linux-kernel - Re: [PATCH] Make bootsector stub 16-bit-only (i386)

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Message-ID: <4640C115.6080300@zytor.com>
Date:	Tue, 08 May 2007 11:27:33 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Alexander van Heukelum <heukelum@...lshack.com>,
	Andi Kleen <ak@...e.de>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Make bootsector stub 16-bit-only (i386)

Andrew Morton wrote:
> 
> Without patch:
> 
> (gdb)  x/20i _start
> 0x0 <_start>:   ljmpw  $0x0,$0x8
> 0x6 <_start+6>: rolb   $0x8c,(%edi)
> 0x9 <start2+1>: enter  $0xd88e,$0x8e
> 0xd <start2+5>: rorb   $0xfb,0x7c00bcd0(%esi)
> 0x14 <start2+12>:       cld    
> 0x15 <start2+13>:       mov    $0x20ac0031,%esi
> 0x1a <msg_loop+2>:      (bad)  
> 0x1b <msg_loop+3>:      je     0x26 <die>
> 0x1d <msg_loop+5>:      mov    $0xe,%ah
> 0x1f <msg_loop+7>:      mov    $0x10cd0007,%ebx
> 0x24 <msg_loop+12>:     jmp    0x18 <msg_loop>
> 0x26 <die>:     xor    %eax,%eax
> 0x28 <die+2>:   int    $0x16
> 
> With patch:
> 
> (gdb) x/20i _start
> 0x0 <_start>:   ljmp   $0xc88c,$0x7c00005
> 0x7 <start2+2>: mov    %eax,%ds
> 0x9 <start2+4>: mov    %eax,%es
> 0xb <start2+6>: mov    %eax,%ss
> 0xd <start2+8>: mov    $0xfcfb7c00,%esp
> 0x12 <start2+13>:       mov    $0x20ac002e,%esi
> 0x17 <msg_loop+2>:      (bad)  
> 0x18 <msg_loop+3>:      je     0x23 <die>
> 0x1a <msg_loop+5>:      mov    $0xe,%ah
> 0x1c <msg_loop+7>:      mov    $0x10cd0007,%ebx
> 0x21 <msg_loop+12>:     jmp    0x15 <msg_loop>
> 
> not sure what's going on there.  We seem to have confused gdb.
> 

You're trying to disassemble 16-bit code as if it was 32-bit code.  The
easiest way to disassemble is to have NASM installed on your machine and
just do:

	ndisasm bzImage

My setup code rewrite creates an intermediate .elf file to retain
debugging information and make 16-bit disassembly
("objdump -d -m i8086") easier.

	-hpa
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