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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-E21E7992-607A-5A49-B022-189ECA9E76D1" xml:lang="en"><title>Code

    13 Paging Overview</title><shortdesc>Overview of demand paging when applied to code paging. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <section id="GUID-254768D6-B78B-5193-8583-2F8CF57614AC"><title>Purpose</title> <p>Demand

    15 paging is a technique where memory appears to application programs to be present

    16 in RAM, but may in fact be stored on some external media and transparently

    17 loaded into RAM when needed. Demand paging trades off increased available

    18 RAM against decreased performance, increased media wear and increased power

    19 usage. More RAM is made available by loading pages only on demand, but a cost

    20 is incurred every time a page is loaded. </p> <p>Demand paging is used to

    21 reduce the amount of RAM that needs to be shipped with a device and so reduce

    22 its cost. </p> <p>For the code paging type of demand paging, the executable

    23 is stored in a ROM. Since the memory locations that will be pointed to cannot

    24 be determined ahead of time, the pointers in the executable have to be modified

    25 after the page-in process. This process is known as 'relocation' and 'fix-up'.

    26 It is usually done by loader. </p> </section>

    27 <section id="GUID-217936E7-4F09-5291-B6C5-FF00BCE1B4BD"><title>Description</title> <p>The

    28 executable is in a ROM and so has to be loaded into RAM before it can be executed.

    29 When the required part of the executable is not present in RAM, then a paging

    30 fault is generated which starts the paging in process of specifying which

    31 part of the ROM is to be paged-in along with which RAM page is to be used. </p> <p>The

    32 above process (in very simple terms) describes how ROM paging works. With

    33 code paging, there is the added complication that the executable is not execute

    34 in place (it is probably stored via the use of an operating system e.g. ROFS)

    35 and so when it is paged in any pointers in the page will not point to any

    36 valid location. Hence a new step has to be carried out that modifies the pointers

    37 in the new page so that will point to meaningful locations once they are RAM.

    38 This process is known as 'relocation' and 'fix-up'. It is usually done by

    39 loader. </p> <p> </p> </section>

    40 <section id="GUID-85164F03-A6B5-5227-A380-5B199C805670"><title>Components</title> <p>These

    41 are the main components of code demand paging: </p> <ul>

    42 <li id="GUID-FB01C6A6-A688-5A2C-85ED-9AA81550031B"><p>Rom image - The executable

    43 is stored in a ROM. </p> </li>

    44 <li id="GUID-1A2D6E09-4A95-5000-B526-A005AD07A2A5"><p>RAM - Where the executable

    45 will be executed. </p> </li>

    46 <li id="GUID-ED582DCB-2018-5E02-8B97-84D41D79F06C"><p>Paging Fault Handler

    47 - To detect that a page-in process is required and to carry it out. </p> </li>

    48 <li id="GUID-ECF22038-E5E2-56BC-9DDF-4EB50ADAFBBC"><p>Loader - This usually

    49 does the 'relocation' and 'fix-up' process after the 'page-in' process. </p> </li>

    50 </ul> </section>

    51 <section id="GUID-A0780848-8A13-41F8-9BCD-C93B6CC7E0B8"><title>Using ROM Paging</title> <p>Which

    52 type of paging is used and for which area of memory is first specified the <codeph>oby</codeph> and <codeph>mmp</codeph> files

    53 and finally build by using specific parameters in the buildrom utility. </p> </section>

    54 </conbody><related-links>

    55 <link href="GUID-BDB847A2-557A-5902-AA6D-C1AE10D8E493.dita"><linktext>Code Paging

    56 Guide</linktext></link>

    57 <link href="GUID-795B8649-B6C3-5540-B52A-9B460F35A5B5.dita"><linktext>ROM Paging</linktext>

    58 </link>

    59 <link href="GUID-B35A70D2-1BC8-51DE-95BF-F315DB394582.dita"><linktext>Demand Paging

    60 Overview</linktext></link>

    61 </related-links></concept>