Hi I saw the Makefile for bootsect.S in arch/i386/boot/
The bootsect.S file is linked at origin 0x00. (-Ttext option)
ld -Ttext 0x00 --oformat binary -o bootsect bootsect.o
There is a small code that prints a string "hello world" in real mode.
When I link this code with the same options as above the string does not
print. That is, I link the code and write it to a boot sector of a
floppy using the dd command and then boot from the floppy. When i link
the code with the option "-Ttext 0x7c00" it works. I guess the reason for
it to work is that ROM will load the boot code at 0x7c00 as mentioned in a
lot of docs.
The problem with my code was that there was code that stores the address of
the string in BP
mov $my_string, %bp ; real mode code
my_string was defined as -
my_string
.ascii "hello world\r\n"
I guess the linker was not adding the extra offset 0x7c00 to the string
address and hence it was not working. Now for some doubts that came up due
to this -
1. Shouldn't the bootsect.S code face the same problems? Where exactly is
the whole code relocated to 0x7c00?
2. I was also calling functions in my code. For example I had code like
call clrscr where clrscr is a label. This code works fine even with -Ttext
0x00. Is there a reason why labels work fine but mov $my_string, %bp does
not?
TIA.
-mohit
Regards,
Mohit Kalra
"It is a capital mistake to theorize before one has data.
Insensibly one begins to twist facts to suit theories, instead of theories
to suit facts."
-SH
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