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

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

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

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

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

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

RAM against decreased performance, increased media wear and increased power

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

</ul> </section>

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

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

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

</conbody><related-links>

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

Guide</linktext></link>

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

</link>

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

Overview</linktext></link>