1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
|
<!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 <em>byte array</em> to store the read data.</dd>
<dt>sector</dt>
<dd>Start sector number in 32-bit LBA.</dd>
<dt>count</dt>
<dd>Number of sectors to read in range of from 1 to 128..</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. If variable sector size is selected (<tt>_MIN_SS < MAX_SS</tt>), FatFs inquires the sector size with <tt>disk_ioctl()</tt> after initialization</tt>.
<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 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>
<ul>
<li>Convert word transfer to byte transfer in this function if needed. - Recommended.</li>
<li>For <tt>f_read()</tt>, avoid long read request that includes a whole of sector. - Direct transfer will never occure.</li>
<li>For <tt>f_read(fp, buff, btr, &br)</tt>, make sure that <tt>(((UINT)buff & 3) == (f_tell(fp) & 3))</tt> is true. - Word aligned direct transfer 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>
|
