“npad”指令是一种汇编宏,用于把下一个指令标签的首地址向指定边界对齐。
被npad指令对齐的标签,通常都是需要被多次跳转到的地址标签。例如,在各种循环体起始地址处的标签之前,我们经常可以看到npad指令。它可通过对齐内存地址、内存总线或缓存线等手段,提高CPU加载数据(或指令代码)的访问效率。
下面这段代码摘自MSVC的文件listing.inc。
顺便提一下,这都是NOP指令的变种。虽然这些指令没有实际的操作意义,但是它们可以占用不同的空间。
出于CPU性能的考虑,下述代码没有使用多条NOP指令,而是使用了单条指令。
;; LISTING.INC
;;
;; This file contains assembler macros and is included by the files created
;; with the -FA compiler switch to be assembled by MASM (Microsoft Macro
;; Assembler).
;;
;; Copyright (c) 1993-2003, Microsoft Corporation. All rights reserved.
;; non destructivenops
npad macro size
if size eq 1
nop
else
if size eq 2
mov edi, edi
else
if size eq 3
; lea ecx, [ecx+00]
DB 8DH, 49H, 00H
else
if size eq 4
; lea esp, [esp+00]
DB 8DH, 64H, 24H, 00H
else
if size eq 5
add eax, DWORD PTR 0
else
if size eq 6
; lea ebx, [ebx+00000000]
DB 8DH, 9BH, 00H, 00H, 00H, 00H
else
if size eq 7
; lea esp, [esp+00000000]
DB 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H
else
if size eq 8
; jmp .+8; .npad 6
DB 0EBH, 06H, 8DH, 9BH, 00H, 00H, 00H, 00H
else
if size eq 9
; jmp .+9; .npad 7
DB 0EBH, 07H, 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H
else
if size eq 10
; jmp .+A; .npad 7; .npad 1
DB 0EBH, 08H, 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H, 90H
else
if size eq 11
; jmp .+B; .npad 7; .npad 2
DB 0EBH, 09H, 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H, 8BH, 0FFH
else
if size eq 12
; jmp .+C; .npad 7; .npad 3
DB 0EBH, 0AH, 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H, 8DH, 49H, 00H
else
if size eq 13
; jmp .+D; .npad 7; .npad 4
DB 0EBH, 0BH, 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H, 8DH, 64H, 24H, 00H
else
if size eq 14
; jmp .+E; .npad 7; .npad 5
DB 0EBH, 0CH, 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H, 05H, 00H, 00H, 00H, 00H
else
if size eq 15
; jmp .+F; .npad 7; .npad 6
DB 0EBH, 0DH, 8DH, 0A4H, 24H, 00H, 00H, 00H, 00H, 8DH, 9BH, 00H, 00H, 00H, 00H
else
%out error: unsupported npad size
.err
endif
endif
endif
endif
endif
endif
endif
endif
endif
endif
endif
endif
endif
endif
endif
endm