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Intel IA-32 Assembly Tutorial - A Guide to the Basics of x86 Assembly - Page 15

Structures

Structures are a grouping of related variables.

Person STRUCT                     ;bytes
    SSN       BYTE "000-00-0000"  ;11
    LastName  BYTE 30 DUP(0)      ;30
    FirstName BYTE 30 DUP(0)      ;30
    Phone     BYTE "000-000-0000" ;12
    History   DWORD 0,0,0,0       ;16
Person ENDS                       ;99 bytes

.data
person1 Person <"123-45-6789","Smith","John","512-555-1212"> ;initialize with <>
person2 Person <,"Jones","David">                            ;skip initializing some elements
person3 Person {"555-66-7777"}                               ;alternatively, use {}
person4 Person <>                                            ;used default initializers

	NumOfEmployees = 10
	Employees Person NumOfEmployees DUP(<>) ;arrays of structures are declared like this

;Referencing structure variables, the following hold true:
TYPE Person             ;99 bytes
SIZEOF Person           ;99 bytes
SIZEOF person1          ;99 bytes

TYPE Person.History     ;4 bytes
LENGTHOF Person.History ;4 elements
TYPE Person.SSN         ;1 byte

Runtime references:

.data
person5 Person <>
.code
mov ebx,offset person5.SSN
mov person5.History,1            ;first element of History
mov person5.History+4,3          ;second element of History
mov person5.History+8,5          ;third element of History
mov esi,offset person5
inc (Person PTR [esi]).History+8

Nested Structures

Point STRUCT
	x WORD ?
	y WORD ?
Point ENDS

Rectangle STRUCT
	TopLeft	 Point <>
	BottomRight Point <>
Rectangle ENDS

.data
rect1 Rectangle < <5,5>, <35,25> >
.code
mov rect1.TopLeft.x,10
mov esi,offset rect1.TopLeft
mov (Point PTR [esi]).y,10

Unions

Each field in a structure has an offset relative to the first byte of the structure.

All fields in a union, however, start at the same offset.

The storage size of a union is equal to the length of its longest field.

Number UNION
	dw DWORD 0
	w  WORD 0
	b  BYTE 0
Number ENDS		;use ENDS for a UNION

A structure can also contain a union.

Identifier STRUCT
	UNION Number
		dw DWORD 0
		w WORD 0
		b BYTE 0
	ENDS
	Text BYTE 8 DUP(?),0
Identifier ENDS

Macros

A macro procedure is a named block of assembly language statements. Wherever the macro is used, a copy of it is inserted into the program.

.686P
.model flat,stdcall
.stack 8192
INCLUDE file_to_include.inc ;macros, structures and prototypes maybe included
.data
hello	 BYTE "HELLO!",0
goodbye BYTE "GOODBYE!",0
.code
MessageBoxA PROTO, hwnd:DWORD, message:PTR BYTE, windowTitle:PTR BYTE, options:DWORD
GetActiveWindow PROTO

popMsg MACRO message:REQ    ;message is a REQuired parameter
    ECHO Expanding popMsg   ;when assembled, this will print
    call GetActiveWindow
    push 0                  ;options (0 = none)
    push 0                  ;windowTitle (0 = error)
    lea esi,message
    push esi                ;message (= message)
    push eax                ;GetActiveWindow returns in EAX (= hwnd)
    call MessageBoxA
ENDM

main PROC
    popMsg hello            ;use macro popMsg
    popMsg goodbye          ;use macro popMsg
    xor eax,eax
    ret
main ENDP

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