问题
In assembly we use the org instruction to set the location counter to a specific location in the memory. This is particularly helpful in making Operating Systems. Here's an example boot loader (From wikibooks):
org 7C00h
jmp short Start ;Jump over the data (the 'short' keyword makes the jmp instruction smaller)
Msg: db "Hello World! "
EndMsg:
Start: mov bx, 000Fh ;Page 0, colour attribute 15 (white) for the int 10 calls below
mov cx, 1 ;We will want to write 1 character
xor dx, dx ;Start at top left corner
mov ds, dx ;Ensure ds = 0 (to let us load the message)
cld ;Ensure direction flag is cleared (for LODSB)
Print: mov si, Msg ;Loads the address of the first byte of the message, 7C02h in this case
;PC BIOS Interrupt 10 Subfunction 2 - Set cursor position
;AH = 2
Char: mov ah, 2 ;BH = page, DH = row, DL = column
int 10h
lodsb ;Load a byte of the message into AL.
;Remember that DS is 0 and SI holds the
;offset of one of the bytes of the message.
;PC BIOS Interrupt 10 Subfunction 9 - Write character and colour
;AH = 9
mov ah, 9 ;BH = page, AL = character, BL = attribute, CX = character count
int 10h
inc dl ;Advance cursor
cmp dl, 80 ;Wrap around edge of screen if necessary
jne Skip
xor dl, dl
inc dh
cmp dh, 25 ;Wrap around bottom of screen if necessary
jne Skip
xor dh, dh
Skip: cmp si, EndMsg ;If we're not at end of message,
jne Char ;continue loading characters
jmp Print ;otherwise restart from the beginning of the message
times 0200h - 2 - ($ - $$) db 0 ;Zerofill up to 510 bytes
dw 0AA55h ;Boot Sector signature
;OPTIONAL:
;To zerofill up to the size of a standard 1.44MB, 3.5" floppy disk
;times 1474560 - ($ - $$) db 0
Is it possible accomplish the task with C++? Is there any command, function etc. like org where i can change the location of the program?
回答1:
No it's not possible to do in any C compiler that I know of. You can however create your own linker script that places the code/data/bss segments at specific addresses.
回答2:
Just for clarity, the org directive does not load the code at the specified address, it merely informs the assembler that the code will be loaded at that address. The code shown appears to be for Nasm (or similar) - in AT&T syntax, the .org directive does something different: it pads the code to that address - similar to the times line in the Nasm code.. Nasm can do this because in -f bin mode, it "acts as it's own linker".
The important thing for the code to know is the address where Msg can be found. The jmps and jnes (and call and ret which your example doesn't have, but a compiler may generate) are relative addressing mode. We code jmp target but the bytes that are actually emitted say jmp distance_to_target (plus or minus) so the address doesn't matter.
Gas doesn't do this, it emits a linkable object file. To use ld without a linker script the command line looks something like:
ld -o boot.bin boot.o -oformat binary -T text=0x7C00
(don't quote me on that exact syntax but "something like that") If you can get a linkable object file from your (16-bit capable!) C++ compiler, you might be able to do the same.
In the case of a bootsector, the code is loaded by the BIOS (or fake BIOS) at 0x7C00 - one of the few things we can assume about the bootsector. The sane thing for a bootsector to do is not fiddle-faddle around printing a message, but to load something else. You'll need to know how to find the something else on the disk and where you want to load it to (perhaps where your C++ compiler wants to put it by default) - and jmp there. This jmp will want to be a far jmp, which does need to know the address.
I'm guessing it's going to be some butt-ugly C++!
来源：https://stackoverflow.com/questions/18526977/org-alternative-for-c