boot0 code mystery

[Jeremy Chadwick]

On Mon, Dec 17, 2007 at 07:16:02PM +0530, M.Girish Rao wrote:
> Whats the memory location of start?

I'm going off of memory of my old x86 days, so be kind to me.  :-) By
the look of it, it's BOOT_BOOT0_ORG, which is 0x600.  I'm basing this on
the flags passed to cc (actually ld) during linktime.

> what's this for incb -0xe(%di)  ?

No idea.

> where are we jumping to in jmp main-LOAD+ORIGIN?

main is below the jmp main-LOAD+ORIGIN jump:

 91                 incb -0xe(%di)                  # Sector number
 92                 jmp main-LOAD+ORIGIN            # To relocated code
 93
 94 main:
 95 #if defined(SIO) && COMSPEED != 0

LOAD is set to 0x7c00:

 27                 .set LOAD,0x7c00                # Load address

You should be able to get the offset of main by looking at boot0.o once
assembled.  The start origin doesn't appear to be included (which is
why it's added manually).

eos# objdump -t boot0.o | grep 'main$'
00000022 l       .text  00000000 main

Thus I'm left to believe main-LOAD+ORIGIN == 0x8a22.  This can be
verified by doing:

eos# objdump -S -M addr16,data16 boot0.o | grep -m 1 'jmp'
  1f:   e9 00 8a                jmp    8a22 <LOAD+0xe22>

> whats ORIGIN?

ORIGIN is an assembler variable set to 0x600:

 25                 .set ORIGIN,0x600               # Execution address

-- 
| Jeremy Chadwick                                    jdc at parodius.com |
| Parodius Networking                           http://www.parodius.com/ |
| UNIX Systems Administrator                      Mountain View, CA, USA |
| Making life hard for others since 1977.                  PGP: 4BD6C0CB |