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Update html and pdf documentation in preparation for release
author William Astle <lost@l-w.ca>
date Fri, 03 May 2019 20:06:17 -0600
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56 >Chapter 3. LWASM</TD
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76 NAME="AEN585"
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, ENDS</DT
106 ><DD
107 ><P
108 >This directive ends the definition of the structure. ENDSTRUCT is the
109 preferred form. Prior to version 3.0 of LWASM, ENDS was used to end a
110 section instead of a structure.</P
111 ></DD
112 ></DL
113 ></DIV
114 ><P
115 >&#13;Within a structure definition, only reservation pseudo ops are permitted.
116 Anything else will cause an assembly error.</P
117 ><P
118 > Once a structure is defined, you can reserve an area of memory in the
119 same structure by using the structure name as the opcode. Structures can
120 also contain fields that are themselves structures. See the example
121 below.</P
122 ><PRE
123 CLASS="PROGRAMLISTING"
124 >tstruct2 STRUCT
125 f1 rmb 1
126 f2 rmb 1
127 ENDSTRUCT
128
129 tstruct STRUCT
130 field1 rmb 2
131 field2 rmb 3
132 field3 tstruct2
133 ENDSTRUCT
134
135 ORG $2000
136 var1 tstruct
137 var2 tstruct2</PRE
138 ><P
139 >Fields are referenced using a dot (.) as a separator. To refer to the
140 generic offset within a structure, use the structure name to the left of the
141 dot. If referring to a field within an actual variable, use the variable's
142 symbol name to the left of the dot.</P
143 ><P
144 >You can also refer to the actual size of a structure (or a variable
145 declared as a structure) using the special symbol sizeof{structname} where
146 structname will be the name of the structure or the name of the
147 variable.</P
148 ><P
149 >Essentially, structures are a shortcut for defining a vast number of
150 symbols. When a structure is defined, the assembler creates symbols for the
151 various fields in the form structname.fieldname as well as the appropriate
152 sizeof{structname} symbol. When a variable is declared as a structure, the
153 assembler does the same thing using the name of the variable. You will see
154 these symbols in the symbol table when the assembler is instructed to
155 provide a listing. For instance, the above listing will create the
156 following symbols (symbol values in parentheses): tstruct2.f1 (0),
157 tstruct2.f2 (1), sizeof{tstruct2} (2), tstruct.field1 (0), tstruct.field2
158 (2), tstruct.field3 (5), tstruct.field3.f1 (5), tstruct.field3.f2 (6),
159 sizeof{tstruct.field3} (2), sizeof{tstruct} (7), var1 {$2000}, var1.field1
160 {$2000}, var1.field2 {$2002}, var1.field3 {$2005}, var1.field3.f1 {$2005},
161 var1.field3.f2 {$2006}, sizeof(var1.field3} (2), sizeof{var1} (7), var2
162 ($2007), var2.f1 ($2007), var2.f2 ($2008), sizeof{var2} (2). </P
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