Date: Thu, 30 Sep 2004 10:34:28 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-hackers@FreeBSD.org Cc: vijay singh <vijju_s@hotmail.com> Subject: Re: about freebsd boot1.S Message-ID: <200409301034.28349.jhb@FreeBSD.org> In-Reply-To: <BAY13-F17DcHqvhRgmX0000f316@hotmail.com> References: <BAY13-F17DcHqvhRgmX0000f316@hotmail.com>
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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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