In every bootloader code that I've seen, there's this line "ORG 0x7c00" at the beginning of the file. As far as I know, BIOS will put this code at address 0x7c00.
So what is the reason of doing the "org 0x7c00"? Isn't it true that if the first instruction is loaded into 0x7c00, the counter will automatically start from there? Isn't the address of every instruction 0x7c00 + offset anyway?
If you have the following code:
org 0x7c00
bits 16
jmp somefunction
somefunction:
xor ax, ax
mov ds, ax
cli
lgdt[gdtr]
mov eax, cr0
or al, 1
mov cr0, eax
jmp 0x08:protectedMode
bits 32
protectedMode:
mov ax, 0x10
mov ds, ax
hlt
gdt_start:
dq 0x0
gdt_code:
dw 0xFFFF ;limit 0-15
dw 0x0 ;base 0-15
db 0x0 ;base 16-23
db 10011010b ;pr, privi (2), s, ex, dc, rw, ac
db 11001111b ;gr, sz, limit 16-19
db 0x0 ;base 24-31
gdt_data:
dw 0xFFFF
dw 0x0
db 0x0
db 10010010b
db 11001111b
db 0x0
gdtr:
dw 24
dd gdt_start
times 510 - ($-$$) db 0
dw 0xAA55
Assemble with nasm -fbin boot.s -oboot.bin. You can disassemble easily the first part (16 bits part) of boot.bin with ndisasm to see what happens. Run the command ndisasm -b16 boot.bin and see the result:
00000000 EB00 jmp short 0x2
00000002 31C0 xor ax,ax
00000004 8ED8 mov ds,ax
00000006 FA cli
00000007 0F0116387C lgdt [0x7c38]
0000000C 0F20C0 mov eax,cr0
0000000F 0C01 or al,0x1
00000011 0F22C0 mov cr0,eax
00000014 EA197C0800 jmp 0x8:0x7c19
00000019 66B810008ED8 mov eax,0xd88e0010
0000001F F4 hlt
00000020 0000 add [bx+si],al
00000022 0000 add [bx+si],al
00000024 0000 add [bx+si],al
00000026 0000 add [bx+si],al
00000028 FF db 0xff
00000029 FF00 inc word [bx+si]
0000002B 0000 add [bx+si],al
0000002D 9ACF00FFFF call 0xffff:0xcf
00000032 0000 add [bx+si],al
00000034 0092CF00 add [bp+si+0xcf],dl
00000038 1800 sbb [bx+si],al
0000003A 207C00 and [si+0x0],bh
0000003D 0000 add [bx+si],al
0000003F 0000 add [bx+si],al
00000041 0000 add [bx+si],al
00000043 0000 add [bx+si],al
00000045 0000 add [bx+si],al
00000047 0000 add [bx+si],al
00000049 0000 add [bx+si],al
0000004B 0000 add [bx+si],al
0000004D 0000 add [bx+si],al
0000004F 0000 add [bx+si],al
00000051 0000 add [bx+si],al
00000053 0000 add [bx+si],al
00000055 0000 add [bx+si],al
00000057 0000 add [bx+si],al
00000059 0000 add [bx+si],al
0000005B 0000 add [bx+si],al
0000005D 0000 add [bx+si],al
0000005F 0000 add [bx+si],al
00000061 0000 add [bx+si],al
00000063 0000 add [bx+si],al
00000065 0000 add [bx+si],al
00000067 0000 add [bx+si],al
00000069 0000 add [bx+si],al
0000006B 0000 add [bx+si],al
0000006D 0000 add [bx+si],al
0000006F 0000 add [bx+si],al
00000071 0000 add [bx+si],al
00000073 0000 add [bx+si],al
00000075 0000 add [bx+si],al
00000077 0000 add [bx+si],al
00000079 0000 add [bx+si],al
0000007B 0000 add [bx+si],al
0000007D 0000 add [bx+si],al
0000007F 0000 add [bx+si],al
00000081 0000 add [bx+si],al
00000083 0000 add [bx+si],al
00000085 0000 add [bx+si],al
00000087 0000 add [bx+si],al
00000089 0000 add [bx+si],al
0000008B 0000 add [bx+si],al
0000008D 0000 add [bx+si],al
0000008F 0000 add [bx+si],al
00000091 0000 add [bx+si],al
00000093 0000 add [bx+si],al
00000095 0000 add [bx+si],al
00000097 0000 add [bx+si],al
00000099 0000 add [bx+si],al
