试图了解二进制文件（NASM）输出的大小(Trying to Understand the size of a binary file (NASM) output)
;---------------------------------------------------------------------- ; A Simple boot program that prints the string 'Hello World' ;---------------------------------------------------------------------- org 0x7c00 ; This is where BIOS loads the bootloader entry: ; Label to Signify entry to program jmp short begin ; Jump over the DOS boot record data ; -------------------------------------------- ; Boot program code begins here ; -------------------------------------------- begin: ; Label to Signify entry to program xor ax, ax ; Zero out ax mov ds, ax ; Set data segment to base of RAM mov si, msg ; Get pointer to string call putstr ; Print the message jmp hang ; Go to infinite loop msg db 'Hello, World',0x00 putstr: ; Function to print the string lodsb ; al = [DS:SI] or al, al ; Set zero flag if al = 0 jz ret ; Jump to end of function if al = 0 mov ah, 0x0e ; Video function 0Eh (print char) mov bx, 0x0007 ; Color int 0x10 jmp putstr ret: retn hang: jmp hang ; just loop forever. ;--------------------------------------------- ; Write Zeros up to end of program - 2 then boot signature ;--------------------------------------------- size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signature
;---------------------------------------------------------------------- ; A Simple boot program that prints the string 'Hello World' ;---------------------------------------------------------------------- org 0x7c00 ; This is where BIOS loads the bootloader entry: ; Label to Signify entry to program jmp short begin ; Jump over the DOS boot record data ; -------------------------------------------- ; Boot program code begins here ; -------------------------------------------- begin: ; Label to Signify entry to program xor ax, ax ; Zero out ax mov ds, ax ; Set data segment to base of RAM mov si, msg ; Get pointer to string call putstr ; Print the message jmp hang ; Go to infinite loop msg db 'Hello, World',0x00 putstr: ; Function to print the string lodsb ; al = [DS:SI] or al, al ; Set zero flag if al = 0 jz ret ; Jump to end of function if al = 0 mov ah, 0x0e ; Video function 0Eh (print char) mov bx, 0x0007 ; Color int 0x10 jmp putstr ret: retn hang: jmp hang ; just loop forever. ;--------------------------------------------- ; Write Zeros up to end of program - 2 then boot signature ;--------------------------------------------- size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signature
我有这两个文件myboot.asm和theirboot.asm（分别列出）：
;---------------------------------------------------------------------- ; A Simple boot program that prints the string 'Hello World' ; Author: Matthew Hoggan 2012 ;---------------------------------------------------------------------- Output db 'Hello',0x00 ; Output string for bios org 0x7c00 ; This is where BIOS loads the bootloader entry: ; Label to Signify entry to program jmp short begin ; Jump over the DOS boot record data ; -------------------------------------------- ; Boot program code begins here ; -------------------------------------------- begin: ; Label to Signify entry to program mov si, Output ; Get pointer to string loop: ; Top of loop to iterate over string mov al, [si] ; Move contents of pointer to al or al, al ; Check if character pointed to is Zero jz hang ; Zero signifies end of string call print_char ; Print current char in al jmp loop ; Repeat ; -------------------------------------------- ; Function to print char ; assume caller has placed char in al ; -------------------------------------------- print_char: mov ah, 0x0e ; Function to print a character to the screen mov bl, 7 ; color/style to use for the character int 0x10 ; print the character hang: jmp hang ; just loop forever. ;--------------------------------------------- ; Write Zeros up to end of program - 2 then boot signature ;--------------------------------------------- size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signature;---------------------------------------------------------------------- ; A