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1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd">
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2 <HTML
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3 ><HEAD
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4 ><TITLE
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5 >Object Files and Sections</TITLE
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6 ><META
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7 NAME="GENERATOR"
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8 CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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9 REL="HOME"
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10 TITLE="LW Tool Chain"
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11 HREF="index.html"><LINK
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12 REL="UP"
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13 TITLE="LWASM"
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14 HREF="c35.html"><LINK
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15 REL="PREVIOUS"
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16 TITLE="Macros"
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17 HREF="x339.html"><LINK
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18 REL="NEXT"
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19 TITLE="Assembler Modes and Pragmas"
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20 HREF="x407.html"></HEAD
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21 ><BODY
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25 LINK="#0000FF"
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26 VLINK="#840084"
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27 ALINK="#0000FF"
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28 ><DIV
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29 CLASS="NAVHEADER"
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30 ><TABLE
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31 SUMMARY="Header navigation table"
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32 WIDTH="100%"
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33 BORDER="0"
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34 CELLPADDING="0"
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35 CELLSPACING="0"
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36 ><TR
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37 ><TH
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38 COLSPAN="3"
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39 ALIGN="center"
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40 >LW Tool Chain</TH
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41 ></TR
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42 ><TR
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43 ><TD
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44 WIDTH="10%"
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45 ALIGN="left"
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46 VALIGN="bottom"
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47 ><A
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48 HREF="x339.html"
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49 ACCESSKEY="P"
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50 >Prev</A
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51 ></TD
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52 ><TD
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53 WIDTH="80%"
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54 ALIGN="center"
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55 VALIGN="bottom"
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56 >Chapter 3. LWASM</TD
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57 ><TD
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58 WIDTH="10%"
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59 ALIGN="right"
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60 VALIGN="bottom"
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61 ><A
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62 HREF="x407.html"
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63 ACCESSKEY="N"
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64 >Next</A
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65 ></TD
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66 ></TR
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67 ></TABLE
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68 ><HR
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69 ALIGN="LEFT"
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70 WIDTH="100%"></DIV
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71 ><DIV
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72 CLASS="SECTION"
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73 ><H1
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74 CLASS="SECTION"
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75 ><A
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76 NAME="AEN361"
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77 >3.8. Object Files and Sections</A
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78 ></H1
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79 ><P
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80 >The object file target is very useful for large project because it allows
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81 multiple files to be assembled independently and then linked into the final
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82 binary at a later time. It allows only the small portion of the project
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83 that was modified to be re-assembled rather than requiring the entire set
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84 of source code to be available to the assembler in a single assembly process.
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85 This can be particularly important if there are a large number of macros,
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86 symbol definitions, or other metadata that uses resources at assembly time.
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87 By far the largest benefit, however, is keeping the source files small enough
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88 for a mere mortal to find things in them.</P
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89 ><P
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90 >With multi-file projects, there needs to be a means of resolving references to
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91 symbols in other source files. These are known as external references. The
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92 addresses of these symbols cannot be known until the linker joins all the
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93 object files into a single binary. This means that the assembler must be
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94 able to output the object code without knowing the value of the symbol. This
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95 places some restrictions on the code generated by the assembler. For
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96 example, the assembler cannot generate direct page addressing for instructions
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97 that reference external symbols because the address of the symbol may not
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98 be in the direct page. Similarly, relative branches and PC relative addressing
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99 cannot be used in their eight bit forms. Everything that must be resolved
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100 by the linker must be assembled to use the largest address size possible to
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101 allow the linker to fill in the correct value at link time. Note that the
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102 same problem applies to absolute address references as well, even those in
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103 the same source file, because the address is not known until link time.</P
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104 ><P
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105 >It is often desired in multi-file projects to have code of various types grouped
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106 together in the final binary generated by the linker as well. The same applies
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107 to data. In order for the linker to do that, the bits that are to be grouped
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108 must be tagged in some manner. This is where the concept of sections comes in.
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109 Each chunk of code or data is part of a section in the object file. Then,
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110 when the linker reads all the object files, it coalesces all sections of the
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111 same name into a single section and then considers it as a unit.</P
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112 ><P
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113 >The existence of sections, however, raises a problem for symbols even
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114 within the same source file. Thus, the assembler must treat symbols from
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115 different sections within the same source file in the same manner as external
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116 symbols. That is, it must leave them for the linker to resolve at link time,
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117 with all the limitations that entails.</P
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118 ><P
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119 >In the object file target mode, LWASM requires all source lines that
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120 cause bytes to be output to be inside a section. Any directives that do
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121 not cause any bytes to be output can appear outside of a section. This includes
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122 such things as EQU or RMB. Even ORG can appear outside a section. ORG, however,
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123 makes no sense within a section because it is the linker that determines
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124 the starting address of the section's code, not the assembler.</P
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125 ><P
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126 >All symbols defined globally in the assembly process are local to the
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127 source file and cannot be exported. All symbols defined within a section are
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128 considered local to the source file unless otherwise explicitly exported.
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129 Symbols referenced from external source files must be declared external,
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130 either explicitly or by asking the assembler to assume that all undefined
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131 symbols are external.</P
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132 ><P
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133 >It is often handy to define a number of memory addresses that will be
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134 used for data at run-time but which need not be included in the binary file.
