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><A
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>3.8. Structures</A
></H1
><P
>&#13;Structures are used to group related data in a fixed structure. A structure
consists a number of fields, defined in sequential order and which take up
specified size.  The assembler does not enforce any means of access within a
structure; it assumes that whatever you are doing, you intended to do.
There are two pseudo ops that are used for defining structures.&#13;</P
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><CODE
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>structname</CODE
> STRUCT</DT
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>&#13;This directive is used to begin the definition of a structure with name
<CODE
CLASS="PARAMETER"
>structname</CODE
>.  Subsequent statements all form part of
the structure definition until the end of the structure is declared.&#13;</P
></DD
><DT
>ENDSTRUCT, ENDS</DT
><DD
><P
>This directive ends the definition of the structure. ENDSTRUCT is the
preferred form. Prior to version 3.0 of LWASM, ENDS was used to end a
section instead of a structure.</P
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>&#13;Within a structure definition, only reservation pseudo ops are permitted.
Anything else will cause an assembly error.</P
><P
> Once a structure is defined, you can reserve an area of memory in the
same structure by using the structure name as the opcode.  Structures can
also contain fields that are themselves structures.  See the example
below.</P
><PRE
CLASS="PROGRAMLISTING"
>tstruct2  STRUCT
f1        rmb 1
f2        rmb 1
          ENDSTRUCT

tstruct   STRUCT
field1    rmb 2
field2    rmb 3
field3    tstruct2
          ENDSTRUCT

          ORG $2000
var1      tstruct
var2      tstruct2</PRE
><P
>Fields are referenced using a dot (.) as a separator. To refer to the
generic offset within a structure, use the structure name to the left of the
dot.  If referring to a field within an actual variable, use the variable's
symbol name to the left of the dot.</P
><P
>You can also refer to the actual size of a structure (or a variable
declared as a structure) using the special symbol sizeof{structname} where
structname will be the name of the structure or the name of the
variable.</P
><P
>Essentially, structures are a shortcut for defining a vast number of
symbols.  When a structure is defined, the assembler creates symbols for the
various fields in the form structname.fieldname as well as the appropriate
sizeof{structname} symbol.  When a variable is declared as a structure, the
assembler does the same thing using the name of the variable.  You will see
these symbols in the symbol table when the assembler is instructed to
provide a listing.  For instance, the above listing will create the
following symbols (symbol values in parentheses): tstruct2.f1 (0),
tstruct2.f2 (1), sizeof{tstruct2} (2), tstruct.field1 (0), tstruct.field2
(2), tstruct.field3 (5), tstruct.field3.f1 (5), tstruct.field3.f2 (6),
sizeof{tstruct.field3} (2), sizeof{tstruct} (7), var1 {$2000}, var1.field1
{$2000}, var1.field2 {$2002}, var1.field3 {$2005}, var1.field3.f1 {$2005},
var1.field3.f2 {$2006}, sizeof(var1.field3} (2), sizeof{var1} (7), var2
($2007), var2.f1 ($2007), var2.f2 ($2008), sizeof{var2} (2).  </P
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author	William Astle <lost@l-w.ca>
date	Sun, 10 Jun 2012 14:14:43 -0600
parents	83b7b4ce3bbd
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