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Chapter 5

Defining and Using Complex
Data Types

With the complex data types available in MASM 6.1 - arrays, strings, records, structures, and unions - you can access data as a unit or as individual elements that make up a unit. The individual elements of complex data types are often the integer types discussed in Chapter 4, "Defining and Using Simple Data Types."

"Arrays and Strings" reviews how to declare, reference, and initialize arrays and strings. This section summarizes the general steps needed to process arrays and strings and describes the MASM instructions for moving, comparing, searching, loading, and storing.

"Structures and Unions" covers similar information for structures and unions: how to declare structure and union types, how to define structure and union variables, and how to reference structures and unions and their fields.

"Records" explains how to declare record types, define record variables, and use record operators.

Arrays and Strings

An array is a sequential collection of variables, all of the same size and type, called "elements." A string is an array of characters. For example, in the string "ABC," each letter is an element. You can access the elements in an array or string relative to the first element. This section explains how to handle arrays and strings in your programs.

Declaring and Referencing Arrays


Array elements occupy memory contiguously, so a program references each element relative to the start of the array. To declare an array, supply a label name, the element type, and a series of initializing values or ? placeholders. The following examples declare the arrays warray and xarray:

warray WORD 1, 2, 3, 4


Initializer lists of array declarations can span multiple lines. The first initializer must appear on the same line as the data type, all entries must be initialized, and, if you want the array to continue to the new line, the line must end with a comma. These examples show legal multiple-line array declarations:

big BYTE 21, 22, 23, 24, 25,
26, 27, 28

somelist WORD 10,

If you do not use the LENGTHOF and SIZEOF operators discussed later in this section, an array may span more than one logical line, although a separate type declaration is needed on each logical line:

var1 BYTE 10, 20, 30
BYTE 40, 50, 60
BYTE 70, 80, 90

The DUP Operator

You can also declare an array with the DUP operator. This operator works with any of the data allocation directives described in "Allocating Memory for Integer Variables" in Chapter 4. In the syntax

count DUP (initialvalue [[, initialvalue]]...)

the count value sets the number of times to repeat all values within the parentheses. The initialvalue can be an integer, character constant, or another DUP operator, and must always appear within parentheses. For example, the statement

barray BYTE 5 DUP (1)

allocates the integer 1 five times for a total of 5 bytes.

The following examples show various ways to allocate data elements with the DUP operator:

array DWORD 10 DUP (1) ; 10 doublewords
; initialized to 1
buffer BYTE 256 DUP (?) ; 256-byte buffer

masks BYTE 20 DUP (040h, 020h, 04h, 02h) ; 80-byte buffer
; with bit masks
three_d DWORD 5 DUP (5 DUP (5 DUP (0))) ; 125 doublewords
; initialized to 0

Referencing Arrays

Each element in an array is referenced with an index number, beginning with zero. The array index appears in brackets after the array name, as in


Assembly-language indexes differ from indexes in high-level languages, where the index number always corresponds to the element’s position. In C, for example, array[9] references the array’s tenth element, regardless of whether each element is 1 byte or 8 bytes in size.

In assembly language, an element’s index refers to the number of bytes between the element and the start of the array. This distinction can be ignored for arrays of byte-sized elements, since an element’s position number matches its index. For example, defining the array

prime BYTE 1, 3, 5, 7, 11, 13, 17

gives a value of 1 to prime[0], a value of 3 to prime[1], and so forth.

However, in arrays with elements larger than 1 byte, index numbers (except zero) do not correspond to an element’s position. You must multiply an element’s position by its size to determine the element’s index. Thus, for the array

wprime WORD 1, 3, 5, 7, 11, 13, 17

wprime[4] represents the third element (5), which is 4 bytes from the beginning of the array. Similarly, the expression wprime[6] represents the fourth element (7) and wprime[10] represents the sixth element (13).

The following example determines an index at run time. It multiplies the position by two (the size of a word element) by shifting it left:

mov si, cx ; CX holds position number
shl si, 1 ; Scale for word referencing
mov ax, wprime[si] ; Move element into AX

The offset required to access an array element can be calculated with the following formula:

nth element of array = array[(n-1) * size of element]

Referencing an array element by distance rather than position is not difficult to master, and is actually very consistent with how assembly language works. Recall that a variable name is a symbol that represents the contents of a particular address in memory. Thus, if the array wprime begins at address DS:2400h, the reference wprime[6] means to the processor "the word value contained in the DS segment at offset 2400h-plus-6-bytes."

As described in "Direct Memory Operands," Chapter 3, you can substitute the plus operator (+) for brackets, as in:


Since brackets simply add a number to an address, you don’t need them when referencing the first element. Thus, wprime and wprime[0] both refer to the first element of the array wprime.

If your program runs only on an 80186 processor or higher, you can use the BOUND instruction to verify that an index value is within the bounds of an array. For a description of BOUND, see the Reference.


When applied to arrays, the LENGTHOF, SIZEOF, and TYPE operators return information about the length and size of the array and about the type of the

The LENGTHOF operator returns the number of elements in the array. The SIZEOF operator returns the number of bytes used by the initializers in the array definition. TYPE returns the size of the elements of the array. The following examples illustrate these operators:

array WORD 40 DUP (5)

larray EQU LENGTHOF array ; 40 elements
sarray EQU SIZEOF array ; 80 bytes
tarray EQU TYPE array ; 2 bytes per element

num DWORD 4, 5, 6, 7, 8, 9, 10, 11

lnum EQU LENGTHOF num ; 8 elements
snum EQU SIZEOF num ; 32 bytes
tnum EQU TYPE num ; 4 bytes per element

warray WORD 40 DUP (40 DUP (5))

len EQU LENGTHOF warray ; 1600 elements
siz EQU SIZEOF warray ; 3200 bytes
typ EQU TYPE warray ; 2 bytes per element

Declaring and Initializing Strings

A string is an array of characters. Initializing a string like "Hello, there" allocates and initializes 1 byte for each character in the string. An initialized string can be no longer than 255 characters.

