Re: about freebsd boot1.S

From: John Baldwin (jhb_at_FreeBSD.org)
Date: 09/30/04 

To: freebsd-hackers@FreeBSD.org
Date: Thu, 30 Sep 2004 10:34:28 -0400

On Wednesday 29 September 2004 03:32 pm, vijay singh wrote:
> hello all, i am trying to understand the freebsd boot code. i saw that the
> 1st stage boot loader relocates itself from 7c00 to 700. why is this done?
> if the run time kernel were to switch to real-mode and transfer control to
> this location (0700) would the 1st stage boot program run again?
>
> for the output below, i used the BIOS debugger and read the 1st sector into
> 7c00, and unassebled the text.
>
> 0000:00007C1D cld
> 0000:00007C1E xor cx, cx /* cx = 0 */
> 0000:00007C20 mov es, cx /* es = 0 */
> 0000:00007C22 mov ds, cx /* ds = 0 */
> 0000:00007C24 mov ss, cx /* ss = 0 */
> 0000:00007C26 mov sp, 7C00 /* set SP to current location */
> 0000:00007C29 mov si, sp
> 0000:00007C2B mov di, 0700 /* DS:SI pair denotes the source string
> and ES:DI pair the destination string 0:7C00 -> 0:700 */
> 0000:00007C2E inc ch /* ch = 1, cx = 100*/
> 0000:00007C30 repe /* repeat the movsw instruction cx number of times */
> 0000:00007C31 movsw /* moves 200h = 512 bytes from 7C00 to 700*/
>
> please cc me in your reply.

We copy ourselves down so that we can use 0x7c00 as a buffer to load sectors
into off of the disk. 

-- 
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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