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3 ><HEAD
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4 ><TITLE
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5 >Structures</TITLE
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10 TITLE="LW Tool Chain"
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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="x473.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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64 >Next</A
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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="AEN495"
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77 >3.8. Structures</A
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78 ></H1
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79 ><P
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80 > Structures are used to group related data in a fixed structure. A structure
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81 consists a number of fields, defined in sequential order and which take up
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82 specified size. The assembler does not enforce any means of access within a
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83 structure; it assumes that whatever you are doing, you intended to do.
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84 There are two pseudo ops that are used for defining structures. </P
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85 ><P
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86 ></P
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87 ><DIV
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88 CLASS="VARIABLELIST"
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89 ><DL
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90 ><DT
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91 ><CODE
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92 CLASS="PARAMETER"
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93 >structname</CODE
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94 > STRUCT</DT
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95 ><DD
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96 ><P
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97 > This directive is used to begin the definition of a structure with name
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98 <CODE
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99 CLASS="PARAMETER"
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100 >structname</CODE
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101 >. Subsequent statements all form part of
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102 the structure definition until the end of the structure is declared. </P
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103 ></DD
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104 ><DT
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105 >ENDSTRUCT, ENDS</DT
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106 ><DD
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107 ><P
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108 >This directive ends the definition of the structure. ENDSTRUCT is the
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109 preferred form. Prior to version 3.0 of LWASM, ENDS was used to end a
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110 section instead of a structure.</P
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111 ></DD
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112 ></DL
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113 ></DIV
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114 ><P
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115 > Within a structure definition, only reservation pseudo ops are permitted.
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116 Anything else will cause an assembly error.</P
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117 ><P
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118 > Once a structure is defined, you can reserve an area of memory in the
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119 same structure by using the structure name as the opcode. Structures can
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120 also contain fields that are themselves structures. See the example
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121 below.</P
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122 ><PRE
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123 CLASS="PROGRAMLISTING"
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124 >tstruct2 STRUCT
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125 f1 rmb 1
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126 f2 rmb 1
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127 ENDSTRUCT
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128
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129 tstruct STRUCT
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130 field1 rmb 2
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131 field2 rmb 3
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132 field3 tstruct2
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133 ENDSTRUCT
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134
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135 ORG $2000
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136 var1 tstruct
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137 var2 tstruct2</PRE
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138 ><P
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139 >Fields are referenced using a dot (.) as a separator. To refer to the
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140 generic offset within a structure, use the structure name to the left of the
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141 dot. If referring to a field within an actual variable, use the variable's
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142 symbol name to the left of the dot.</P
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143 ><P
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144 >You can also refer to the actual size of a structure (or a variable
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145 declared as a structure) using the special symbol sizeof{structname} where
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146 structname will be the name of the structure or the name of the
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147 variable.</P
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148 ><P
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149 >Essentially, structures are a shortcut for defining a vast number of
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150 symbols. When a structure is defined, the assembler creates symbols for the
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151 various fields in the form structname.fieldname as well as the appropriate
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152 sizeof{structname} symbol. When a variable is declared as a structure, the
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153 assembler does the same thing using the name of the variable. You will see
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154 these symbols in the symbol table when the assembler is instructed to
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155 provide a listing. For instance, the above listing will create the
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156 following symbols (symbol values in parentheses): tstruct2.f1 (0),
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157 tstruct2.f2 (1), sizeof{tstruct2} (2), tstruct.field1 (0), tstruct.field2
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158 (2), tstruct.field3 (5), tstruct.field3.f1 (5), tstruct.field3.f2 (6),
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159 sizeof{tstruct.field3} (2), sizeof{tstruct} (7), var1 {$2000}, var1.field1
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160 {$2000}, var1.field2 {$2002}, var1.field3 {$2005}, var1.field3.f1 {$2005},
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161 var1.field3.f2 {$2006}, sizeof(var1.field3} (2), sizeof{var1} (7), var2
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162 ($2007), var2.f1 ($2007), var2.f2 ($2008), sizeof{var2} (2). </P
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163 ></DIV
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