For data directives other than BYTE, a string may initialize only the first element. The initializer value must fit into the specified size and conform to the expression word size in effect (see "Integer Constants and Constant Expressions" in Chapter 1), as shown in these examples:

wstr WORD "OK"
dstr DWORD "DATA" ; Legal under EXPR32 only

As with arrays, string initializers can span multiple lines. The line must end with a comma if you want the string to continue to the next line.

str1 BYTE "This is a long string that does not ",
"fit on one line."

You can also have an array of pointers to strings.

msg1 BYTE "Operation completed successfully."
msg2 BYTE "Unknown command"
msg3 BYTE "File not found"
pmsg PBYTE msg1 ; pmsg is an array
PBBYTE msg2 ; of pointers to
PBYTE msg3 ; above messages

Strings must be enclosed in single (') or double (") quotation marks. To put a single quotation mark inside a string enclosed by single quotation marks, use two single quotation marks. Likewise, if you need quotation marks inside a string enclosed by double quotation marks, use two sets. These examples show the various uses of quotation marks:

char BYTE 'a'
message BYTE "That's the message." ; That's the message.
warn BYTE 'Can''t find file.' ; Can't find file.
string BYTE "This ""value"" not found." ; This "value" not found.

You can always use single quotation marks inside a string enclosed by double quotation marks, as the initialization for message shows, and vice versa.

The ? Initializer

You do not have to initialize an array. The ? operator lets you allocate space for the array without placing specific values in it. Object files contain records for initialized data. Unspecified space left in the object file means that no records contain initialized data for that address. The actual values stored in arrays allocated with ? depend on certain conditions. The ? initializer is treated as a zero in a DUP statement that contains initializers in addition to the ? initializer. If the ? initializer does not appear in a DUP statement, or if the DUP statement contains only ? initializers, the assembler leaves the allocated space unspecified.

LENGTHOF, SIZEOF, and TYPE for Strings

Because strings are simply arrays of byte elements, the LENGTHOF, SIZEOF, and TYPE operators behave as you would expect, as illustrated in this example:

msg BYTE "This string extends ",
"over three ",

lmsg EQU LENGTHOF msg ; 37 elements
smsg EQU SIZEOF msg ; 37 bytes
tmsg EQU TYPE msg ; 1 byte per element

Processing Strings

The 8086-family instruction set has seven string instructions for fast and efficient processing of entire strings and arrays. The term "string" in "string instructions" refers to a sequence of elements, not just character strings. These instructions work directly only on arrays of bytes and words on the 8086-80486 processors, and on arrays of bytes, words, and doublewords on the 80386/486 processors. Processing larger elements must be done indirectly with loops.

The following list gives capsule descriptions of the five instructions discussed in this section.

Instruction Description

MOVS Copies a string from one location to another

STOS Stores contents of the accumulator register to a string

CMPS Compares one string with another

LODS Loads values from a string to the accumulator register

SCAS Scans a string for a specified value


All of these instructions use registers in a similar way and have a similar syntax. Most are used with the repeat instruction prefixes REP, REPE (or REPZ), and REPNE (or REPNZ). REPZ is a synonym for REPE (Repeat While Equal) and REPNZ is a synonym for REPNE (Repeat While Not Equal).

This section first explains the general procedures for using all string instructions. It then illustrates each instruction with an example.

Overview of String Instructions

The string instructions have specific requirements for the location of strings and the use of registers. To operate on any string, follow these three steps:

1. Set the direction flag to indicate the direction in which you want to process the string. The STD instruction sets the flag, while CLD clears it.

If the direction flag is clear, the string is processed upward (from low addresses to high addresses, which is from left to right through the string). If the direction flag is set, the string is processed downward (from high addresses to low addresses, or from right to left). Under MS-DOS, the direction flag is normally clear if your program has not changed it.

2. Load the number of iterations for the string instruction into the CX register.

If you want to process 100 elements in a string, move 100 into CX. If you wish the string instruction to terminate conditionally (for example, during a search when a match is found), load the maximum number of iterations that can be performed without an error.

3. Load the starting offset address of the source string into DS:SI and the starting address of the destination string into ES:DI. Some string instructions take only a destination or source, not both (see Table 5.1).

Normally, the segment address of the source string should be DS, but you can use a segment override to specify a different segment for the source operand. You cannot override the segment address for the destination string. Therefore, you may need to change the value of ES. For information on changing segment registers, see "Programming Segmented Addresses" in Chapter 3.



Although you can use a segment override on the source operand, a segment override combined with a repeat prefix can cause problems in certain situations on all processors except the 80386/486. If an interrupt occurs during the string operation, the segment override is lost and the rest of the string operation processes incorrectly. Segment overrides can be used safely when interrupts are turned off or with the 80386/486 processors.


