4.29 Records

4.29 Records

Another major composite data structure is the Pascal record or C/C++ structure.^[30] The Pascal terminology is probably better, because it tends to avoid confusion with the more general term data structure. Because HLA uses the term "record" we'll adopt that term here.

Whereas an array is homogeneous, whose elements are all the same, the elements in a record can have different types. Arrays let you select a particular element via an integer index. With records, you must select an element (known as a field) by name.

The whole purpose of a record is to let you encapsulate different, but logically related, data into a single package. The Pascal record declaration for a student is a typical example:

 student =      record           Name: string [64];           Major: integer;           SSN: string[11];           Midterm1: integer;           Midterm2: integer;           Final: integer;           Homework: integer;           Projects: integer;      end; 

Most Pascal compilers allocate each field in a record to contiguous memory locations. This means that Pascal will reserve the first 65 bytes for the name^[31]; the next two bytes hold the major code, the next 12 the Social Security Number, and so on.

In HLA, you can also create structure types using the record/endrecord declaration. You would encode the above record in HLA as follows:

 type      student:   xrecord           Name: char[65];           Major: int16;           SSN: char[12];           Midterm1: int16;           Midterm2: int16;           Final: int16;           Homework: int16;           Projects: int16; endrecord; 

As you can see, the HLA declaration is very similar to the Pascal declaration. Note that, to be true to the Pascal declaration, this example uses character arrays rather than strings for the Name and SSN (U.S. Social Security Number) fields. In a real HLA record declaration you'd probably use a string type for at least the name (keeping in mind that a string variable is only a four-byte pointer).

The field names within the record must be unique. That is, the same name may not appear two or more times in the same record. However, all field names are local to that record. Therefore, you may reuse those field names elsewhere in the program or in different records.

The record/endrecord declaration may appear in a variable declaration section (e.g., static or var) or in a type declaration section. In the previous example the Student declaration appears in the type section, so this does not actually allocate any storage for a Student variable. Instead, you have to explicitly declare a variable of type Student. The following example demonstrates how to do this:

 var      John: Student; 

This allocates 81 bytes of storage laid out in memory as shown in Figure 4-10.

Figure 4-10: Student Data Structure Storage in Memory.

If the label John corresponds to the base address of this record, then the Name field is at offset John+0, the Major field is at offset John+65, the SSN field is at offset John+67, and so on.

To access an element of a structure you need to know the offset from the beginning of the structure to the desired field. For example, the Major field in the variable John is at offset 65 from the base address of John. Therefore, you could store the value in AX into this field using the instruction

 mov( ax, (type word John[65]) ); 

Unfortunately, memorizing all the offsets to fields in a record defeats the whole purpose of using them in the first place. After all, if you've got to deal with these numeric offsets why not just use an array of bytes instead of a record?

Well, as it turns out, HLA lets you refer to field names in a record using the same mechanism C/C++/C# and Pascal use: the dot operator. To store AX into the Major field, you could use "mov( ax, John.Major );" instead of the previous instruction. This is much more readable and certainly easier to use.

Note that the use of the dot operator does not introduce a new addressing mode. The instruction "mov( ax, John.Major );" still uses the displacement only addressing mode. HLA simply adds the base address of John with the offset to the Major field (65) to get the actual displacement to encode into the instruction.

Like any type declaration, HLA requires all record type declarations to appear in the program before you use them. However, you don't have to define all records in the type section to create record variables. You can use the record/endrecord declaration directly in a variable declaration section. This is convenient if you have only one instance of a given record object in your program. The following example demonstrates this:

 storage      OriginPoint:      record           x: uns8;           y: uns8;           z: uns8;      endrecord; 

^[30]It also goes by some other names in other languages, but most people recognize at least one of these names.

^[31]Strings require an extra byte, in addition to all the characters in the string, to encode the length.