From: Aleph One <amerei@gmail.com>
To: "Marcin Kościelnicki" <markosc@interia.pl>
Cc: linux-assembly@vger.kernel.org
Subject: Re: grub thing
Date: Thu, 2 Mar 2006 06:46:34 +0000 [thread overview]
Message-ID: <20060302064634.5acb6f7c.amerei@gmail.com> (raw)
In-Reply-To: <200603011542.51608.markosc@interia.pl>
On Wed, 1 Mar 2006 15:42:50 +0100
Marcin Ko≈õcielnicki <markosc@interia.pl> wrote:
~> 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.
ahh! i see i see. it's for protection.
~
~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.
one note. why must (ss*0x10)?
~
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prev parent reply other threads:[~2006-03-02 6:46 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
2006-03-01 18:08 ` Ricardo Nabinger Sanchez
2006-03-02 6:46 ` Aleph One [this message]
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