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135 These memory addresses are not initialized until run-time, either by the
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136 program itself or by the program loader, depending on the operating environment.
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137 Such sections are often known as BSS sections. LWASM supports generating
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138 sections with a BSS attribute set which causes the section definition including
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139 symbols exported from that section and those symbols required to resolve
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140 references from the local file, but with no actual code in the object file.
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141 It is illegal for any source lines within a BSS flagged section to cause any
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142 bytes to be output.</P
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143 ><P
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144 >The following directives apply to section handling.</P
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145 ><P
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146 ></P
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147 ><DIV
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148 CLASS="VARIABLELIST"
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149 ><DL
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150 ><DT
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151 >SECTION <CODE
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152 CLASS="PARAMETER"
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153 >name[,flags]</CODE
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154 >, SECT <CODE
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155 CLASS="PARAMETER"
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156 >name[,flags]</CODE
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157 ></DT
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158 ><DD
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159 ><P
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160 >Instructs the assembler that the code following this directive is to be
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161 considered part of the section <CODE
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162 CLASS="PARAMETER"
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163 >name</CODE
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164 >. A section name
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165 may appear multiple times in which case it is as though all the code from
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166 all the instances of that section appeared adjacent within the source file.
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167 However, <CODE
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168 CLASS="PARAMETER"
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169 >flags</CODE
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170 > may only be specified on the first
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171 instance of the section.</P
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172 ><P
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173 >There is a single flag supported in <CODE
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174 CLASS="PARAMETER"
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175 >flags</CODE
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176 >. The
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177 flag <CODE
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178 CLASS="PARAMETER"
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179 >bss</CODE
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180 > will cause the section to be treated as a BSS
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181 section and, thus, no code will be included in the object file nor will any
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182 bytes be permitted to be output.</P
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183 ><P
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184 >If assembly is already happening within a section, the section is implicitly
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185 ended and the new section started. This is not considered an error although
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186 it is recommended that all sections be explicitly closed.</P
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187 ></DD
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188 ><DT
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189 >ENDSECTION, ENDSECT, ENDS</DT
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190 ><DD
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191 ><P
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192 >This directive ends the current section. This puts assembly outside of any
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193 sections until the next SECTION directive.</P
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194 ></DD
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195 ><DT
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196 ><CODE
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197 CLASS="PARAMETER"
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198 >sym</CODE
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199 > EXTERN, <CODE
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200 CLASS="PARAMETER"
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201 >sym</CODE
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202 > EXTERNAL, <CODE
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203 CLASS="PARAMETER"
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204 >sym</CODE
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205 > IMPORT</DT
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206 ><DD
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207 ><P
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208 >This directive defines <CODE
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209 CLASS="PARAMETER"
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210 >sym</CODE
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211 > as an external symbol.
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212 This directive may occur at any point in the source code. EXTERN definitions
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213 are resolved on the first pass so an EXTERN definition anywhere in the
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214 source file is valid for the entire file. The use of this directive is
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215 optional when the assembler is instructed to assume that all undefined
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216 symbols are external. In fact, in that mode, if the symbol is referenced
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217 before the EXTERN directive, an error will occur.</P
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218 ></DD
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219 ><DT
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220 ><CODE
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221 CLASS="PARAMETER"
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222 >sym</CODE
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223 > EXPORT</DT
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224 ><DD
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225 ><P
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226 >This directive defines <CODE
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227 CLASS="PARAMETER"
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228 >sym</CODE
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229 > as an exported symbol.
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230 This directive may occur at any point in the source code, even before the
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231 definition of the exported symbol.</P
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232 ></DD
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233 ></DL
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234 ></DIV
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235 ></DIV
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236 ><DIV
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237 CLASS="NAVFOOTER"
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238 ><HR
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239 ALIGN="LEFT"
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240 WIDTH="100%"><TABLE
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241 SUMMARY="Footer navigation table"
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245 CELLSPACING="0"
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246 ><TR
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247 ><TD
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248 WIDTH="33%"
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249 ALIGN="left"
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250 VALIGN="top"
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251 ><A
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252 HREF="x339.html"
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253 ACCESSKEY="P"
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254 >Prev</A
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255 ></TD
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256 ><TD
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257 WIDTH="34%"
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258 ALIGN="center"
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259 VALIGN="top"
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260 ><A
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261 HREF="index.html"
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262 ACCESSKEY="H"
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263 >Home</A
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264 ></TD
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265 ><TD
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266 WIDTH="33%"
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267 ALIGN="right"
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268 VALIGN="top"
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269 ><A
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270 HREF="x407.html"
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271 ACCESSKEY="N"
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272 >Next</A
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273 ></TD
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274 ></TR
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275 ><TR
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276 ><TD
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277 WIDTH="33%"
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278 ALIGN="left"
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279 VALIGN="top"
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280 >Macros</TD
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281 ><TD
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282 WIDTH="34%"
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283 ALIGN="center"
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284 VALIGN="top"
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285 ><A
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286 HREF="c35.html"
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287 ACCESSKEY="U"
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288 >Up</A
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289 ></TD
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290 ><TD
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291 WIDTH="33%"
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292 ALIGN="right"
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293 VALIGN="top"
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294 >Assembler Modes and Pragmas</TD
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295 ></TR
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296 ></TABLE
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297 ></DIV
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298 ></BODY
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299 ></HTML
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300 > |
