fatfs/doc/en/dread.html

   1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
   2 <html lang="en">
   3 <head>
   4 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
   5 <meta http-equiv="Content-Style-Type" content="text/css">
   6 <link rel="up" title="FatFs" href="../00index_e.html">
   7 <link rel="alternate" hreflang="ja" title="Japanese" href="../ja/dread.html">
   8 <link rel="stylesheet" href="../css_e.css" type="text/css" media="screen" title="ELM Default">
   9 <title>FatFs - disk_read</title>
  10 </head>
  11
  12 <body>
  13
  14 <div class="para func">
  15 <h2>disk_read</h2>
  16 <p>The disk_read function reads sector(s) from the storage device.</p>
  17 <pre>
  18 DRESULT disk_read (
  19   BYTE <span class="arg">pdrv</span>,     <span class="c">/* [IN] Physical drive number */</span>
  20   BYTE* <span class="arg">buff</span>,    <span class="c">/* [OUT] Pointer to the read data buffer */</span>
  21   DWORD <span class="arg">sector</span>,  <span class="c">/* [IN] Start sector number */</span>
  22   UINT <span class="arg">count</span>     <span class="c">/* [IN] Number of sectros to read */</span>
  23 );
  24 </pre>
  25 </div>
  26
  27 <div class="para arg">
  28 <h4>Parameters</h4>
  29 <dl class="par">
  30 <dt>pdrv</dt>
  31 <dd>Physical drive number to identify the target device.</dd>
  32 <dt>buff</dt>
  33 <dd>Pointer to the <em>byte array</em> to store the read data.</dd>
  34 <dt>sector</dt>
  35 <dd>Start sector number in logical block address (LBA).</dd>
  36 <dt>count</dt>
  37 <dd>Number of sectors to read. FatFs specifis it in range of from 1 to 128.</dd>
  38 </dl>
  39 </div>
  40
  41
  42 <div class="para ret">
  43 <h4>Return Value</h4>
  44 <dl class="ret">
  45 <dt>RES_OK (0)</dt>
  46 <dd>The function succeeded.</dd>
  47 <dt>RES_ERROR</dt>
  48 <dd>Any hard error occured during the read operation and could not recover it.</dd>
  49 <dt>RES_PARERR</dt>
  50 <dd>Invalid parameter.</dd>
  51 <dt>RES_NOTRDY</dt>
  52 <dd>The device has not been initialized.</dd>
  53 </dl>
  54 </div>
  55
  56
  57 <div class="para desc">
  58 <h4>Description</h4>
  59 <p>The memory address specified by <tt class="arg">buff</tt> is not that always aligned to word boundary because the type of argument is defined as <tt>BYTE*</tt>. The misaligned read/write request can occure at <a href="appnote.html#fs1">direct transfer</a>. If the bus architecture, especially DMA controller, does not allow misaligned memory access, it should be solved in this function. There are some workarounds described below to avoid this issue.</p>
  60 <ul>
  61 <li>Convert word transfer to byte transfer in this function. - Recommended.</li>
  62 <li>For <tt>f_read()</tt>, avoid long read request that includes a whole of sector. - Direct transfer will never occure.</li>
  63 <li>For <tt>f_read(fp, buff, btr, &amp;br)</tt>, make sure that <tt>(((UINT)buff &amp; 3) == (f_tell(fp) &amp; 3))</tt> is true. - Word aligned direct transfer is guaranteed.</li>
  64 </ul>
  65 <p>Generally, a multiple sector transfer request must not be split into single sector transactions to the storage device, or you will not get good read throughput.</p>
  66 </div>
  67
  68
  69 <p class="foot"><a href="../00index_e.html">Return</a></p>
  70 </body>
  71 </html>