Reboots after ''OR AL,1 MOV CR0, EAX'' on some computers.

[john at essenz.com]

Ha...

I thought this email was spam, with a subject resembling ORAL MOVIES !!


-john

----------------------------------------------------
>From : breath <breath at unix.net>
To : freebsd-hackers at freebsd.org
Subject : Reboots after "OR AL,1  MOV CR0, EAX" on some computers.
Date : Fri, 16 Jul 2004 00:34:46 +0400
> Hi. I need some help. I think you'll be interested. I develop a little 
OS 
> that will boot from floppy. It's still under construction and has about
> five thousand lines of assembly source now. I use Bochs emulator to 
debug 
> it and after that I test it on my real computer. I have all working
> correctly on my compter. But I started to test it around other machines
> recently and encountered a problem I can't solve. But I'm sure there are
> lot of minds in FreeBSD world who know how to help me. I can't figure 
out 
> what happens because on my computer I get all things well, but I can't
> conduct test of this kind on other computers.
> 
> Here is the problem. My OS's boot loader (after loading the main module)
> switches CPU to very limited protected mode and jumps to module's entry
> point. After it, main module reloads all tables like GDT etc to make a
> good free environment for initialization and further running. Shortly: 
ON 
> SOME COMPUTERS (MAYBE CPUS) I GET REBOOTING JUST ON 'JMP' INSTRUCTION
> AFTER PE BIT IS ENABLED. I've got no reboots on all i386, i486, i586
> computers that I tryed to boot from. I have a Pentium III Celeron
> (Coppermine) 900MHz - no reboots. Also tested on some Pentium II 400MHz 
- 
> no reboots. But on other side Pentium IV (don't remember speed) gave me 
a 
> reboot. And other computer I was not able to see processor model (maybe
> PentiumIV !?) gave me a reboot too. Using endless loop stop points I
> figured out that reboot is before any instruction pointed by 'protected'
> label and that reboot happens after setting the PE bit.
> 
> Here's very diminished boot sector code that reflects the error. It must
> be compiled with nasm as a binary so that result will be a floppy disk
> image file. Then it must be written to a floppy from scratch (from boot
> sector). Created floppy is expected to be bootable disk.
> 
> Normal:
> When you boot from the floppy you get something like black screen and
> computer hangs up (but not reboots)
> 
> Abnormal:
> When you boot from the floppy you get a screen splash and then back to
> reboot POST procedure.
> 
> Here goes the code. PLEASE, even if you're not interested in this
> 'puzzle':( MAKE AND TEST IT ON YOUR COMPUTER. EMAIL ME RESULTS AND
> COMPUTER MODEL.
> 
> It was HARD work to make this OS. I believe that some people understand 
me 
> and will give me a clue. From my side I'll show them the full version.
> 
> ;---------------------------------------------------------------------
> 
> ; Compile: %nasm thisfile.asm -o fd
> ; Insert a floppy
> ; Write (as root): #dd if=fd of=/dev/fd0
> 
> org 0x7C00	; Expecting to be loaded at 7C00 by BIOS
> 
> bits 16
> 
> real:
> 	cli
> 	xor	ax, ax
> 	mov	ss, ax
> 	mov	sp, 0x7C00	; Temp stack just under myself
> 
> 	call	real_open_A20	; For 32 bit address space
> 	call	real_init_gdt	; Load GDTR
> 
> 	mov	eax, cr0
> 	or	al, 0x1		; cr0 |= PE
> 	mov	cr0, eax
> ; If I place 'jmp $' here all computers stop here normally
> 	jmp	0x10: protected
> 
> real_open_A20:
> .l1:	in al, 0x64
> 	test al, 0x2
> 	jnz .l1
> 	mov al, 0xD1
> 	out 0x64, al
> .l2:	in al, 0x64
> 	test al, 0x2
> 	jnz .l2
> 	mov al, 0xDF
> 	out 0x60, al
> 	ret
> 
> real_init_gdt:
> 	lgdt	[.gdtr]
> 	ret
> .gdt0	dw	0x0000, 0x0000, 0x0000, 0x0000
> .data	dw	0xFFFF, 0x0000, 0x9200, 0x00CF
> .code	dw	0xFFFF, 0x0000, 0x9800, 0x00CF
> .gdtr	dw	$ - .gdt0 - 1
> 	dd	.gdt0
> 
> bits 32
> 
> protected:
> ; Wherever I place 'jmp $' after the 'protected' label,
> ; on some computers I get reboot to hell.
> ; (In that case CS:IP is never points to 'jmp $'
> ; - something happens before)
> 	mov	ax, 0x8
> 	mov	ds, ax
> 	mov	es, ax
> 	mov	fs, ax
> 	mov	gs, ax
> 	mov	ss, ax
> 	mov	esp, 0x7C00
> 	jmp	$
> 
> times 512 - 2 - ($ - $$) db 0	; Fill the rest of sector
> dw 0xAA55	; Bootable sector sign
> 
> times 1474560 - ($ - $$) db 0	; Fill the rest of floppy
> 
> ;---------------------------------------------------------------------
> 
> Best regards,
> Yuri Grebenkin
> breath at unix.net
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