0000009B 0000 add [bx+si],al
0000009D 0000 add [bx+si],al
0000009F 0000 add [bx+si],al
000000A1 0000 add [bx+si],al
000000A3 0000 add [bx+si],al
000000A5 0000 add [bx+si],al
000000A7 0000 add [bx+si],al
000000A9 0000 add [bx+si],al
000000AB 0000 add [bx+si],al
000000AD 0000 add [bx+si],al
000000AF 0000 add [bx+si],al
000000B1 0000 add [bx+si],al
000000B3 0000 add [bx+si],al
000000B5 0000 add [bx+si],al
000000B7 0000 add [bx+si],al
000000B9 0000 add [bx+si],al
000000BB 0000 add [bx+si],al
000000BD 0000 add [bx+si],al
000000BF 0000 add [bx+si],al
000000C1 0000 add [bx+si],al
000000C3 0000 add [bx+si],al
000000C5 0000 add [bx+si],al
000000C7 0000 add [bx+si],al
000000C9 0000 add [bx+si],al
000000CB 0000 add [bx+si],al
000000CD 0000 add [bx+si],al
000000CF 0000 add [bx+si],al
000000D1 0000 add [bx+si],al
000000D3 0000 add [bx+si],al
000000D5 0000 add [bx+si],al
000000D7 0000 add [bx+si],al
000000D9 0000 add [bx+si],al
000000DB 0000 add [bx+si],al
000000DD 0000 add [bx+si],al
000000DF 0000 add [bx+si],al
000000E1 0000 add [bx+si],al
000000E3 0000 add [bx+si],al
000000E5 0000 add [bx+si],al
000000E7 0000 add [bx+si],al
000000E9 0000 add [bx+si],al
000000EB 0000 add [bx+si],al
000000ED 0000 add [bx+si],al
000000EF 0000 add [bx+si],al
000000F1 0000 add [bx+si],al
000000F3 0000 add [bx+si],al
000000F5 0000 add [bx+si],al
000000F7 0000 add [bx+si],al
000000F9 0000 add [bx+si],al
000000FB 0000 add [bx+si],al
000000FD 0000 add [bx+si],al
000000FF 0000 add [bx+si],al
00000101 0000 add [bx+si],al
00000103 0000 add [bx+si],al
00000105 0000 add [bx+si],al
00000107 0000 add [bx+si],al
00000109 0000 add [bx+si],al
0000010B 0000 add [bx+si],al
0000010D 0000 add [bx+si],al
0000010F 0000 add [bx+si],al
00000111 0000 add [bx+si],al
00000113 0000 add [bx+si],al
00000115 0000 add [bx+si],al
00000117 0000 add [bx+si],al
00000119 0000 add [bx+si],al
0000011B 0000 add [bx+si],al
0000011D 0000 add [bx+si],al
0000011F 0000 add [bx+si],al
00000121 0000 add [bx+si],al
00000123 0000 add [bx+si],al
00000125 0000 add [bx+si],al
00000127 0000 add [bx+si],al
00000129 0000 add [bx+si],al
0000012B 0000 add [bx+si],al
0000012D 0000 add [bx+si],al
0000012F 0000 add [bx+si],al
00000131 0000 add [bx+si],al
00000133 0000 add [bx+si],al
00000135 0000 add [bx+si],al
00000137 0000 add [bx+si],al
00000139 0000 add [bx+si],al
0000013B 0000 add [bx+si],al
0000013D 0000 add [bx+si],al
0000013F 0000 add [bx+si],al
00000141 0000 add [bx+si],al
00000143 0000 add [bx+si],al
00000145 0000 add [bx+si],al
00000147 0000 add [bx+si],al
00000149 0000 add [bx+si],al
0000014B 0000 add [bx+si],al
0000014D 0000 add [bx+si],al
0000014F 0000 add [bx+si],al
00000151 0000 add [bx+si],al
00000153 0000 add [bx+si],al
00000155 0000 add [bx+si],al
00000157 0000 add [bx+si],al
00000159 0000 add [bx+si],al
0000015B 0000 add [bx+si],al
0000015D 0000 add [bx+si],al
0000015F 0000 add [bx+si],al
00000161 0000 add [bx+si],al
00000163 0000 add [bx+si],al
00000165 0000 add [bx+si],al
00000167 0000 add [bx+si],al
00000169 0000 add [bx+si],al
0000016B 0000 add [bx+si],al
0000016D 0000 add [bx+si],al
0000016F 0000 add [bx+si],al
00000171 0000 add [bx+si],al
00000173 0000 add [bx+si],al
00000175 0000 add [bx+si],al
