Date: Mon, 17 Dec 2007 19:16:02 +0530 From: "M.Girish Rao" <m.girish.rao@gmail.com> To: freebsd-hackers@freebsd.org Subject: boot0 code mystery Message-ID: <47667D9A.8070300@gmail.com>
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Hi,
I am reading the code for boot0 (/usr/src/sys/boot/i386/boot0/boot0.S).
This is the part i am trying to understand:
--------------------------------------------------------------------------------------------------------
/*
* Initialise segments and registers to known values.
* segments start at 0.
* The stack is immediately below the address we were loaded to.
*/
start: cld # String ops inc
xorw %ax,%ax # Zero
movw %ax,%es # Address
movw %ax,%ds # data
movw %ax,%ss # Set up
movw $LOAD,%sp # stack
/*
* Copy this code to the address it was linked for
*/
movw %sp,%si # Source
movw $start,%di # Destination
movw $0x100,%cx # Word count
rep # Relocate
movsw # code
/*
* Set address for variable space beyond code, and clear it.
* Notice that this is also used to point to the values embedded in the
block,
* by using negative offsets.
*/
movw %di,%bp # Address variables
movb $0x8,%cl # Words to clear
rep # Zero
stosw # them
/*
* Relocate to the new copy of the code.
*/
incb -0xe(%di) # Sector number
jmp main-LOAD+ORIGIN # To relocated code
----------------------------------------------------------------------------------------------------------------
This is all the information I could gather:
From this first disk's first sector, 512 bytes are read into the memory
location of 0x7C00. After that, the BIOS will check for the number
0xAA55 at the memory location of 0x7DFE (the last two bytes of the boot
block code). After the boot0 program is loaded and control is
transferred to it, it will set up its registers and stack information.
Then, boot0 relocates itself into a lower memory location and jumps to
the new address offset to its main routine.
Whats the memory location of start?
what's this for incb -0xe(%di) ?
where are we jumping to in jmp main-LOAD+ORIGIN?
whats ORIGIN?
I would really appreciate if some could kindly help me out with this.
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