You can adapt these steps to the requirements of any particular string operation. The syntax for the string instructions is:

[[prefix]] CMPS [[segmentregister:]] source, [[ES:]] destination
LODS [[segmentregister:]] source
[[prefix]] MOVS [[ES:]] destination, [[segmentregister:]] source
[[prefix]] SCAS [[ES:]] destination
[[prefix]] STOS [[ES:]] destination

Some instructions have special forms for byte, word, or doubleword operands. If you use the form of the instruction that ends in B (BYTE), W (WORD), or D (DWORD) with LODS, SCAS, and STOS, the assembler knows whether the element is in the AL, AX, or EAX register. Therefore, these instruction forms do not require operands.


Table 5.1 lists each string instruction with the type of repeat prefix it uses and indicates whether the instruction works on a source, a destination, or both.

Table 5.1 Requirements for String Instructions

Instruction Repeat Prefix Source/Destination Register Pair




LODS None Source DS:SI

STOS REP Destination ES:DI

INS REP Destination ES:DI



The repeat prefix causes the instruction that follows it to repeat for the number of times specified in the count register or until a condition becomes true. After each iteration, the instruction increments or decrements SI and DI so that it points to the next array element. The direction flag determines whether SI and DI are incremented (flag clear) or decremented (flag set). The size of the instruction determines whether SI and DI are altered by 1, 2, or 4 bytes each time.

Each prefix governs the number of repetitions as follows:

Prefix Description

REP Repeats instruction CX times

REPE, REPZ Repeats instruction maximum CX times while values are equal

REPNE, REPNZ Repeats instruction maximum CX times while values are not equal


The prefixes apply to only one string instruction at a time. To repeat a block of instructions, use a loop construction. (See "Loops" in Chapter 7.)

At run time, if a string instruction is preceded by a repeat sequence, the processor:

1. Checks the CX register and exits if CX is 0.

2. Performs the string operation once.

3. Increases SI and/or DI if the direction flag is clear. Decreases SI and/or DI if the direction flag is set. The amount of increase or decrease is 1 for byte operations, 2 for word operations, and 4 for doubleword operations.

4. Decrements CX without modifying the flags.

5. Checks the zero flag (for SCAS or CMPS) if the REPE or REPNE prefix is used. If the repeat condition holds, loops back to step 1. Otherwise, the loop ends and execution proceeds to the next instruction.


When the repeat loop ends, SI (or DI) points to the position following a match (when using SCAS or CMPS), so you need to decrement or increment DI or SI to point to the element where the last match occurred.

Although string instructions (except LODS) are used most often with repeat prefixes, they can also be used by themselves. In these cases, the SI and/or DI registers are adjusted as specified by the direction flag and the size of operands.

Using String Instructions

To use the 8086-family string instructions, follow the steps outlined in the previous section. Examples in this section illustrate each instruction.

You can also use the techniques in this section with structures and unions, since arrays and strings can be fields in structures and unions. (See the section "Structures and Unions," following.)

Moving Array Data

The MOVS instruction copies data from one area of memory to another. To move data, first load the count, source and destination addresses into the appropriate registers. Then use REP with the MOVS instruction.

.MODEL small
source BYTE 10 DUP ('0123456789')
destin BYTE 100 DUP (?)
mov ax, @data ; Load same segment
mov ds, ax ; to both DS
mov es, ax ; and ES
cld ; Work upward
mov cx, LENGTHOF source ; Set iteration count to 100
mov si, OFFSET source ; Load address of source
mov di, OFFSET destin ; Load address of destination
rep movsb ; Move 100 bytes

Filling Arrays

The STOS instruction stores a specified value in each position of a string. The string is the destination, so it must be pointed to by ES:DI. The value to store must be in the accumulator.

The next example stores the character 'a' in each byte of a 100-byte string, filling the entire string with "aaaa...." Notice how the code stores 50 words rather than

100 bytes. This makes the fill operation faster by reducing the number of iterations. To fill an odd number of bytes, you need to adjust for the last byte.

.MODEL small, C
destin BYTE 100 DUP (?)
ldestin EQU (LENGTHOF destin) / 2
. ; Assume ES = DS
cld ; Work upward
mov ax, 'aa' ; Load character to fill
mov cx, ldestin ; Load length of string
mov di, OFFSET destin ; Load address of destination
rep stosw ; Store 'aa' into array

Comparing Arrays

The CMPS instruction compares two strings and points to the address after which a match or nonmatch occurs. If the values are the same, the zero flag is set. Either string can be considered the destination or the source unless a segment override is used. This example using CMPSB assumes that the strings are in different segments. Both segments must be initialized to the appropriate segment register.

.MODEL large, C
string1 BYTE "The quick brown fox jumps over the lazy dog"
string2 BYTE "The quick brown dog jumps over the lazy fox"
lstring EQU LENGTHOF string2
mov ax, @data ; Load data segment
mov ds, ax ; into DS
mov ax, @fardata ; Load far data segment
mov es, ax ; into ES
cld ; Work upward
mov cx, lstring ; Load length of string
mov si, OFFSET string1 ; Load offset of string1
mov di, OFFSET string2 ; Load offset of string2
repe cmpsb ; Compare
je allmatch ; Jump if all match
allmatch: ; Special case for all match

Loading Data from Arrays

The LODS instruction loads a value from a string into the accumulator register. This instruction is not used with a repeat instruction prefix, since continually reloading the accumulator serves no purpose.

The code in this example loads, processes, and displays each byte in a string.

info BYTE 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
linfo WORD LENGTHOF info
cld ; Work upward
mov cx, linfo ; Load length
mov si, OFFSET info ; Load offset of source
mov ah, 2 ; Display character function

lodsb ; Get a character
add al, '0' ; Convert to ASCII
mov dl, al ; Move to DL
int 21h ; Call DOS to display character
loop get ; Repeat

Searching Arrays

The SCAS instruction compares the value pointed to by ES:DI with the value in the accumulator. If both values are the same, it sets the zero flag.

