From: "Marcin Kościelnicki" <markosc@interia.pl>
To: linux-assembly@vger.kernel.org
Subject: Re: grub thing
Date: Wed, 1 Mar 2006 15:42:50 +0100 [thread overview]
Message-ID: <200603011542.51608.markosc@interia.pl> (raw)
In-Reply-To: <20060301071033.0c9eb1d4.amerei@gmail.com>
> i'm looking at a disassembly of grub's stage 1. and i wanna know why it
> need's to CLI before setting up the environment??
It's because it sets the stack address -- otherwise, interrupt could occur
between mov to ss and mov to sp, which would use invalid stack address and be
lethal.
Also, it has NOTHING to do with lack of interrupt handlers -- BIOS has
handlers for all interrupts needed for bootstrap, and the others are
disabled. No bad thing can happen. In fact, it re-enables interrupts right
after setting the stack (sti)
> i tried to interpret it
> line by line and this is what i have so far
>
> 00007C4A FA cli ; disable interrupt
> 00007C4B 80CA80 or dl,0x80 ; correct boot drive byte
> 00007C4E EA537C0000 jmp 0x0:0x7c53 ; jmp to next inst
> 00007C53 31C0 xor ax,ax ; zero ax
> 00007C55 8ED8 mov ds,ax ; zero data segment
> 00007C57 8ED0 mov ss,ax ; zero stack segment
> 00007C59 BC0020 mov sp,0x2000 ; 8 kilobytes
> 00007C5C FB sti ; enable interrupt
>
> also, since the origin is at 0000:7c00h i figured that must imply that "mov
> sp, 0x2000" means that the stack starts at 0x7c00+0x2000 = 9c000 right?
No.
> 00007C57 8ED0 mov ss,ax ; zero stack segment
> 00007C59 BC0020 mov sp,0x2000 ; 8 kilobytes
Here, ss==0 and sp==0x2000. Stack starts at ss*0x10+sp == 0x0*0x10+0x2000 ==
0x2000. Also, remember it grows down.
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next prev parent reply other threads:[~2006-03-01 14:42 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-02-22 21:26 nasm -f bin / Elf format Thiago Silva
2006-02-23 0:32 ` Brian Raiter
2006-03-01 7:10 ` grub thing Aleph One
2006-03-01 13:34 ` Ricardo Nabinger Sanchez
2006-03-02 6:49 ` Aleph One
2006-03-01 14:42 ` Marcin Kościelnicki [this message]
2006-03-01 18:08 ` Ricardo Nabinger Sanchez
2006-03-02 6:46 ` Aleph One
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