Date: Wed, 29 Sep 2004 12:32:53 -0700 From: "vijay singh" <vijju_s@hotmail.com> To: freebsd-hackers@freebsd.org Subject: about freebsd boot1.S Message-ID: <BAY13-F17DcHqvhRgmX0000f316@hotmail.com>
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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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