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1 <refentry id="dcheck">
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2 <refnamediv>
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3 <refname>DCHECK</refname>
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4 <refpurpose>Check Disk File Structure</refpurpose>
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5 </refnamediv>
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6
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7 <refsynopsisdiv>
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8 <cmdsynopsis>
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9 <command>DCHECK</command>
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10 <arg choice="opt">
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11 <option>-opts</option>
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12 </arg>
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13 <arg choice="plain">
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14 &replstart;devnam&replend;
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15 </arg>
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16 </cmdsynopsis>
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17 </refsynopsisdiv>
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18
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19 <refsect1><title>Description</title>
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20 <para>
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21 It is possible for sectors on a disk to be marked as being allocated
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22 but in fact are not actually associated with a file or the disk's
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23 free space. This can happen if a disk is removed from a drive while
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24 files are still open, or if a directory which still contains files
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25 is deleted (see 3.5). DCHECK is a diagnostic that can be used to
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26 detect this condition, as well as the general integrity of the directory/file linkages.
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27 </para>
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28 <para>
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29 DCHECK is given as a parameter the name of the disk device to be
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30 checked. After verifying and printing some vital file structure
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31 parameters, DCHECK follows pointers down the disk's file system tree
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32 to all directories and files on the disk. As it does so, it
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33 verifies the integrity of the file descriptor sectors, reports any
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34 discrepancies in the directory/file linkages, and builds a sector
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35 allocation map from the segment list associated with each file. If
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36 any file descriptor sectors (FDs) describe a segment with a cluster
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37 not within the file structure of the disk, a message is reported
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38 like:
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39 </para>
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40 <screen>
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41 *** Bad FD segment ($xxxxxx-$yyyyyy) for file: &replstart;pathlist&replend;
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42 </screen>
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43 <para>
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44 This indicates that a segment starting at sector xxxxxx and ending
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45 at sector yyyyyy cannot really be on this disk. Because there is a
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46 good chance the entire FD is bad if any of it's segment descriptors
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47 are bad, the allocation map is <emphasis>not</emphasis> updated for corrupt FDs.
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48 </para>
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49 <para>
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50 While building the allocation map, DCHECK also makes sure that each
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51 disk cluster appears only once and only once in the file structure.
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52 If this condition is detected, DCHECK will display a message like:
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53 </para>
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54 <screen>
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55 Cluster $xxxxxx was previously allocated
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56 </screen>
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57 <para>
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58 This message indicates that cluster xxxxxx has been found at least
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59 once before in the file structure. The message may be printed more
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60 than once if a cluster appears in a segment in more than one file.
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61 </para>
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62 <para>
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63 The newly created allocation map is then compared to the allocation
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64 map stored on the disk, and any differences are reported in messages
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65 like:
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66 </para>
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67 <screen>
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68 Cluster $xxxxxx in allocation map but not in file structure
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69 Cluster $xxxxxx in file structure but not in allocation map
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70 </screen>
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71 <para>
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72 The first message indicates sector number xxxxxx (hexadecimal) was
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73 found not to be part of the file system, but was marked as allocated
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74 in the disk's allocation map. In addition to the causes mentioned
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75 in the first paragraph, some sectors may have been excluded from the
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76 allocation map by the FORMAT program because they were defective or
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77 they may be the last few sectors of the disk, the sum of which was
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78 two small to comprise a cluster.
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79 </para>
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80 <para>
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81 The second message indicates that the cluster starting at sector
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82 xxxxxx is part of the file structure but is <emphasis>not</emphasis>
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83 marked as allocated
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84 in the disk's allocation map. It is possible that this cluster may
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85 be allocated to another file later, overwriting the contents of the
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86 cluster with data from the newly allocated file. Any clusters that
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87 have been reported as "previously allocated" by DCHECK as described
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88 above surely have this problem.
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89 </para>
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90 <para>
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91 Available DCHECK options are:
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92 </para>
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93
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94 <informaltable frame="none">
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95 <tgroup cols="2">
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96 <colspec colwidth="1.3in">
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97 <colspec colwidth="3in">
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98 <tbody>
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99 <row>
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100 <entry>-w=&replstart;path&replend;</entry>
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101 <entry>pathlist to directory for work files</entry>
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102 </row>
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103 <row>
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104 <entry>-p</entry>
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105 <entry>print pathlists for questionable clusters</entry>
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106 </row>
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107 <row>
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108 <entry>-m</entry>
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109 <entry>save allocation map work files</entry>
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110 </row>
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111 <row>
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112 <entry>-b</entry>
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113 <entry>suppress listing of unused clusters</entry>
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114 </row>
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115 <row>
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116 <entry>-s</entry>
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117 <entry>display count of files and directories only</entry>
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118 </row>
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119 <row>
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120 <entry>-o</entry>
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121 <entry>print DCHECK's valid options</entry>
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122 </row>
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123 </tbody>
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124 </tgroup>
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125 </informaltable>
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126
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127 <para>
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128 The "-s" option causes DCHECK to display a count of files and
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129 directories only; only FDs are checked for validity. The "-b" option
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130 suppresses listing of clusters allocated but not in file structure.
