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=utf-8"> 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 is called to read data from the sector(s) of storage device.</p> 17<pre> 18DRESULT 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 < 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 & 3) == (f_tell(fp) & 3))</tt> is true. - Word alignment of <tt class="arg">buff</tt> is guaranteed.</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> 68 69 70<p class="foot"><a href="../00index_e.html">Return</a></p> 71</body> 72</html> 73