Structures and Records

For what purpose are structured data types included in assembly language?
A structured data type is one in which each data item is a collection of other data items. In a structured data type, the entire collection uses a single identifier (name). The purpose of structured data types is to group related data of various types for convenient access using the same identifier.

What structured data types do MASM and TASM support?
MASM and TASM support the following structured data types:
arrays;
structures;
associations;
records.

Def.1 Structure is a type of data, which consists of fixed number of elements of different characteristics.
Structures can be considered as pictures with descriptions of data formats, which may be patched (наложены) on different areas of memory with a purpose – to have a possibility for addressing to fields of these areas with help of mnemonic names, which are determined in the structure description.
Structures are especially useful in cases, when it is necessary to address for memory areas, which haven’t been included in program’s segments (i.e. to such fields, which are not possible to describe with help of symbolic names). Structures are also used in such situations, when a program includes complex collections of data, which repeat many times and have different meanings.
A data structure is a collection of data values, the relationships among them, and the functions or operations that can be applied to the data.

For using structures in the program, it is necessary:
To set a picture of the structure, i.e. to determine a new type of data, which will be used in future for determination of variables of this type;
To determine an exemplar of the structure, i.e. to initialize a certain variable with the beforehand determined (with help of the picture) structure;
To organize access (addressing) to this variable.

Structure may be described only once, but it can be determined many times.
The description of a structure has got the following syntax:
name_of_structure STRUC
name_of_structure ENDS

workers STRUC ; information concerning some worker
name DB 30 DUP (“ “); surname, name, patronymic
position DB 30 DUP (“ “)
age DW ?
salary DD ?
birthdate DB 30 DUP(“ “)
workers ENDS

data segment
Sotr1 workers <”Kozlov Susik Musikovich”, , ‘artist’,99,1000000,’01.10.1917’>
Sotr2 workers<”Frackinbok Matilda Karlsovna”,‘dancer’,18,’90000000’,’11.12.2000’>
Sotr3 workers <>; here all meanings are on default
data ends

address_expression. name_of_the_structure
sotr2.salary
[bx].age

Records.
Def. Record is a structured type of data, which consists of fixed number of elements of the size from one up to a number of bits.

The using of records in a program are organized in three stages:
Setting a picture of record, i.e. determination of bits fields, their lengths, and, if it’s necessary, initialization of the fields;
Determination of record exemplar. As it is made for structures, this stage means initialization of certain variable by type of beforehand determined record with help of picture.
Organization of addressing to the record’s elements.

The using of records in a program are organized in three stages:
Determination of record exemplar. As it is made for structures, this stage means initialization of certain variable by type of beforehand determined record with help of picture.
Organization of addressing to the record’s elements.
Setting a picture of record, i.e. determination of bits fields, their lengths, and, if it’s necessary, initialization of the fields;

Record Description.
The description of picture has got the following syntax:
name_of_record RECORD of elements>
here
is a sequence of different elements descriptions according the syntax diagram

…………
name_1 RECORD fld_1:5=8, fld_2:6=4, fld_3:2, fld_4:6=6
………
name_10 name_1?;there is no necessity to initialize fields
…………
name_1 RECORD fld_1:5=8, fld_2:1=4, fld_3:2, fld_4:1=6
………
name_10 name_1 <, ,7, >; before the third position there are “, ,”
or
…………
name_1 RECORD fld_1:5=8, fld_2:1=4, fld_3:2, fld_4:1=6
………
name_10 name_1{fld_3=7}

Work with Records.
It is not possible to use usual mechanisms for addressing to record’s elements
to each element of record the translator (assembly) puts in correspondence (assigns) a numeric value, which is equal to a number of shifts to the right;
the shift to the right is performed with help of instruction shr ;
with help of operator width it is possible to determine a size of element (or the record as a whole) in bits. There are the next variants of using this operator
width name_of_record’s_element – the meaning of this operator will be a size of the element in bits;
width name_of_record’s_examplar
or
width name_of_record’s_type
the Assembly has an operator mask, which allows to localize bits of record’s element;
all operations, concerned with transformation record’s elements are executed with help of logical instructions.
Addressing a specific field in a record is reduced to working with a separate register.
How is the Record bit-field addressed in assembler?