实模式下 一个汇编小程序

主要通过它来验证汇编中的标号实际是表示段内偏移呢,还是实际的内存地址。 

原代码:

 

 

org 07c00h mov ax, cs mov es, ax mov bx, 000ch mov cx, 13 ; the string length mov dh, 1 mov dl, 1 mov bp, ScreenMsg mov ax, 1301h ; show the string int 10h jmp $ ScreenMsg: db "hello wupeng!" times 510-($-$) db 0 dw 0xaa55  
nasm boot.asm -o boot.bin
执行ndisasm -o 0x7c00 boot.bin 反汇编后的代码:
00007C00 8CC8 mov ax,cs 00007C02 8EC0 mov es,ax 00007C04 BB0C00 mov bx,0xc 00007C07 B90D00 mov cx,0xd 00007C0A B601 mov dh,0x1 00007C0C B201 mov dl,0x1 00007C0E BD197C mov bp,0x7c19 00007C11 B80113 mov ax,0x1301 00007C14 CD10 int 0x10 00007C16 E9FDFF jmp word 0x7c16 00007C19 68656C push word 0x6c65 00007C1C 6C insb 00007C1D 6F outsw 00007C1E 207775 and [bx+0x75],dh 00007C21 7065 jo 0x7c88 00007C23 6E outsb 00007C24 672100 and [eax],ax 00007C27 0000 add [bx+si],al 00007C29 0000 add [bx+si],al 00007C2B 0000 add [bx+si],al 00007C2D 0000 add [bx+si],al 00007C2F 0000 add [bx+si],al 00007C31 0000 add [bx+si],al 00007C33 0000 add [bx+si],al 00007C35 0000 add [bx+si],al 00007C37 0000 add [bx+si],al 00007C39 0000 add [bx+si],al 00007C3B 0000 add [bx+si],al 00007C3D 0000 add [bx+si],al 00007C3F 0000 add [bx+si],al 00007C41 0000 add [bx+si],al 00007C43 0000 add [bx+si],al 00007C45 0000 add [bx+si],al 00007C47 0000 add [bx+si],al 00007C49 0000 add [bx+si],al 00007C4B 0000 add [bx+si],al 00007C4D 0000 add [bx+si],al 00007C4F 0000 add [bx+si],al 00007C51 0000 add [bx+si],al 00007C53 0000 add [bx+si],al 00007C55 0000 add [bx+si],al 00007C57 0000 add [bx+si],al 00007C59 0000 add [bx+si],al 00007C5B 0000 add [bx+si],al 00007C5D 0000 add [bx+si],al 00007C5F 0000 add [bx+si],al 00007C61 0000 add [bx+si],al 00007C63 0000 add [bx+si],al 00007C65 0000 add [bx+si],al 00007C67 0000 add [bx+si],al 00007C69 0000 add [bx+si],al 00007C6B 0000 add [bx+si],al 00007C6D 0000 add [bx+si],al 00007C6F 0000 add [bx+si],al 00007C71 0000 add [bx+si],al 00007C73 0000 add [bx+si],al 00007C75 0000 add [bx+si],al 00007C77 0000 add [bx+si],al 00007C79 0000 add [bx+si],al 00007C7B 0000 add [bx+si],al 00007C7D 0000 add [bx+si],al 00007C7F 0000 add [bx+si],al 00007C81 0000 add [bx+si],al 00007C83 0000 add [bx+si],al 00007C85 0000 add [bx+si],al 00007C87 0000 add [bx+si],al 00007C89 0000 add [bx+si],al 00007C8B 0000 add [bx+si],al 00007C8D 0000 add [bx+si],al 00007C8F 0000 add [bx+si],al 00007C91 0000 add [bx+si],al 00007C93 0000 add [bx+si],al 00007C95 0000 add [bx+si],al 00007C97 0000 add [bx+si],al 00007C99 0000 add [bx+si],al 00007C9B 0000 add [bx+si],al 00007C9D 0000 add [bx+si],al 00007C9F 0000 add [bx+si],al 00007CA1 0000 add [bx+si],al 00007CA3 0000 add [bx+si],al 00007CA5 0000 add [bx+si],al 00007CA7 0000 add [bx+si],al 