У меня есть два файла myboot.asm и ихboot.asm (перечислены соответственно):
;----------------------------------------------------------------------
; 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
Сборка обоих этих файлов с использованием hexdump и перечисление файлов вкаталог, чтобы увидеть их размер показывает:
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
Почему размер файлов уменьшается на 6 байт?