A repeat prefix lets SCAS work on an entire string, scanning (from which SCAS gets its name) for a particular value called the target. REPNE SCAS sets the zero flag if it finds the target value in the array. REPE SCAS sets the zero flag if the scanned array contains nothing but the target value.


This example assumes that ES is not the same as DS and that the address of the string is stored in a pointer variable. The LES instruction loads the far address of the string into ES:DI.

string BYTE "The quick brown fox jumps over the lazy dog"
pstring PBYTE string ; Far pointer to string
lstring EQU LENGTHOF string ; Length of string
cld ; Work upward
mov cx, lstring ; Load length of string
les di, pstring ; Load address of string
mov al, 'z' ; Load character to find
repne scasb ; Search
jne notfound ; Jump if not found
. ; ES:DI points to character
. ; after first 'z'
notfound: ; Special case for not found

Translating Data in Byte Arrays

The XLAT (Translate) instruction copies a byte from an array of bytes into the AL register. The instruction takes its name from its ability to translate an element’s number into the element itself. For example, given the number 7, XLAT returns byte #7 from the array. The array may hold byte-sized integers or, very often, a table or list of characters. The syntax for XLAT is:

XLAT[[B]] [[[[segment:]]memory]]

The optional B suffix (for "byte") reflects the size of data the instruction handles. Both XLAT and XLATB assemble to exactly the same machine code.

To use XLAT, place the offset of the start of the array in the BX register and the desired index value in AL. Array indexes always begin with 0 in assembly language. To retrieve the first byte of the array, set AL to 0; to retrieve the second byte, set AL to 1, and so forth. XLAT returns the byte element in AL, overwriting the index number.

By default, the DS register contains the segment of the table, but you can use a segment override to specify a different segment. You need not give an operand except when specifying a segment override. (For information about the segment override operator, see "Direct Memory Operands" in Chapter 3.)

This example illustrates XLAT by looking up hexadecimal characters in a list. The code converts an eight-bit binary number to a string representing a hexadecimal number.

; Table of hexadecimal digits
hex BYTE "0123456789ABCDEF"
convert BYTE "You pressed the key with ASCII code "
key BYTE ?,?,"h",13,10,"$"
mov ah, 8 ; Get a key in AL
int 21h ; Call DOS
mov bx, OFFSET hex ; Load table address
mov ah, al ; Save a copy in high byte
and al, 00001111y ; Mask out top character
xlat ; Translate
mov key[1], al ; Store the character
mov cl, 12 ; Load shift count
shr ax, cl ; Shift high char into position
xlat ; Translate
mov key, al ; Store the character
mov dx, OFFSET convert ; Load message
mov ah, 9 ; Display character
int 21h ; Call DOS

Although AL cannot contain an index value greater than 255, you can use XLAT with arrays containing more than 256 elements. Simply treat each 256-byte block of the array as a smaller sub-array. For example, to retrieve the 260th element of an array, add 256 to BX and set AL=3 (260-256-1).

Structures and Unions

A structure is a group of possibly dissimilar data types and variables that can be accessed as a unit or by any of its components. The fields within the structure can have different sizes and data types.

Unions are identical to structures, except that the fields of a union overlap in memory, which allows you to define different data formats for the same memory space. Unions can store different types of data depending on the situation. They also can store data as one data type and retrieve it as another data type.

Whereas each field in a structure has an offset relative to the first byte of the structure, all the fields in a union start at the same offset. The size of a structure is the sum of its components; the size of a union is the length of the longest field.

A MASM structure is similar to a struct in the C language, a STRUCTURE in FORTRAN, and a RECORD in Pascal. Unions in MASM are similar to unions in C and FORTRAN, and to variant records in Pascal.

Follow these steps when using structures and unions:

1. Declare a structure (or union) type.

2. Define one or more variables having that type.

3. Reference the fields directly or indirectly with the field (dot) operator.


You can use the entire structure or union variable or just the individual fields as operands in assembler statements. This section explains the allocating, initializing, and nesting of structures and unions.

MASM 6.1 extends the functionality of structures and also makes some changes to MASM 5.1 behavior. If you prefer, you can retain MASM 5.1 behavior by specifying OPTION OLDSTRUCTS in your program.

Declaring Structure and Union Types

When you declare a structure or union type, you create a template for data. The template states the sizes and, optionally, the initial values in the structure or union, but allocates no memory.

The STRUCT keyword marks the beginning of a type declaration for a structure. (STRUCT and STRUC are synonyms.) The format for STRUCT and UNION type declarations is:

name {STRUCT | UNION} [[alignment]] [[,NONUNIQUE ]]

The fielddeclarations is a series of one or more variable declarations. You can declare default initial values individually or with the DUP operator. (See "Defining Structure and Union Variables," following.) "Referencing Structures, Unions, and Fields," later in this chapter, explains the NONUNIQUE keyword. You can nest structures and unions, as explained in "Nested Structures and Unions," also later in this chapter.

Initializing Fields

If you provide initializers for the fields of a structure or union when you declare the type, these initializers become the default value for the fields when you define a variable of that type. "Defining Structure and Union Variables," following, explains default initializers.

When you initialize the fields of a union type, the type and value of the first field become the default value and type for the union. In this example of an initialized union declaration, the default type for the union is DWORD:

w WORD ?
b BYTE ?

