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date Fri, 23 Jul 2010 17:18:52 -0600
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77 >3.8. Structures</A
78 ></H1
79 ><P
80 >&#13;Structures are used to group related data in a fixed structure. A structure
81 consists a number of fields, defined in sequential order and which take up
82 specified size. The assembler does not enforce any means of access within a
83 structure; it assumes that whatever you are doing, you intended to do.
84 There are two pseudo ops that are used for defining structures.&#13;</P
85 ><P
86 ></P
87 ><DIV
88 CLASS="VARIABLELIST"
89 ><DL
90 ><DT
91 ><CODE
92 CLASS="PARAMETER"
93 >structname</CODE
94 > STRUCT</DT
95 ><DD
96 ><P
97 >&#13;This directive is used to begin the definition of a structure with name
98 <CODE
99 CLASS="PARAMETER"
100 >structname</CODE
101 >. Subsequent statements all form part of
102 the structure definition until the end of the structure is declared.&#13;</P
103 ></DD
104 ><DT
105 >ENDSTRUCT</DT
106 ><DD
107 ><P
108 >This directive ends the definition of the structure.</P
109 ></DD
110 ></DL
111 ></DIV
112 ><P
113 >&#13;Within a structure definition, only reservation pseudo ops are permitted.
114 Anything else will cause an assembly error.</P
115 ><P
116 > Once a structure is defined, you can reserve an area of memory in the
117 same structure by using the structure name as the opcode. Structures can
118 also contain fields that are themselves structures. See the example
119 below.</P
120 ><PRE
121 CLASS="PROGRAMLISTING"
122 >tstruct2 STRUCT
123 f1 rmb 1
124 f2 rmb 1
125 ENDSTRUCT
126
127 tstruct STRUCT
128 field1 rmb 2
129 field2 rmb 3
130 field3 tstruct2
131 ENDSTRUCT
132
133 ORG $2000
134 var1 tstruct
135 var2 tstruct2</PRE
136 ><P
137 >Fields are referenced using a dot (.) as a separator. To refer to the
138 generic offset within a structure, use the structure name to the left of the
139 dot. If referring to a field within an actual variable, use the variable's
140 symbol name to the left of the dot.</P
141 ><P
142 >You can also refer to the actual size of a structure (or a variable
143 declared as a structure) using the special symbol sizeof{structname} where
144 structname will be the name of the structure or the name of the
145 variable.</P
146 ><P
147 >Essentially, structures are a shortcut for defining a vast number of
148 symbols. When a structure is defined, the assembler creates symbols for the
149 various fields in the form structname.fieldname as well as the appropriate
150 sizeof{structname} symbol. When a variable is declared as a structure, the
151 assembler does the same thing using the name of the variable. You will see
152 these symbols in the symbol table when the assembler is instructed to
153 provide a listing. For instance, the above listing will create the
154 following symbols (symbol values in parentheses): tstruct2.f1 (0),
155 tstruct2.f2 (1), sizeof{tstruct2} (2), tstruct.field1 (0), tstruct.field2
156 (2), tstruct.field3 (5), tstruct.field3.f1 (5), tstruct.field3.f2 (6),
157 sizeof{tstruct.field3} (2), sizeof{tstruct} (7), var1 {$2000}, var1.field1
158 {$2000}, var1.field2 {$2002}, var1.field3 {$2005}, var1.field3.f1 {$2005},
159 var1.field3.f2 {$2006}, sizeof(var1.field3} (2), sizeof{var1} (7), var2
160 ($2007), var2.f1 ($2007), var2.f2 ($2008), sizeof{var2} (2). </P
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