Simple boot program that prints the string 'Hello World' ;---------------------------------------------------------------------- org 0x7c00 ; This is where BIOS loads the bootloader entry: ; Label to Signify entry to program jmp short begin ; Jump over the DOS boot record data ; -------------------------------------------- ; Boot program code begins here ; -------------------------------------------- begin: ; Label to Signify entry to program xor ax, ax ; Zero out ax mov ds, ax ; Set data segment to base of RAM mov si, msg ; Get pointer to string call putstr ; Print the message jmp hang ; Go to infinite loop msg db 'Hello, World',0x00 putstr: ; Function to print the string lodsb ; al = [DS:SI] or al, al ; Set zero flag if al = 0 jz ret ; Jump to end of function if al = 0 mov ah, 0x0e ; Video function 0Eh (print char) mov bx, 0x0007 ; Color int 0x10 jmp putstr ret: retn hang: jmp hang ; just loop forever. ;--------------------------------------------- ; Write Zeros up to end of program - 2 then boot signature ;--------------------------------------------- size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signature
构建这两个文件并在其上运行hexdump并列出目录中的文件以查看其大小显示：
mehoggan@mehoggan-laptop:~/Code/play/asm$ nasm myboot.asm -f bin -o boot.bin && hexdump boot.bin && ls -l && echo "------" && nasm bootloader1.asm -f bin -o boot.bin && hexdump boot.bin && ls -l 0000000 6548 6c6c 006f 00eb 00be 8a7c 0804 74c0 0000010 e80c 0003 f4e9 b4ff b30e cd07 e910 fffd 0000020 0000 0000 0000 0000 0000 0000 0000 0000 * 0000200 0000 0000 aa55 0000206 total 20 -rw-r--r-- 1 mehoggan mehoggan 518 2012-02-29 21:57 boot.bin -rw-r--r-- 1 mehoggan mehoggan 2290 2012-02-29 20:23 bootloader0.asm -rw-r--r-- 1 mehoggan mehoggan 1661 2012-02-29 21:55 bootloader1.asm -rw-r--r-- 1 mehoggan mehoggan 1786 2012-02-29 21:49 myboot.asm -rw-r--r-- 1 mehoggan mehoggan 1065 2012-02-29 20:14 ourbootloader.asm ------ 0000000 00eb c031 d88e 0fbe e87c 0010 1de9 4800 0000010 6c65 6f6c 202c 6f57 6c72 0064 08ac 74c0 0000020 b40a bb0e 0007 10cd f1e9 c3ff fde9 00ff 0000030 0000 0000 0000 0000 0000 0000 0000 0000 * 00001f0 0000 0000 0000 0000 0000 0000 0000 aa55 0000200 total 20 -rw-r--r-- 1 mehoggan mehoggan 512 2012-02-29 21:57 boot.bin -rw-r--r-- 1 mehoggan mehoggan 2290 2012-02-29 20:23 bootloader0.asm -rw-r--r-- 1 mehoggan mehoggan 1661 2012-02-29 21:55 bootloader1.asm -rw-r--r-- 1 mehoggan mehoggan 1786 2012-02-29 21:49 myboot.asm -rw-r--r-- 1 mehoggan mehoggan 1065 2012-02-29 20:14 ourbootloader.asm为什么文件大小减去6个字节？
I have these two files myboot.asm and theirboot.asm (listed respectively):
;---------------------------------------------------------------------- ; A Simple boot program that prints the string 'Hello World' ; Author: Matthew Hoggan 2012 ;---------------------------------------------------------------------- Output db 'Hello',0x00 ; Output string for bios org 0x7c00 ; This is where BIOS loads the bootloader entry: ; Label to Signify entry to program jmp short begin ; Jump over the DOS boot record data ; -------------------------------------------- ; Boot program code begins here ; -------------------------------------------- begin: ; Label to Signify entry to program mov si, Output ; Get pointer to string loop: ; Top of loop to iterate over string mov al, [si] ; Move contents of pointer to al or al, al ; Check if character pointed to is Zero jz hang ; Zero signifies end of string call print_char ; Print current char in al jmp loop ; Repeat ; -------------------------------------------- ; Function to print char ; assume caller has placed char in al ; -------------------------------------------- print_char: mov ah, 0x0e ; Function to print a character to the screen mov bl, 7 ; color/style to use for the character int 0x10 ; print the character hang: jmp hang ; just loop forever. ;--------------------------------------------- ; Write Zeros up to end of program - 2 then boot signature ;--------------------------------------------- size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signature;---------------------------------------------------------------------- ; A Simple boot program that prints the string 'Hello World' ;---------------------------------------------------------------------- org 0x7c00 ; This is where BIOS loads the bootloader entry: ; Label to Signify entry to program jmp short begin ; Jump over the DOS boot record data ; -------------------------------------------- ; Boot program code begins here ; -------------------------------------------- begin: ; Label to Signify entry to program xor ax, ax ; Zero out ax mov ds, ax ; Set data segment to base of RAM mov si, msg ; Get pointer to string call putstr ; Print the message jmp hang ; Go to infinite loop msg db 'Hello, World',0x00 putstr: ; Function to print the string lodsb ; al = [DS:SI] or al, al ; Set zero flag if al = 0 jz ret ; Jump to end of function if al = 0 mov ah, 0x0e ; Video function 0Eh (print char) mov bx, 0x0007 ; Color int 0x10 jmp putstr ret: retn hang: jmp hang ; just loop forever. ;--------------------------------------------- ; Write Zeros up to end of program - 2 then boot signature ;--------------------------------------------- size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signature
Building both of these files running hexdump on them and listing the files in the directory to see their size reveals:
mehoggan@mehoggan-laptop:~/Code/play/asm$ nasm myboot.asm -f bin -o boot.bin && hexdump boot.bin && ls -l && echo "------" && nasm bootloader1.asm -f bin -o boot.bin && hexdump boot.bin && ls -l 0000000 6548 6c6c 006f 00eb 00be 8a7c 0804 74c0 0000010 e80c 0003 f4e9 b4ff b30e cd07 e910 fffd 0000020 0000 0000 0000 0000 0000 0000 0000 0000 * 0000200 0000 0000 aa55 0000206 total 20 -rw-r--r-- 1 mehoggan mehoggan 518 2012-02-29 21:57 boot.bin -rw-r--r-- 1 mehoggan mehoggan 2290 2012-02-29 20:23 bootloader0.asm -rw-r--r-- 1 mehoggan mehoggan 1661 2012-02-29 21:55 bootloader1.asm -rw-r--r-- 1 mehoggan mehoggan 1786 2012-02-29 21:49 myboot.asm -rw-r--r-- 1 mehoggan mehoggan 1065 2012-02-29 20:14 ourbootloader.asm ------ 0000000 00eb c031 d88e 0fbe e87c 0010 1de9 4800 0000010 6c65 6f6c 202c 6f57 6c72 0064 08ac 74c0 0000020 b40a bb0e 0007 10cd f1e9 c3ff fde9 00ff 0000030 0000 0000 0000 0000 0000 0000 0000 0000 * 00001f0 0000 0000 0000 0000 0000 0000 0000 aa55 0000200 total 20 -rw-r--r-- 1 mehoggan mehoggan 512 2012-02-29 21:57 boot.bin -rw-r--r-- 1 mehoggan mehoggan 2290 2012-02-29 20:23 bootloader0.asm -rw-r--r-- 1 mehoggan mehoggan 1661 2012-02-29 21:55 bootloader1.asm -rw-r--r-- 1 mehoggan mehoggan 1786 2012-02-29 21:49 myboot.asm -rw-r--r-- 1 mehoggan mehoggan 1065 2012-02-29 20:14 ourbootloader.asmWhy are the files sizes off by 6 bytes?
最满意答案
查看那里的最后一小段汇编代码：
size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signature这段代码计算代码的大小（从entry到当前位置），然后用最后两个位置将其填充为总共512字节的零和一个签名0x55 0xAA 。 那是：
entry: Some code . . . Some zeroes . . . 0x55 0xAA那个小的汇编块意味着从entry标签到0x55 0xAA的输出大小总是 512字节。 在第一个示例中，在entry之前有一个六字节字符串Hello\0 。 在你的第二个例子中没有。 因此，第一个程序比第二个程序长六个字节。 您可能希望在entry之后和填充块之前将该字符串移动到某个位置。
如果你在二进制文件上使用hexump -C ，你会在第一个二进制文件的顶部看到字符串。
Check out the last little block of assembly code there:
size equ $ - entry times (512 - size - 2) db 0 db 0x55, 0xAA ;2 byte boot signatureThis block of code calculates how big the code is (from entry to the current location), then pads it out to a total of 512 bytes with zeroes and a signature 0x55 0xAA in the last two positions. That is:
entry: Some code . . . Some zeroes . . . 0x55 0xAAThat little assembly block means the output size from the entry label to 0x55 0xAA is always 512 bytes. In your first example, there is a six byte string Hello\0 before entry. In your second example there's not. Therefore, the first program is six bytes longer than the second. You probably want to move that string to someplace after entry and before the padding block.
If you use hexump -C on your binaries, you'll see the string right up at the top of the first binary.
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