If the size of the first member is less than the size of the union, the assembler initializes the rest of the union to zeros. When initializing strings in a type, make sure the initial values are long enough to accommodate the largest possible string.

Field Names

Structure and union field names must be unique within a nesting level because they represent the offset from the beginning of the structure to the corresponding field.

A label elsewhere in the code may have the same name as a structure field, but a text macro cannot. Also, field names between structures need not be unique. Field names must be unique if you place OPTION M510 or OPTION OLDSTRUCTS in your code or use the /Zm option from the command line, since versions of MASM prior to 6.0 require unique field names. (See Appendix A.)

Alignment Value and Offsets for Structures

Data access to structures is faster on aligned fields than on unaligned fields. Therefore, alignment gains speed at the cost of space. Alignment improves access on 16-bit and 32-bit processors but makes no difference in programs executing on an 8-bit 8088 processor.

The way the assembler aligns structure fields determines the amount of space required to store a variable of that type. Each field in a structure has an offset relative to 0. If you specify an alignment in the structure declaration (or with the /Zpn command-line option), the offset for each field may be modified by the alignment (or n).

The only values accepted for alignment are 1, 2, and 4. The default is 1. If the type declaration includes an alignment, each field is aligned to either the field’s size or the alignment value, whichever is less. If the field size in bytes is greater than the alignment value, the field is padded so that its offset is evenly divisible by the alignment value. Otherwise, the field is padded so that its offset is evenly divisible by the field size.

Any padding required to reach the correct offset for the field is added prior to allocating the field. The padding consists of zeros and always precedes the aligned field. The size of the structure must also be evenly divisible by the structure alignment value, so zeros may be added at the end of the structure.

If neither the alignment nor the /Zp command-line option is used, the offset is incremented by the size of each data directive. This is the same as a default alignment equal to 1. The alignment specified in the type declaration overrides the /Zp command-line option.

These examples show how the assembler determines offsets:

STUDENT2 STRUCT 2 ; Alignment value is 2
score WORD 1 ; Offset = 0
id BYTE 2 ; Offset = 2 (1 byte padding added)
year DWORD 3 ; Offset = 4
sname BYTE 4 ; Offset = 8 (1 byte padding added)

One byte of padding is added at the end of the first byte-sized field. Otherwise, the offset of the year field would be 3, which is not divisible by the alignment value of 2. The size of this structure is now 9 bytes. Since 9 is not evenly divisible by 2, 1 byte of padding is added at the end of student2.

STUDENT4 STRUCT 4 ; Alignment value is 4
sname BYTE 1 ; Offset = 0 (1 byte padding added)
score WORD 10 DUP (100) ; Offset = 2
year BYTE 2 ; Offset = 22 (1 byte padding
; added so offset of next field
; is divisible by 4)
id DWORD 3 ; Offset = 24

The alignment value affects the alignment of structure variables, so adding an alignment value affects memory usage. This feature provides compatibility with structures in Microsoft C. MASM 6.1 provides an improved H2INC utility, which C programmers can use to translate C structures to assembly. (See Environment and Tools, Chapter 20.)

The ALIGN, EVEN, and ORG directives can modify how field offsets are placed during structure definition. The EVEN and ALIGN directives insert padding bytes to round the field offset up to the specified alignment boundary. The ORG directive changes the offset of the next field to a given value, either positive or negative. If you use ORG when declaring a structure, you cannot define a structure of that type. ORG is useful when accessing existing data structures, such as a stack frame created by a high-level language.


Defining Structure and Union Variables

Once you have declared a structure or union type, you can define variables of that type. For each variable defined, memory is allocated in the current segment in the format declared by the type. The syntax for defining a structure or union variable is:

[[name]] typename < [[initializer [[,initializer]]...]] >

[[name]] typename { [[initializer [[,initializer]]...]] }

[[name]] typename constant DUP ({ [[initializer [[,initializer]]...]] })

The name is the label assigned to the variable. If you do not provide a name, the assembler allocates space for the variable but does not give it a symbolic name. The typename is the name of a previously declared structure or union type.

You can give an initializer for each field. Each initializer must correspond in type with the field defined in the type declaration. For unions, the type of the initializer must be the same as the type for the first field. An initialization list can also use the DUP operator.

The list of initializers can be broken only after a comma unless you end the line with a continuation character (\). The last curly brace or angle bracket must appear on the same line as the last initializer. You can also use the line continuation character to extend a line as shown in the Item4 declaration that follows. Angle brackets and curly braces can be intermixed in an initialization as long as they match. This example illustrates the options for initializing lists in structures of type ITEMS:

Iname BYTE 'Item Name'
Inum WORD ?
UNION ITYPE ; UNION keyword appears first
oldtype BYTE 0 ; when nested in structure.
newtype WORD ? ; (See "Nested Structures
ENDS ; and Unions," following ).
Item1 ITEMS < > ; Accepts default initializers
Item2 ITEMS { } ; Accepts default initializers
Item3 ITEMS <'Bolts', 126> ; Overrides default value of first
; 2 fields; use default of
; the third field
Item4 ITEMS { \
'Bolts', ; Item name
126 \ ; Part number

The example defines - that is, allocates space for - four structures of the ITEMS type. The structures are named Item1 through Item4. Each definition requires the angle brackets or curly braces even when not initialized. If you initialize more than one field, separate the values with commas, as shown in Item3 and Item4.

You need not initialize all fields in a structure. If a field is blank, the assembler uses the structure’s initial value given for that field in the declaration. If there is no default value, the field value is left unspecified.

