Re: GRUB sample kernel question

[ram <ram@curvesoft.com>]

Alexander Jänicke wrote:

>Am Samstag, 1. November 2003 18:07 schrieb ram:
>  
>
>>Hi,
>>
>>I was looking at the toy kernel in the grub distribution (multiboot.h,
>>kernel.c, boot.S
>>in the 'docs' subdirectory) and noticed boot.S starts out like this:
>>
>>    .globl    start, _start
>>start:
>>_start:
>>    jmp    multiboot_entry
>>    ....
>>multiboot_entry:
>>
>>I'm wondering why this jump is needed; couldn't the start/_start labels
>>simply have
>>been placed adjacent to the multiboot_entry label thereby avoiding the
>>jump ?
>>
>>Thanks.
>>
>>Ram
>>    
>>
>
>
>No, this jump is necessary for making sure that the cs register is initialised 
>correctly. Every BIOS loads the bootloader to physical address 0x07c00. 
>Normally cs=0 and ip=0x7c00, but to be on the safe side, you should make a 
>far-jump. That's why code in most bootloaders begins with a 
>jmp 0x0000:StartAddr or jmp 0x07c0:StartAddr, dependent on whether the 
>location counter started at 0x0000 or 0x07c0.
>If you are interested in os development, have a look at 
>http://www.nondot.org/sabre/os/articles
>
>Alex
>-
>To unsubscribe from this list: send the line "unsubscribe linux-assembly" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>  
>
Thanks for the response but I'm still a little confused: my impression 
is that this
code is not a bootloader but a sample 32-bit OS loaded by GRUB, so GRUB has
already switched to protected mode when this code starts to execute ?

Ram

-
To unsubscribe from this list: send the line "unsubscribe linux-assembly" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html