Message-Id: <199612032147.QAA27120@delorie.com>
From: Kuang-Chun Cheng <kccheng AT hep2 DOT phys DOT sinica DOT edu DOT tw>
Subject: boot loader problem, need advise
To: djgpp AT delorie DOT com
Date: Wed, 4 Dec 96 5:49:27 EAT
Mailer: Elm [revision: 70.85]

Hi,

    Recently, I am interesting about boot loader (specially for COFF
format file).  I got one from Bill Currie which build bootstrap image
by using djasm.  I also went through the method Linux used which build
bootstrap image by as86.  Well, I ask myself why not just use GNU as
to build bootstrap image?  Why use two different assembler??  So I did
some tests and I failed while switching to protected mode :(  And worst,
I don't know why it failed :(

    First, I add .code16 directive at the beginning of the asm code.
If I didn't add .code16, I can't print out string by calling BIOS video
services.   I want the 512 bytes bootstrap image can switch cpu to pmode
before pass control to COFF file (just as Bill's version did).  I set
up the IDT and GDT (I believe they are setup correctly).  The problem
show up while executing (I use objcopy -O binary coff_out bin_out to
generate image)

	lidt   idt_lable

This instruction causes a reboot cycle.  That meant when I try to boot
from floppy (with the image I wrote), system reboot again when it meet
instruction lidt.  I did disable interrupt, NMI and try to reset 8259
as Linux kernel did ...  No matter how I try, it just doesn't work :(

If I comment out lidt and switch to pmode blinding by 

        movw    %cr0, %eax
        orb     %al, $0x01
        movw    %eax, %cr0     # (1)
or
        xorw    %ax, %ax
        inc     %ax
        lmsw    %ax            # (2)

The same problem as lidt appear at (1) and (2).  Well, I just can't figure
out what's wrong.  Could someone explain what's wrong for me?  Is it
possible to write a bootstrap loader by GNU as?  And how ?

     Thanks a lot

     Kuang-chun Cheng
     kccheng AT hep2 DOT phys DOT sinica DOT edu DOT tw