For nested structures or unions, however, these are equivalent:

Item5 ITEMS {'Bolts', , }
Item6 ITEMS {'Bolts', , { } }

A variable and an array of union type WB look like this:

w WORD ?
b BYTE ?

num WB {0Fh} ; Store 0Fh
array WB (40 / SIZEOF WB) DUP ({2}) ; Allocates and
; initializes 20 unions

Arrays as Field Initializers

The size of the initializer determines the length of the array that can override the contents of a field in a variable definition. The override cannot contain more elements than the default. Specifying fewer override array elements changes the first n values of the default where n is the number of values in the override. The rest of the array elements take their default values from the initializer.

Strings as Field Initializers

If the override is shorter, the assembler pads the override with spaces to equal the length of the initializer. If the initializer is a string and the override value is not a string, the override value must be enclosed in angle brackets or curly braces.

A string can override any member of type BYTE (or SBYTE). You need not enclose the string in angle brackets or curly braces unless mixed with other override methods.

If a structure has an initialized string field or an array of bytes, any new string assigned to a variable of the field that is smaller than the default is padded with spaces. The assembler adds four spaces at the end of 'Bolts' in the variables of type ITEMS previously shown. The Iname field in the ITEMS structure cannot contain a field initializer longer than 'Item Name'.

Structures as Field Initializers

Initializers for structure variables must be enclosed in curly braces or angle brackets, but you can specify overrides with fewer elements than the defaults.

This example illustrates the use of default values with structures as field

a1 BYTE ?
b1 BYTE ?
c1 BYTE ?

buffer BYTE 100 DUP (?)
crlf BYTE 13, 10
query BYTE 'Filename: ' ; String <= can override
endmark BYTE 36
drives DISKDRIVES <0, 1, 1>

info1 INFO { , , 'Dir' }

; Next line illegal since name in query field is too long:
; info2 INFO {"TESTFILE", , "DirectoryName"}

lotsof INFO { , , 'file1', , {0,0,0} },
{ , , 'file2', , {0,0,1} },
{ , , 'file3', , {0,0,2} }

The following diagram shows how the assembler stores info1.

The initialization for drives gives default values for all three fields of the structure. The fields left blank in info1 use the default values for those fields. The info2 declaration is illegal because "DirectoryName" is longer than the initial string for that field.

Arrays of Structures and Unions

You can define an array of structures using the DUP operator (see "Declaring and Referencing Arrays," earlier in this chapter) or by creating a list of structures. For example, you can define an array of structure variables like this:

Item7 ITEMS 30 DUP ({,,{10}})

The Item7 array defined here has 30 elements of type ITEMS, with the third field of each element (the union) initialized to 10.

You can also list array elements as shown in the following example.

Item8 ITEMS {'Bolts', 126, 10},
{'Pliers',139, 10},
{'Saws', 414, 10}

Redeclaring a Structure

The assembler generates an error when you declare a structure more than once unless the following are the same:

Field names

Offsets of named fields

Initialization lists

Field alignment value


LENGTHOF, SIZEOF, and TYPE for Structures

The size of a structure determined by SIZEOF is the offset of the last field, plus the size of the last field, plus any padding required for proper alignment. (For information about alignment, see "Declaring Structure and Union Types," earlier in this chapter.)

This example, using the preceding data declarations, shows how to use the LENGTHOF, SIZEOF, and TYPE operators with structures.

buffer BYTE 100 DUP (?)
crlf BYTE 13, 10
query BYTE 'Filename: '
endmark BYTE 36
drives DISKDRIVES <0, 1, 1>

info1 INFO { , , 'Dir' }
lotsof INFO { , , 'file1', , {0,0,0} },
{ , , 'file2', , {0,0,1} },
{ , , 'file3', , {0,0,2} }

sinfo1 EQU SIZEOF info1 ; 116 = number of bytes in
; initializers
linfo1 EQU LENGTHOF info1 ; 1 = number of items
tinfo1 EQU TYPE info1 ; 116 = same as size

slotsof EQU SIZEOF lotsof ; 116 * 3 = number of bytes in
; initializers
llotsof EQU LENGTHOF lotsof ; 3 = number of items
tlotsof EQU TYPE lotsof ; 116 = same as size for structure
; of type INFO


The size of a union determined by SIZEOF is the size of the longest field plus any padding required. The length of a union variable determined by LENGTHOF equals the number of initializers defined inside angle brackets or curly braces. TYPE returns a value indicating the type of the longest field.

w WORD ?
b BYTE ?

num DWB {0FFFFh}
array DWB (100 / SIZEOF DWB) DUP ({0})

snum EQU SIZEOF num ; = 4
lnum EQU LENGTHOF num ; = 1
tnum EQU TYPE num ; = 4
sarray EQU SIZEOF array ; = 100 (4*25)
larray EQU LENGTHOF array ; = 25
tarray EQU TYPE array ; = 4

Referencing Structures, Unions, and Fields

Like other variables, structure variables can be accessed by name. You can access fields within structure variables with this syntax:

variable. field

References to fields must always be fully qualified, with the structure or union names and the dot operator preceding the field name. The assembler requires that you use the dot operator only with structure fields, not as an alternative to the plus operator; nor can you use the plus operator as an alternative to the dot operator.

The following example shows several ways to reference the fields of a structure of type DATE.

