Structures

+Structures + + +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. + + + + + +structname STRUCT + + + +This directive is used to begin the definition of a structure with name +structname. Subsequent statements all form part of +the structure definition until the end of the structure is declared. + + + + + +ENDSTRUCT + + +This directive ends the definition of the structure. + + + + + + + +Within a structure definition, only reservation pseudo ops are permitted. +Anything else will cause an assembly error. + + + 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. + + +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 + + +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. + +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. + +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). + +