| 123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"><html lang="en"><head><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><meta http-equiv="Content-Style-Type" content="text/css"><link rel="up" title="FatFs" href="../00index_e.html"><link rel="alternate" hreflang="ja" title="Japanese" href="../ja/dread.html"><link rel="stylesheet" href="../css_e.css" type="text/css" media="screen" title="ELM Default"><title>FatFs - disk_read</title></head><body><div class="para func"><h2>disk_read</h2><p>The disk_read function reads sector(s) from the storage device.</p><pre>DRESULT disk_read (  BYTE <span class="arg">pdrv</span>,     <span class="c">/* [IN] Physical drive number */</span>  BYTE* <span class="arg">buff</span>,    <span class="c">/* [OUT] Pointer to the read data buffer */</span>  DWORD <span class="arg">sector</span>,  <span class="c">/* [IN] Start sector number */</span>  UINT <span class="arg">count</span>     <span class="c">/* [IN] Number of sectros to read */</span>);</pre></div><div class="para arg"><h4>Parameters</h4><dl class="par"><dt>pdrv</dt><dd>Physical drive number to identify the target device.</dd><dt>buff</dt><dd>Pointer to the first item of the <em>byte array</em> to store read data.</dd><dt>sector</dt><dd>Start sector number in 32-bit LBA.</dd><dt>count</dt><dd>Number of sectors to read.</dd></dl></div><div class="para ret"><h4>Return Value</h4><dl class="ret"><dt>RES_OK (0)</dt><dd>The function succeeded.</dd><dt>RES_ERROR</dt><dd>Any hard error occured during the read operation and could not recover it.</dd><dt>RES_PARERR</dt><dd>Invalid parameter.</dd><dt>RES_NOTRDY</dt><dd>The device has not been initialized.</dd></dl></div><div class="para desc"><h4>Description</h4><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><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><ul><li>Convert word transfer to byte transfer in this function if needed. - Recommended.</li><li>On the <tt>f_read</tt> calls, avoid long read request that includes a whole of sector. - Any direct transfer never occures.</li><li>On the <tt>f_read</tt> calls, make sure that <tt>(((UINT)data & 3) == (f_tell(fp) & 3))</tt> is true. - Word alignment of <tt>buff</tt> is guaranteed.</li></ul><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></div><p class="foot"><a href="../00index_e.html">Return</a></p></body></html>
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