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   9 <title>FatFs - disk_read</title>
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  13
  14 <div class="para func">
  15 <h2>disk_read</h2>
  16 <p>The disk_read function is called to read data from the sector(s) of 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 first item of the <em>byte array</em> to store read data. Size of read data will be the sector size * <tt class="arg">count</tt> bytes.</dd>
  34 <dt>sector</dt>
  35 <dd>Start sector number in 32-bit LBA.</dd>
  36 <dt>count</dt>
  37 <dd>Number of sectors to read.</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>An unrecoverable hard error occured during the read operation.</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>Read/write operation to the generic storage devices, such as memory card, hadddisk and optical disk, is done in unit of block of data bytes called <em>sector</em>. FatFs supports the sector size in range of 512 to 4096 bytes. When FatFs is configured for fixed sector size (<tt>FF_MIN_SS == FF_MAX_SS</tt>, this is the most case), the read/write function must work at that sector size. When FatFs is configured for variable sector size (<tt>FF_MIN_SS &lt; FF_MAX_SS</tt>), the sector size of medium is inquired with <tt>disk_ioctl</tt> function immediately following <tt>disk_initialize</tt> function succeeded.</p>
  60 <p>The memory address specified by <tt class="arg">buff</tt> is not that always aligned to word boundary because the argument is defined as <tt>BYTE*</tt>. The unaligned read/write request can occure at <a href="appnote.html#fs1">direct transfer</a>. If the bus architecture, especially DMA controller, does not allow unaligned memory access, it should be solved in this function. There are some workarounds described below to avoid this issue.</p>
  61 <ul>
  62 <li>Convert word transfer to byte transfer in this function if needed. - Recommended.</li>
  63 <li>On the <tt>f_read()</tt> calls, avoid long read request that includes a whole of sector. - Any direct transfer never occures.</li>
  64 <li>On the <tt>f_read(fp, dat, btw, bw)</tt> calls, make sure that <tt>(((UINT)dat &amp; 3) == (f_tell(fp) &amp; 3))</tt> is true. - Word alignment of <tt class="arg">buff</tt> is guaranteed no matter dat is not word aligned.</li>
  65 </ul>
  66 <p>Generally, a multiple sector read request must not be split into single sector transactions to the storage device, or read throughput gets worse.</p>
  67 </div>
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