Assembly Language for Intel
Assembly Language for Intel - - Based Based Computers, 4
Computers, 4 th th Edition Edition
Chapter 10: Structures and Macros
Kip R. Irvine
Chapter Overview Chapter Overview
• Structures
• Macros
• Conditional-Assembly Directives
• Defining Repeat Blocks
Structure Structure
• A template or pattern given to a logically related group of variables.
• field - structure member containing data
• Program access to a structure:
• entire structure as a complete unit
• individual fields
• Useful way to pass multiple related arguments to a procedure
• example: file directory information
Using a Structure Using a Structure
Using a structure involves three sequential steps:
1. Define the structure.
2. Declare one or more variables of the structure type, called structure variables.
3. Write runtime instructions that access the structure.
Structure Definition Syntax Structure Definition Syntax
name STRUCT
field-declarations name ENDS
• Field-declarations are identical to variable
declarations
COORD Structure COORD Structure
• The COORD structure used by the MS-Windows programming library identifies X and Y screen coordinates
COORD STRUCT
X WORD ? ; offset 00 Y WORD ? ; offset 02 COORD ENDS
Employee Structure Employee Structure
Employee STRUCT
IdNum BYTE "000000000"
LastName BYTE 30 DUP(0) Years WORD 0
SalaryHistory DWORD 0,0,0,0 Employee ENDS
A structure is ideal for combining fields of different types:
"000000000" (null) 0 0 0 0 0
SalaryHistory Lastname
Years Idnum
Declaring Structure Variables Declaring Structure Variables
• Structure name is a user-defined type
• Insert replacement initializers between brackets:
< . . . >
• Empty brackets <> retain the structure's default field initializers
• Examples:
.data
point1 COORD <5,10>
point2 COORD <>
worker Employee <>
Initializing Array Fields Initializing Array Fields
• Use the DUP operator to initialize one or more elements of an array field:
.data
emp Employee <,,,4 DUP(20000)>
Array of Structures Array of Structures
• An array of structure objects can be defined using the DUP operator.
• Initializers can be used
NumPoints = 3
AllPoints COORD NumPoints DUP(<0,0>) RD_Dept Employee 20 DUP(<>)
accounting Employee 10 DUP(<,,,4 DUP(20000) >)
Referencing Structure Variables Referencing Structure Variables
.data
worker Employee <>
mov eax,TYPE Employee ; 57
mov eax,SIZEOF Employee ; 57
mov eax,SIZEOF worker ; 57
mov eax,TYPE Employee.SalaryHistory ; 4 mov eax,LENGTHOF Employee.SalaryHistory ; 4
Employee STRUCT ; bytes
IdNum BYTE "000000000" ; 9 LastName BYTE 30 DUP(0) ; 30
Years WORD 0 ; 2
SalaryHistory DWORD 0,0,0,0 ; 16
Employee ENDS ; 57
. . . continued . . . continued
mov dx,worker.Years
mov worker.SalaryHistory,20000 ; first salary mov worker.SalaryHistory+4,30000 ; second salary mov edx,OFFSET worker.LastName
mov esi,OFFSET worker
mov ax,(Employee PTR [esi]).Years
mov ax,[esi].Years ; invalid operand (ambiguous)
Looping Through an Array of Points Looping Through an Array of Points
.data
NumPoints = 3
AllPoints COORD NumPoints DUP(<0,0>) .code
mov edi,0 ; array index
mov ecx,NumPoints ; loop counter
mov ax,1 ; starting X, Y values L1:
mov (COORD PTR AllPoints[edi]).X,ax mov (COORD PTR AllPoints[edi]).Y,ax add edi,TYPE COORD
inc ax
Sets the X and Y coordinates of the AllPoints array to
sequentially increasing values (1,1), (2,2), ...
Example: Displaying the System Time
Example: Displaying the System Time (1 of 3) (1 of 3)
• Retrieves and displays the system time at a selected screen location.
• Uses COORD and SYSTEMTIME structures:
SYSTEMTIME STRUCT wYear WORD ? wMonth WORD ?
wDayOfWeek WORD ? wDay WORD ?
wHour WORD?
wMinute WORD ?
Example: Displaying the System Time
Example: Displaying the System Time (2 of 3) (2 of 3)
• Uses a Windows API call to get the standard console output handle. SetConsoleCursorPosition positions the cursor. GetLocalTime gets the current time of day:
.data
sysTime SYSTEMTIME <>
XYPos COORD <10,5>
consoleHandle DWORD ? colonStr BYTE “:”,0 .code
INVOKE GetStdHandle, STD_OUTPUT_HANDLE mov consoleHandle,eax
INVOKE SetConsoleCursorPosition,
Example: Displaying the System Time
Example: Displaying the System Time (3 of 3) (3 of 3)
• Display the time using library calls:
movzx eax,sysTime.wHour ; hours call WriteDec
mov edx,offset colonStr ; ":"
call WriteString
movzx eax,sysTime.wMinute ; minutes call WriteDec
mov edx,offset colonStr ; ":"
call WriteString
movzx eax,sysTime.wSecond ; seconds
Nested Structures
Nested Structures (1 of 2) (1 of 2)
Rectangle STRUCT
UpperLeft COORD <>
LowerRight COORD <>
Rectangle ENDS .code
rect1 Rectangle { {10,10}, {50,20} } rect2 Rectangle < <10,10>, <50,20> >
• Define a structure that contains other structures.
• Used nested braces (or brackets) to initialize each COORD structure.
COORD STRUCT X WORD ? Y WORD ? COORD ENDS
Nested Structures
Nested Structures (2 of 2) (2 of 2)
mov rect1.UpperLeft.X, 10 mov esi,OFFSET rect1
mov (Rectangle PTR [esi]).UpperLeft.Y, 10 // use the OFFSET operator
mov edi,OFFSET rect2.LowerRight mov (COORD PTR [edi]).X, 50
mov edi,OFFSET rect2.LowerRight.X
• Use the dot (.) qualifier to access nested fields.
• Use indirect addressing to access the overall
structure or one of its fields
Example: Drunkard's Walk Example: Drunkard's Walk
• Random-path simulation
• Uses a nested structure to accumulate path data as the simulation is running
• Uses a multiple branch structure to choose the direction
WalkMax = 50
DrunkardWalk STRUCT
path COORD WalkMax DUP(<0,0>) pathsUsed WORD 0
DrunkardWalk ENDS
Declaring and Using Unions Declaring and Using Unions
• A union is similar to a structure in that it contains multiple fields
• All of the fields in a union begin at the same offset
• (differs from a structure)
• Provides alternate ways to access the same data
• Syntax:
unionname UNION
union-fields
Integer Union Example Integer Union Example
Integer UNION D DWORD 0 W WORD 0 B BYTE 0 Integer ENDS
The Integer union consumes 4 bytes (equal to the largest field)
.data
val1 Integer <12345678h>
val2 Integer <100h>
val3 Integer <>
D, W, and B are often called variant fields.
Integer can be used to define data:
Union Inside a Structure Union Inside a Structure
Integer UNION D DWORD 0 W WORD 0 B BYTE 0 Integer ENDS FileInfo STRUCT
FileID Integer <>
FileName BYTE 64 DUP(?) FileInfo ENDS
.data