DATE STRUCT ; Defines structure type
month BYTE ?
day BYTE ?
year WORD ?

yesterday DATE {1, 20, 1993} ; Declare structure
; variable
mov al, ; Use structure variables
mov bx, OFFSET yesterday ; Load structure address
mov al, (DATE PTR [bx]).month ; Use as indirect operand
mov al, [bx].date.month ; This is necessary only if
; month is already a
; field in a different
; structure

Under OPTION M510 or OPTION OLDSTRUCTS, unique structure names do not need to be qualified. However, if the NONUNIQUE keyword appears in a structure definition, all fields of the structure must be fully qualified when referenced, even if the OPTION OLDSTRUCTS directive appears in the code. Also, you must qualify all references to a field. (For information on the OPTION directive, see Chapter 1.)

Even if the initialized union is the size of a WORD or DWORD, members of structures or unions are accessible only through the field’s names.


In the following example, the two MOV statements show how you can access the elements of an array of unions.

w WORD ?
b BYTE ?

array WB (100 / SIZEOF WB) DUP ({0})

mov array[12].w, 40h
mov array[32].b, 2

As the preceding code illustrates, you can use unions to access the same data in more than one form. One application of structures and unions is to simplify the task of reinitializing a far pointer. For a far pointer declared as


WordPtr FPWORD ?

you must follow these steps to point WordPtr to a word value named ThisWord in the current data segment.

mov WORD PTR WordPtr[2], ds
mov WORD PTR WordPtr, OFFSET ThisWord

The preceding method requires that you remember whether the segment or the offset is stored first. However, if your program declares a union like this:

uptr UNION
dwptr FPWORD 0
offs WORD 0
segm WORD 0
uptr ENDS


You can initialize a far pointer with these steps:

WrdPtr2 uptr <>
mov WrdPtr2.segm, ds
mov WrdPtr2.offs, OFFSET ThisWord

This code moves the segment and the offset into the pointer and then moves the pointer into a register with the other field of the union. Although this technique does not reduce the code size, it avoids confusion about the order for loading the segment and offset.

Nested Structures and Unions

You can nest structures and unions in several ways. This section explains how to refer to the fields in a nested structure or union. The following example illustrates the four techniques for nesting, and how to reference the fields. Note the syntax for nested structures. The techniques are reviewed following the example.

Inum WORD ?
Iname BYTE 'Item Name'

UpDate WORD ?
oldItem ITEMS { \
'AF8' \ ; Named variable of
} ; existing structure
ITEMS { ?, '94C' } ; Unnamed variable of
; existing type
STRUCT ups ; Named nested structure
source WORD ?
shipmode BYTE ?
STRUCT ; Unnamed nested structure
f1 WORD ?
f2 WORD ?


yearly INVENTORY { }

; Referencing each type of data in the yearly structure:

mov ax, yearly.oldItem.Inum
mov, 'A'
mov yearly.Inum, 'C'
mov ax, yearly.f1

To nest structures and unions, you can use any of these techniques:

The field of a structure or union can be a named variable of an existing structure or union type, as in the oldItem field. Because INVENTORY contains two structures of type ITEMS , the field names in oldItem are not unique. Therefore, you must use the full field names when referencing those fields, as in the statement

mov ax, yearly.oldItem.Inum

To declare a named structure or union inside another structure or union, give the STRUCT or UNION keyword first and then define a label for it. Fields of the nested structure or union must always be qualified:

mov, 'A'

As shown in the Items field of Inventory, you also can use unnamed variables of existing structures or unions inside another structure or union. In these cases, you can reference fields directly:

mov yearly.Inum, 'C'
mov ax, yearly.f1


Records are similar to structures, except that fields in records are bit strings. Each bit field in a record variable can be used separately in constant operands or expressions. The processor cannot access bits individually at run time, but it can access bit fields with instructions that manipulate bits.

Records are bytes, words, or doublewords in which the individual bits or groups of bits are considered fields. In general, the three steps for using record variables are the same as those for using other complex data types:

1. Declare a record type.

2. Define one or more variables having the record type.

3. Reference record variables using shifts and masks.


Once it is defined, you can use the record variable as an operand in assembler statements.

This section explains the record declaration syntax and the use of the MASK and WIDTH operators. It also shows some applications of record variables and constants.

Declaring Record Types

A record type creates a template for data with the sizes and, optionally, the initial values for bit fields in the record. It does not allocate memory space for the

The RECORD directive declares a record type for an 8-bit, 16-bit, or 32-bit record that contains one or more bit fields. The maximum size is based on the expression word size. See OPTION EXPR16 and OPTION EXPR32 in Chapter 1. The syntax is:

recordname RECORD field [[, field]]...

The field declares the name, width, and initial value for the field. The syntax for each field is:


Global labels, macro names, and record field names must all be unique, but record field names can have the same names as structure field names. Width is the number of bits in the field, and expression is a constant giving the initial (or default) value for the field. Record definitions can span more than one line if the continued lines end with commas.

If expression is given, it declares the initial value for the field. The assembler generates an error message if an initial value is too large for the width of its field.

The first field in the declaration always goes into the most significant bits of the record. Subsequent fields are placed to the right in the succeeding bits. If the fields do not total exactly 8, 16, or 32 bits as appropriate, the entire record is shifted right, so the last bit of the last field is the lowest bit of the record. Unused bits in the high end of the record are initialized to 0.

The following example creates a byte record type COLOR having four fields: blink, back, intense, and fore. The contents of the record type are shown after the example. Since no initial values are given, all bits are set to 0. Note that this is only a template maintained by the assembler. It allocates no space in the data segment.