00007CA9 0000 add [bx+si],al 00007CAB 0000 add [bx+si],al 00007CAD 0000 add [bx+si],al 00007CAF 0000 add [bx+si],al 00007CB1 0000 add [bx+si],al 00007CB3 0000 add [bx+si],al 00007CB5 0000 add [bx+si],al 00007CB7 0000 add [bx+si],al 00007CB9 0000 add [bx+si],al 00007CBB 0000 add [bx+si],al 00007CBD 0000 add [bx+si],al 00007CBF 0000 add [bx+si],al 00007CC1 0000 add [bx+si],al 00007CC3 0000 add [bx+si],al 00007CC5 0000 add [bx+si],al 00007CC7 0000 add [bx+si],al 00007CC9 0000 add [bx+si],al 00007CCB 0000 add [bx+si],al 00007CCD 0000 add [bx+si],al 00007CCF 0000 add [bx+si],al 00007CD1 0000 add [bx+si],al 00007CD3 0000 add [bx+si],al 00007CD5 0000 add [bx+si],al 00007CD7 0000 add [bx+si],al 00007CD9 0000 add [bx+si],al 00007CDB 0000 add [bx+si],al 00007CDD 0000 add [bx+si],al 00007CDF 0000 add [bx+si],al 00007CE1 0000 add [bx+si],al 00007CE3 0000 add [bx+si],al 00007CE5 0000 add [bx+si],al 00007CE7 0000 add [bx+si],al 00007CE9 0000 add [bx+si],al 00007CEB 0000 add [bx+si],al 00007CED 0000 add [bx+si],al 00007CEF 0000 add [bx+si],al 00007CF1 0000 add [bx+si],al 00007CF3 0000 add [bx+si],al 00007CF5 0000 add [bx+si],al 00007CF7 0000 add [bx+si],al 00007CF9 0000 add [bx+si],al 00007CFB 0000 add [bx+si],al 00007CFD 0000 add [bx+si],al 00007CFF 0000 add [bx+si],al 00007D01 0000 add [bx+si],al 00007D03 0000 add [bx+si],al 00007D05 0000 add [bx+si],al 00007D07 0000 add [bx+si],al 00007D09 0000 add [bx+si],al 00007D0B 0000 add [bx+si],al 00007D0D 0000 add [bx+si],al 00007D0F 0000 add [bx+si],al 00007D11 0000 add [bx+si],al 00007D13 0000 add [bx+si],al 00007D15 0000 add [bx+si],al 00007D17 0000 add [bx+si],al 00007D19 0000 add [bx+si],al 00007D1B 0000 add [bx+si],al 00007D1D 0000 add [bx+si],al 00007D1F 0000 add [bx+si],al 00007D21 0000 add [bx+si],al 00007D23 0000 add [bx+si],al 00007D25 0000 add [bx+si],al 00007D27 0000 add [bx+si],al 00007D29 0000 add [bx+si],al 00007D2B 0000 add [bx+si],al 00007D2D 0000 add [bx+si],al 00007D2F 0000 add [bx+si],al 00007D31 0000 add [bx+si],al 00007D33 0000 add [bx+si],al 00007D35 0000 add [bx+si],al 00007D37 0000 add [bx+si],al 00007D39 0000 add [bx+si],al 00007D3B 0000 add [bx+si],al 00007D3D 0000 add [bx+si],al 00007D3F 0000 add [bx+si],al 00007D41 0000 add [bx+si],al 00007D43 0000 add [bx+si],al 00007D45 0000 add [bx+si],al 00007D47 0000 add [bx+si],al 00007D49 0000 add [bx+si],al 00007D4B 0000 add [bx+si],al 00007D4D 0000 add [bx+si],al 00007D4F 0000 add [bx+si],al 00007D51 0000 add [bx+si],al 00007D53 0000 add [bx+si],al 00007D55 0000 add [bx+si],al 00007D57 0000 add [bx+si],al 00007D59 0000 add [bx+si],al 00007D5B 0000 add [bx+si],al 00007D5D 0000 add [bx+si],al 00007D5F 0000 add [bx+si],al 00007D61 0000 