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  9. <title>FatFs - disk_read</title>
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  12. <div class="para func">
  13. <h2>disk_read</h2>
  14. <p>The disk_read function reads sector(s) from the storage device.</p>
  15. <pre>
  16. DRESULT disk_read (
  17. BYTE <span class="arg">pdrv</span>, <span class="c">/* [IN] Physical drive number */</span>
  18. BYTE* <span class="arg">buff</span>, <span class="c">/* [OUT] Pointer to the read data buffer */</span>
  19. DWORD <span class="arg">sector</span>, <span class="c">/* [IN] Start sector number */</span>
  20. UINT <span class="arg">count</span> <span class="c">/* [IN] Number of sectros to read */</span>
  21. );
  22. </pre>
  23. </div>
  24. <div class="para arg">
  25. <h4>Parameters</h4>
  26. <dl class="par">
  27. <dt>pdrv</dt>
  28. <dd>Physical drive number to identify the target device.</dd>
  29. <dt>buff</dt>
  30. <dd>Pointer to the first item of the <em>byte array</em> to store read data.</dd>
  31. <dt>sector</dt>
  32. <dd>Start sector number in 32-bit LBA.</dd>
  33. <dt>count</dt>
  34. <dd>Number of sectors to read.</dd>
  35. </dl>
  36. </div>
  37. <div class="para ret">
  38. <h4>Return Value</h4>
  39. <dl class="ret">
  40. <dt>RES_OK (0)</dt>
  41. <dd>The function succeeded.</dd>
  42. <dt>RES_ERROR</dt>
  43. <dd>Any hard error occured during the read operation and could not recover it.</dd>
  44. <dt>RES_PARERR</dt>
  45. <dd>Invalid parameter.</dd>
  46. <dt>RES_NOTRDY</dt>
  47. <dd>The device has not been initialized.</dd>
  48. </dl>
  49. </div>
  50. <div class="para desc">
  51. <h4>Description</h4>
  52. <p>The data read/write operation to the storage devices is done in unit of <em>sector</em>. FatFs supports the sector size in range of from 512 to 4096 bytes. When FatFs is configured to fixed sector size (<tt>_MIN_SS == MAX_SS</tt>, this will be the most case), the read/write function must work at that sector size. When FatFs is configured to variable sector size (<tt>_MIN_SS != MAX_SS</tt>), sector size is inquired with <tt>disk_ioctl</tt> function following <tt>disk_initialize</tt> function.</p>
  53. <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>
  54. <ul>
  55. <li>Convert word transfer to byte transfer in this function if needed. - Recommended.</li>
  56. <li>On the <tt>f_read</tt> calls, avoid long read request that includes a whole of sector. - Any direct transfer never occures.</li>
  57. <li>On the <tt>f_read</tt> calls, make sure that <tt>(((UINT)data &amp; 3) == (f_tell(fp) &amp; 3))</tt> is true. - Word alignment of <tt>buff</tt> is guaranteed.</li>
  58. </ul>
  59. <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>
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