers

Memory protection

implemented by associating the protection bit with each frame to indicate if access is allowed

Valid-invalid

bit attached to each entry in the page table:

valid

that the associated page is in the process’ logical address space, and is thus a legal page

invalid

indicates that the page is not in the process’ logical address space

Private code and data

Each process keeps a separate copy of the code and data

True

True or False

• The pages for the private code and data can appear anywhere in the logical address space

Hierarchical Page Tables

Break up the logical address space into multiple page tables

Two-level Page Example

A logical address (on 32-bit machine with 1K page size)

12 but page number

10 bit page offset

64-Bit Logical Address Space

• Even a two-level paging scheme not sufficient

Hashed Page Tables

Used in architecture with address spaces > 32 bits

Virtual Page Number

hashed into a page table

Swapping

• A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution

Backing store

ast 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; the lower-priority process is swapped out so the higher-priority process can b

True

True or false:

The major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swappe