about freebsd boot1.S

From: vijay singh (vijju_s_at_hotmail.com)
Date: 09/29/04

  • Next message: John Baldwin: "Re: about freebsd boot1.S" 
    To: freebsd-hackers@freebsd.org
Date: Wed, 29 Sep 2004 12:32:53 -0700

    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.
    thanks
    vijay

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  • Next message: John Baldwin: "Re: about freebsd boot1.S"