COLOR RECORD blink:1, back:3, intense:1, fore:3

The next example creates a record type CW that has six fields. Each record declared with this type occupies 16 bits of memory. Initial (default) values are given for each field. You can use them when declaring data for the record. The bit diagram after the example shows the contents of the record type.

CW RECORD r1:3=0, ic:1=0, rc:2=0, pc:2=3, r2:2=1, masks:6=63

Defining Record Variables

Once you have declared a record type, you can define record variables of that type. For each variable, the assembler allocates memory in the format declared by the type. The syntax is:

[[name]] recordname <[[initializer [[,initializer]]...]] >

[[name]] recordname { [[initializer [[,initializer]]...]] }

[[name]] recordname constant DUP ( [[initializer [[,initializer]]...]] )

The recordname is the name of a record type previously declared with the RECORD directive.

A fieldlist for each field in the record can be a list of integers, character constants, or expressions that correspond to a value compatible with the size of the field. You must include curly braces or angle brackets even when you do not specify an initial value.

If you use the DUP operator (see "Declaring and Referencing Arrays," earlier in this chapter) to initialize multiple record variables, only the angle brackets and any initial values need to be enclosed in parentheses. For example, you can define an array of record variables with

xmas COLOR 50 DUP ( <1, 2, 0, 4> )

You do not have to initialize all fields in a record. If an initial value is blank, the assembler automatically stores the default initial value of the field. If there is no default value, the assembler clears each bit in the field.

The definition in the following example creates a variable named warning whose type is given by the record type COLOR. The initial values of the fields in the variable are set to the values given in the record definition. The initial values override any default record values given in the declaration.

COLOR RECORD blink:1,back:3,intense:1,fore:3 ; Record
; declaration
warning COLOR <1, 0, 1, 4> ; Record
; definition

LENGTHOF, SIZEOF, and TYPE with Records

The SIZEOF and TYPE operators applied to a record name return the number of bytes used by the record. SIZEOF returns the number of bytes a record variable occupies. You cannot use LENGTHOF with a record declaration, but you can use it with defined record variables. LENGTHOF returns the number of records in an array of records, or 1 for a single record variable. The following example illustrates these points.

; Record definition
; 9 bits stored in 2 bytes
RGBCOLOR RECORD red:3, green:3, blue:3

mov ax, RGBCOLOR ; Equivalent to "mov ax, 01FFh"
; mov ax, LENGTHOF RGBCOLOR ; Illegal since LENGTHOF can
; apply only to data label
mov ax, SIZEOF RGBCOLOR ; Equivalent to "mov ax, 2"
mov ax, TYPE RGBCOLOR ; Equivalent to "mov ax, 2"

; Record instance
; 8 bits stored in 1 byte
RGBCOLOR2 RECORD red:3, green:3, blue:2
rgb RGBCOLOR2 <1, 1, 1> ; Initialize to 00100101y

mov ax, RGBCOLOR2 ; Equivalent to
; "mov ax, 00FFh"
mov ax, LENGTHOF rgb ; Equivalent to "mov ax, 1"
mov ax, SIZEOF rgb ; Equivalent to "mov ax, 1"
mov ax, TYPE rgb ; Equivalent to "mov ax, 1"

Record Operators

The WIDTH operator (used only with records) returns the width in bits of a record or record field. The MASK operator returns a bit mask for the bit positions occupied by the given record field. A bit in the mask contains a 1 if that bit corresponds to a bit field. The following example shows how to use MASK and WIDTH.

COLOR RECORD blink:1, back:3, intense:1, fore:3
message COLOR <1, 5, 1, 1>
wblink EQU WIDTH blink ; "wblink" = 1
wback EQU WIDTH back ; "wback" = 3
wintens EQU WIDTH intense ; "wintens" = 1
wfore EQU WIDTH fore ; "wfore" = 3
wcolor EQU WIDTH COLOR ; "wcolor" = 8
mov ah, message ; Load initial 1101 1001
and ah, NOT MASK back ; Turn off AND 1000 1111
; "back" ---------
; 1000 1001
or ah, MASK blink ; Turn on OR 1000 0000
; "blink" ---------
; 1000 1001
xor ah, MASK intense ; Toggle XOR 0000 1000
; "intense" ---------
; 1000 0001

IF (WIDTH COLOR) GT 8 ; If color is 16 bit, load
mov ax, message ; into 16-bit register
ELSE ; else
mov al, message ; load into low 8-bit register
xor ah, ah ; and clear high 8-bits

The example continues by illustrating several ways in which record fields can serve as operands and expressions:

; Rotate "back" of "message" without changing other values

mov al, message ; Load value from memory
mov ah, al ; Save a copy for work 1101 1001=ah/al
and al, NOT MASK back; Mask out old bits AND 1000 1111=mask
; to save old message ---------
; 1000 1001=al
mov cl, back ; Load bit position
shr ah, cl ; Shift to right 0000 1101=ah
inc ah ; Increment 0000 1110=ah

shl ah, cl ; Shift left again 1110 0000=ah
and ah, MASK back ; Mask off extra bits AND 0111 0000=mask
; to get new message ---------
; 0110 0000 ah
or ah, al ; Combine old and new OR 1000 1001 al
; ---------
mov message, ah ; Write back to memory 1110 1001 ah

Record variables are often used with the logical operators to perform logical operations on the bit fields of the record, as in the previous example using the MASK operator.

See also:

file: /Techref/language/masm/masmc05.htm, 98KB, , updated: 2008/5/21 07:40, local time: 2018/2/23 04:35,

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