00000177 0000 add [bx+si],al
00000179 0000 add [bx+si],al
0000017B 0000 add [bx+si],al
0000017D 0000 add [bx+si],al
0000017F 0000 add [bx+si],al
00000181 0000 add [bx+si],al
00000183 0000 add [bx+si],al
00000185 0000 add [bx+si],al
00000187 0000 add [bx+si],al
00000189 0000 add [bx+si],al
0000018B 0000 add [bx+si],al
0000018D 0000 add [bx+si],al
0000018F 0000 add [bx+si],al
00000191 0000 add [bx+si],al
00000193 0000 add [bx+si],al
00000195 0000 add [bx+si],al
00000197 0000 add [bx+si],al
00000199 0000 add [bx+si],al
0000019B 0000 add [bx+si],al
0000019D 0000 add [bx+si],al
0000019F 0000 add [bx+si],al
000001A1 0000 add [bx+si],al
000001A3 0000 add [bx+si],al
000001A5 0000 add [bx+si],al
000001A7 0000 add [bx+si],al
000001A9 0000 add [bx+si],al
000001AB 0000 add [bx+si],al
000001AD 0000 add [bx+si],al
000001AF 0000 add [bx+si],al
000001B1 0000 add [bx+si],al
000001B3 0000 add [bx+si],al
000001B5 0000 add [bx+si],al
000001B7 0000 add [bx+si],al
000001B9 0000 add [bx+si],al
000001BB 0000 add [bx+si],al
000001BD 0000 add [bx+si],al
000001BF 0000 add [bx+si],al
000001C1 0000 add [bx+si],al
000001C3 0000 add [bx+si],al
000001C5 0000 add [bx+si],al
000001C7 0000 add [bx+si],al
000001C9 0000 add [bx+si],al
000001CB 0000 add [bx+si],al
000001CD 0000 add [bx+si],al
000001CF 0000 add [bx+si],al
000001D1 0000 add [bx+si],al
000001D3 0000 add [bx+si],al
000001D5 0000 add [bx+si],al
000001D7 0000 add [bx+si],al
000001D9 0000 add [bx+si],al
000001DB 0000 add [bx+si],al
000001DD 0000 add [bx+si],al
000001DF 0000 add [bx+si],al
000001E1 0000 add [bx+si],al
000001E3 0000 add [bx+si],al
000001E5 0000 add [bx+si],al
000001E7 0000 add [bx+si],al
000001E9 0000 add [bx+si],al
000001EB 0000 add [bx+si],al
000001ED 0000 add [bx+si],al
000001EF 0000 add [bx+si],al
000001F1 0000 add [bx+si],al
000001F3 0000 add [bx+si],al
000001F5 0000 add [bx+si],al
000001F7 0000 add [bx+si],al
000001F9 0000 add [bx+si],al
000001FB 0000 add [bx+si],al
000001FD 0055AA add [di-0x56],dl
A short jump like a jmp to a label is relative. This means that the jump will work even if you don't have the org directive. In the meantime, a far jump is absolute. It jumps to an address specified as a segment:offset pair. The org directive tells the assembler to add the org (origin) value to the offset. This allows that when you far jump to a label with an unknown position (instead of hardcoding the position of the label), the offset will be calculated properly by the assembler considering the actual position of the code in RAM.
With the example above, the org directive is useful only for the far jump to protected mode and for the lgdt instruction. You see that the far jump is disassembled to the line 00000014 EA197C0800 jmp 0x8:0x7c19. The far jump's address is absolute meaning that the address is calculated directly. If you don't have the org directive, the disassembled far jump will instead be jmp 0x8:0x19. This is not the right address. It would jump to address 0x19 which doesn't work with the fact that your bootloader code will be loaded at 0x7c00. You need to tell the assembler to add 0x7c00 to all absolute addresses (mostly far jumps and the lgdt instruction).