结构和联合分别用 struct、union 定义, 都是 ends 结束定义.
它们使用方法相同, 并可以互相嵌套; 主要区别是后者的各成员共用同一个地址.
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结构示例:
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.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

;定义结构
MyPoint struct
    X dd ?
    Y dd ?
MyPoint ends

.data
    ;声明结构, 使用 <>、{} 均可
    pt1 MyPoint <>
    pt2 MyPoint {}
    pt3 MyPoint <33,44>
    pt4 MyPoint {55,66}
    pt5 MyPoint {77,}
    pt6 MyPoint {,88}
    pt7 MyPoint {,}
.code
main proc
    mov pt1.X, 11
    mov pt1.Y, 22
    inc pt1.X
    PrintDec pt1.X ;12
    PrintDec pt1.Y ;22
    ;
    PrintDec pt2.X ;0
    PrintDec pt2.Y ;0
    ;
    PrintDec pt3.X ;33
    PrintDec pt3.Y ;44
    ;
    PrintDec pt4.X ;55
    PrintDec pt4.Y ;66
    ;
    PrintDec pt5.X ;77
    PrintDec pt5.Y ;0
    ;
    PrintDec pt6.X ;0
    PrintDec pt6.Y ;88
    ;
    PrintDec pt7.X ;0
    PrintDec pt7.Y ;0
    ret
main endp
end main
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联合示例:
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.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

;定义联合
MyUnion union
    D dd ?
    W dw ?
    B db ?
MyUnion ends

.data
    u1 MyUnion <>
    u2 MyUnion {1122h} ;只能有一个初始值
.code
main proc
    mov u1.D, 12345678h
    PrintHex u1.D ;12345678
    PrintHex u1.W ;5678
    PrintHex u1.B ;78
    ;
    PrintHex u2.D ;00001122
    PrintHex u2.W ;1122
    PrintHex u2.B ;22
    ret
main endp
end main
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用寄存器引用结构变量:
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.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

MyPoint struct
    X dd ?
    Y dd ?
MyPoint ends

.data
    pt1 MyPoint <11,22>
.code
main proc
    lea ebx, pt1
    PrintDec (MyPoint ptr [ebx]).X ;11
    PrintDec (MyPoint ptr [ebx]).Y ;22
    ret
main endp
end main
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定义时的数据初始化:
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.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

MyStruct struct
    FName db 12 dup(0)
    FAge  db 66
MyStruct ends

.data
    u1 MyStruct <>
    u2 MyStruct <'张三', 88>
.code
main proc
    PrintString u1.FName ;
    PrintDec    u1.FAge  ;66
   
    PrintString u2.FName ;张三
    PrintDec    u2.FAge  ;88
    ret
main endp
end main
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使用 SYSTEMTIME 结构获取系统时间的例子:
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.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

;SYSTEMTIME 结构定义在 windows.inc
;GetLocalTime 函数声明在 kernel32.inc
.data
    sysTime SYSTEMTIME <>
.code
main proc
    invoke GetLocalTime, addr sysTime
    PrintDec sysTime.wYear  ;2010
    PrintDec sysTime.wMonth ;4
    PrintDec sysTime.wDay   ;22
    ret
main endp
end main
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结构嵌套:
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.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

Pt struct
   X dd ?
   Y dd ?
Pt ends

Rect struct
    Left  Pt <>
    Right Pt <>
Rect ends

.data
    r1 Rect <>
    r2 Rect <<55,66>, <77,88>>
.code
main proc
    mov r1.Left.X, 11
    mov r1.Left.Y, 22
    mov r1.Right.X, 33
    mov r1.Right.Y, 44
   
    PrintDec r1.Left.X  ;11
    PrintDec r1.Left.Y  ;22
    PrintDec r1.Right.X ;33
    PrintDec r1.Right.Y ;44
   
    PrintDec r2.Left.X  ;55
    PrintDec r2.Left.Y  ;66
    PrintDec r2.Right.X ;77
    PrintDec r2.Right.Y ;88
    ret
main endp
end main