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    10 <!DOCTYPE concept

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    12 <concept id="GUID-6C74A8B4-50B6-486C-A75F-A50636F4C566" xml:lang="en"><title>Register Access Overview</title><shortdesc>Provides a summary of the Register Access platform service.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    13 <p>The Register Access platform service is intended for use in writing

    14 device drivers. Writing device drivers involves frequent access to

    15 hardware registers for reading, writing and modifying them. </p>

    16 <section id="GUID-65EDE628-937F-4472-9662-8FE36D77B12D">         

    17    <title>What is a register</title>             <p>A register is

    18 a memory location on the ASSP hardware to store data that relates

    19 to the operation of that hardware. For example, a register can be

    20 a counter, or a bit field, or the next sequence number or the next

    21 byte or word of data that has arrived over a bus.</p><p>The Symbian

    22 platform provides access functions for registers that have the following

    23 sizes:<ul>

    24 <li><p>8–bit</p></li>

    25 <li><p>16–bit</p></li>

    26 <li><p>32–bit</p></li>

    27 <li><p>64–bit</p></li>

    28 </ul></p>         </section>

    29 <section id="GUID-25E7482D-B837-4228-9BE5-836DF48AD664"><title>What

    30 functions are available</title><p>There are three types of function

    31 provided for register access:<ul>

    32 <li><p><codeph>Read</codeph> - get the value of a register</p></li>

    33 <li><p><codeph>Write</codeph> - write a value to a register</p></li>

    34 <li><p><codeph>Modify</codeph> - use bitmasks to clear or set specific

    35 bits in a register</p></li>

    36 </ul></p><p>Each function takes a first argument of a <xref href="GUID-7452AD53-A7EE-3B1E-BC3D-C4202E5DAEBC.dita"><apiname>TLinAddr</apiname></xref> which is an address. The device driver only needs to know the address,

    37 and the size of the register. The implementation of the Register Access

    38 must map the exposed physical hardware register to a linear address.</p><p>Each type of function (read, write, modify) has 8–bit, 16–bit,

    39 32–bit and 64–bit versions.</p><p>The <codeph>Modify</codeph> functions

    40 take two bitmasks as parameters, to specify which bits to clear to

    41 zero (clear mask) and which bits to set to one (set mask). This means

    42 that you could call <codeph>Modify16(myaddress, 0xC000,0x000F)</codeph> and the top two bits would be set to zero, the bottom four bits

    43 would be set to one, and the ten bits in the middle would retain their

    44 current value.</p><p>See <xref href="GUID-DB55C1A0-2901-4661-B6B1-3B61BF6FF4FA.dita">Register Access Client

    45 Interface Guide</xref> for more details on each function and how to

    46 use them.</p></section>

    47 </conbody></concept>