32 位的寄存器容量是 4 字節, 如果內存中的數據都按 4*n 字節對齊, 肯定會加快吞吐速度;
但事實并非如此, 不同大小的數據可能會讓寄存器別別扭扭地去處理, 從而降低了運行速度!

如果使用對齊, 就會浪費掉一些內存空間; 其實這是一個需要權衡 "速度" 與 "內存" 得失的問題.

準備使用的測試文件:

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.586
.modelflat, stdcall

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

.data
?? ;聲明三個字節變量
??? v1 db 0
??? v2 db 0
??? v3 db 0
.code
main proc
?? ;三個字節變量的默認偏移地址如下(在沒有對齊約束的情況下, 它們各占一個字節):
??? PrintDec offset v1 ;4206592
??? PrintDec offset v2 ;4206593
??? PrintDec offset v3 ;4206594
??? ret
main endp
end main

------------------------------------------------------------------------------------------------------------------------------------------
ALIGN: 指定對齊邊界

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; Test11_2.asm
.586
.modelflat, stdcall

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

.data
??? v1 db0
??? align 4 ;讓下一個變量的起始地址保證是 4 的倍數
??? v2 db0
??? v3 db0
.code
main proc
??? PrintDec offset v1 ;4206592
??? PrintDec offset v2 ;4206596 (!)
??? PrintDec offset v3 ;4206597
??? ret
main endp
end main

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;align 后面的參數是 2ⁿ, 還可測試下: 1、2、8、16

EVEN 是偶對齊, 相當于 ALIGN 2

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; Test11_3.asm
.586
.modelflat, stdcall

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

.data
??? v1 db0
??? even?? ;(!)
??? v2 db0
??? v3 db0
.code
main proc
??? PrintDec offset v1 ;4206592
??? PrintDec offset v2 ;4206594 (!)
??? PrintDec offset v3 ;4206595
??? ret
main endp
end main

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ORG 可以指定從當前位置跨越指定書目的字節再安排下一個數據:

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; Test11_4.asm
.586
.modelflat, stdcall

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

.data
??? v1 db0
??? org100 ;(!)
??? v2 db0
??? v3 db0
.code
main proc
??? PrintDec offset v1 ;4206592
??? PrintDec offset v2 ;4206692 (!)
??? PrintDec offset v3 ;4206693
??? ret
main endp
end main

------------------------------------------------------------------------------------------------------------------------------------------

這些偽指令不對局部變量產生影響, 因為偽指令作用在編譯之前, 局部變量使用內存是程序運行之后的事.
關于局部變量的對齊和速度, 除了編譯的工作外, 我們在安排變量類型和次序時也應該考慮到 "對齊" 的問題.

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轉載于：----博客圓---------