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131 The "-p" option causes DCHECK to make a second pass through the file
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132 structure printing the pathlists for any clusters that DCHECK finds
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133 as "already allocated" or "in file structure but not in allocation
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134 map". The "-w=" option tells DCHECK where to locate it's allocation
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135 map work file(s). The pathlist specified must be a FULL pathlist to
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136 a <emphasis>directory</emphasis>. The directory "/D0" is
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137 used is used if "-w" is not
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138 specified. It is recommended that this pathlist NOT be located on
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139 the disk being DCHECKed if the disk's file structure integrity is in
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140 doubt.
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141 </para>
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142 <para>
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143 DCHECK builds its disk allocation map in a file called
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144 &replstart;pathlist&replend;/DCHECKppO, where &replstart;pathlist&replend;
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145 is as specified by the
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146 "-w=" option and pp is the process number in hexadecimal. Each bit
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147 in this bitmap file corresponds to a cluster of sectors on the disk.
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148 If the "-p" option appears on the command line, DCHECK creates a
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149 second bitmap file (&replstart;pathlist&replend;/DCHECKpp1) that has a bit set for
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150 each cluster DCHECK finds as "previously allocated" or "in file
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151 structure but not in allocation map" while building the allocation
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152 map. DCHECK them makes another pass through the directory structure
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153 to determine the pathlists for these questionable clusters. These
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154 bitmap work files may be saved by specifying the "-m" option on the
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155 command line.
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156 </para>
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157 </refsect1>
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158 <refsect1><title>Restrictions</title>
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159 <para>
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160 For best results, DCHECK should have exclusive access to the disk
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161 being checked. Otherwise DCHECK may be fooled if the disk allocation map
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162 changes while it is building its bitmap file from the
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163 changing file structure. DCHECK cannot process disks with a directory
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164 depth greater than 39 levels.
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165 </para>
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166 <para>
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167 For more information see: 3.10, 3.5, FORMAT,
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168 6.1 of OS-9 Systems Programmer's Manual
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169 </para>
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170 </refsect1>
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171 <refsect1><title>Examples</title>
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172 <screen>
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173 OS9: dcheck /d2 (workfile is on /D0)
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174
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175 Volume - 'My system disk' on device /d2
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176 $009A bytes in allocation map
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177 1 sector per cluster
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178 $0004D0 total sectors on media
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179 Sector $000002 is start of root directory FD
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180 $0010 sectors used for id, allocation map and root directory
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181 Building allocation map work file...
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182 Checking allocation map file...
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183
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184
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185 'My system disk' file structure is intact
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186 1 directory
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187 2 files
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188
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189 OS9: dcheck -mpw=/d2 /d0
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190 Volume - 'System disk' on device /d0
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191 $0046 bytes in allocation map
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192 1 sector per cluster
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193 $00022A total sectors on media
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194 Sector $000002 is start of root directory FD
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195 $0010 sectors used for id, allocation map and root directory
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196 Building allocation map work file...
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197 Cluster $00040 was previously allocated
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198 *** Bad FD segment ($111111-$23A6F0) for file: /d0/test/junky.file
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199 Checking allocation map file...
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200 Cluster $000038 in file structure but not in allocation map
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201 Cluster $00003B in file structure but not in allocation map
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202 Cluster $0001B9 in allocation map but not in file structure
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203 Cluster $0001BB in allocation map but not in file structure
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204
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205 Pathlists for questionable clusters:
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206 Cluster $000038 in path: /d0/OS9boot
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207 Cluster $00003B in path: /d0/OS9boot
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208 Cluster $000040 in path: /d0/OS9boot
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209 Cluster $000040 in path: /d0/test/double.file
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210
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211 1 previously allocated clusters found
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212 2 clusters in file structure but not in allocation map
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213 2 clusters in allocation map but not in file structure
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214 1 bad file descriptor sector
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215
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216 'System disk' file structure is not intact
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217 5 directories
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218 25 files
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219 </screen>
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220 </refsect1>
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221 </refentry>
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222
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