add [bx+si],al 00007D63 0000 add [bx+si],al 00007D65 0000 add [bx+si],al 00007D67 0000 add [bx+si],al 00007D69 0000 add [bx+si],al 00007D6B 0000 add [bx+si],al 00007D6D 0000 add [bx+si],al 00007D6F 0000 add [bx+si],al 00007D71 0000 add [bx+si],al 00007D73 0000 add [bx+si],al 00007D75 0000 add [bx+si],al 00007D77 0000 add [bx+si],al 00007D79 0000 add [bx+si],al 00007D7B 0000 add [bx+si],al 00007D7D 0000 add [bx+si],al 00007D7F 0000 add [bx+si],al 00007D81 0000 add [bx+si],al 00007D83 0000 add [bx+si],al 00007D85 0000 add [bx+si],al 00007D87 0000 add [bx+si],al 00007D89 0000 add [bx+si],al 00007D8B 0000 add [bx+si],al 00007D8D 0000 add [bx+si],al 00007D8F 0000 add [bx+si],al 00007D91 0000 add [bx+si],al 00007D93 0000 add [bx+si],al 00007D95 0000 add [bx+si],al 00007D97 0000 add [bx+si],al 00007D99 0000 add [bx+si],al 00007D9B 0000 add [bx+si],al 00007D9D 0000 add [bx+si],al 00007D9F 0000 add [bx+si],al 00007DA1 0000 add [bx+si],al 00007DA3 0000 add [bx+si],al 00007DA5 0000 add [bx+si],al 00007DA7 0000 add [bx+si],al 00007DA9 0000 add [bx+si],al 00007DAB 0000 add [bx+si],al 00007DAD 0000 add [bx+si],al 00007DAF 0000 add [bx+si],al 00007DB1 0000 add [bx+si],al 00007DB3 0000 add [bx+si],al 00007DB5 0000 add [bx+si],al 00007DB7 0000 add [bx+si],al 00007DB9 0000 add [bx+si],al 00007DBB 0000 add [bx+si],al 00007DBD 0000 add [bx+si],al 00007DBF 0000 add [bx+si],al 00007DC1 0000 add [bx+si],al 00007DC3 0000 add [bx+si],al 00007DC5 0000 add [bx+si],al 00007DC7 0000 add [bx+si],al 00007DC9 0000 add [bx+si],al 00007DCB 0000 add [bx+si],al 00007DCD 0000 add [bx+si],al 00007DCF 0000 add [bx+si],al 00007DD1 0000 add [bx+si],al 00007DD3 0000 add [bx+si],al 00007DD5 0000 add [bx+si],al 00007DD7 0000 add [bx+si],al 00007DD9 0000 add [bx+si],al 00007DDB 0000 add [bx+si],al 00007DDD 0000 add [bx+si],al 00007DDF 0000 add [bx+si],al 00007DE1 0000 add [bx+si],al 00007DE3 0000 add [bx+si],al 00007DE5 0000 add [bx+si],al 00007DE7 0000 add [bx+si],al 00007DE9 0000 add [bx+si],al 00007DEB 0000 add [bx+si],al 00007DED 0000 add [bx+si],al 00007DEF 0000 add [bx+si],al 00007DF1 0000 add [bx+si],al 00007DF3 0000 add [bx+si],al 00007DF5 0000 add [bx+si],al 00007DF7 0000 add [bx+si],al 00007DF9 0000 add [bx+si],al 00007DFB 0000 add [bx+si],al 00007DFD 0055AA add [di-0x56],dl 
看其中一行: 
jmp word 0x7c16
比较确定: 标号代表实际的物理地址。
可是仔细一想, 实际的物理地址不是20位的吗? 这里怎么才16位?
看了书才知道:段基址是0x0000, 所以这个还是段内偏移。
为什么需要在最前面加上一条"org 7c00h"?
从后面的代码可以看到, 所有的地址都是以7c00h为基准的。 如果在最前面改为“org 0000h”,那么刚才的
“jmp word 0x7c16”就会变成“jmp word 0x0016”
可是引导代码都是被BIOS加载到0x7c00执行的, 如果执行上述的那条跳转指令, 程序就走到0000:0016去了,显然跑飞了。
所以, 在编写汇编代码时, 一定要知道你的代码在内存中的首地址,否则一旦执行跳转指令,肯定跑飞。

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