need help switching back to real mode

[packet50071]

I spent enough time in kernel programming i am getting bored now so i want work on eye candy for a fun

but i cannot seem to get back to real mode

when i run it vmware says

*** VMware Server internal monitor error ***
vcpu-0:SyncCB failure: 6f76665 (bug #4938)
There is a problem in this version of VMware Server.
We rely on your feedback to improve the quality of our product. Please submit a support request that describes the problem at our Web page "http://www.vmware.com/info?id=8". Do not forget to attach the log file (C:\djgpp\bin\vm\vmware.log) and the core file (c:\djgpp\bin\vm\vmware-core.gz).
To collect files to submit to VMware support, run vm-support.vbs.
We appreciate your feedback,
-- the VMware Server team.

obviously its not their problem. It must be my code

my code :

Code: Select all

[BITS 32]


;part of this code is base on http://www.osdever.net/bkerndev/
global _etry, _multi_b, _halt, _kernel_stack

extern _Gptr,_link_text, _link_data, _link_bss,_PTRidt,_EX_handler ,__irq_handler,_kernal

MULTIBOOT_PAGE_ALIGN     equ 1<<0
MULTIBOOT_AOUT_KLUDGE    equ 1<<16
MULTIBOOT_HEADER_MAGIC   equ 0x1BADB002
MULTIBOOT_HEADER_FLAGS   equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_AOUT_KLUDGE
MULTIBOOT_CHECKSUM       equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)

STACKSZ equ 0x4000 

SECTION .text
	_etry:
        jmp short _startup
        align 4
    _multi_b:
        dd MULTIBOOT_HEADER_MAGIC
        dd MULTIBOOT_HEADER_FLAGS
        dd MULTIBOOT_CHECKSUM
        dd _multi_b
        dd  _link_text
        dd _link_data
        dd  _link_bss
        dd _etry
     _startup:
     mov esp, _kernel_stack+STACKSZ      
     push eax                         
     push ebx 
     ;call _kernal       
     mov eax, cr0
     xor eax, eax
     mov cr0, eax    
	
[BITS 16]
	mov al,65
	int 0x10                
    _halt:
          hlt
    SECTION .bss
        resb STACKSZ
    _kernel_stack:
 

as you can see grub loads my kernel
any help would be great pls
thx in advance

I did a search and googled but nothing help all returned same result
[btw the search function in this site need some repairing (no offense) cause the results were totally unrelated ]

[Masterkiller]

Well I see you clear the whole cr0 register (only the PE needs to be cleared), but I don't believe that to be problem. After changing value of cr0, espesially PE bit, only next ONE instruction is loaded to processor and it is recommended (even it must be) far JMP instruction to preload CS register or the real mode will use the 16-bit value of visible part of CS, multiply by 16, add IP and hop, you are going too far from the next instruction

[Combuster]

Clearing all of CR0 is bad practice. You will kill some bits now that will later cause you trouble. Maybe that's exactly why VMware crashes because you tried to clear the NE (and ET) bits, both of which should be set.

Also, you will need to be in a 16-bit code segment to allow PE to be disabled safely. I think that that is the real culprit at the moment, but please fix the other one too if you don't want to screw up your floating point unit.

[packet50071]

Code: Select all

[BIT 32]  
        cli    
        mov eax,cr0
	or al,0x1
	mov cr0,eax
	jmp   0x0009:_16bitc
[BITS 16]
_16bitc:
	mov al,65

I hope i am not clearing the cr0 now

Also, you will need to be in a 16-bit code segment to allow PE

HUH! which other part should be in 16 bit

Forgive my ignorance

[edit] btw it still doesn't work. I know some thinging is missing but quite cannot get my head around it

[AJ]

Hi,

In your code snippet above, you are actually setting the PE bit. You need to AND CR0 with the inverse (NOT) of the bit you want to clear.

Cheers,
Adam

[Dex]

Here is a demo http://www.dex4u.com/demos/DemoVesa.zip
that i wrote many moons ago, it goes to realmode from pmode to switch between vesa mode and text mode.
It should help you.
Note: I was called "ASHLEY4" when i wrote it.

[packet50071]

that example was Seriously good Well commented
I will ask if I still have problems

[packet50071]

my linker doesn't like it now - I have no clue why
My code now ( i just copy paste ur code to mine For testing )

Code: Select all

[BITS 32]


;part of this code is base on http://www.osdever.net/bkerndev/   &
;http://www.dex4u.com/demos/DemoVesa.zip [ temporarily ]
global _etry, _multi_b, _halt, _kernel_stack,_do_16v,_RealModeCS,_do_rm

extern _linkd,_linkb, _linkt,kernel

MULTIBOOT_PAGE_ALIGN     equ 1<<0
MULTIBOOT_AOUT_KLUDGE    equ 1<<16
MULTIBOOT_HEADER_MAGIC   equ 0x1BADB002
MULTIBOOT_HEADER_FLAGS   equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_AOUT_KLUDGE
MULTIBOOT_CHECKSUM       equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)

STACKSZ equ 0x4000 

[SECTION .text]
   _RealModeCS:
	    dw 0
	_etry:
        jmp short _startup
        align 4
    _multi_b:
        dd MULTIBOOT_HEADER_MAGIC
        dd MULTIBOOT_HEADER_FLAGS
        dd MULTIBOOT_CHECKSUM
        dd _multi_b
        dd _etry
        dd _linkd
        dd _linkb
        dd _linkt
     _startup:
     mov esp, _kernel_stack+STACKSZ      
     push eax                         
     push ebx 
     ;call _kernel   
	   pushad
     jmp   20h:_do_16v
        
	[BITS 16]
_do_16v:	 
    mov   ax,28h
	mov   ds,ax
	mov   ss,ax
	nop                                         
        mov   bx,[_RealModeCS]                  ; push real-mode CS:IP
	push  bx
	lea   bx,[_do_rm]
	push  bx                                   ; clear PE [protected mode enable] bit and return to real mode
    mov   eax,cr0
	and   al,0xFE
	mov   cr0,eax
	retf	
  _do_rm:		                        ; jumps to do_rm
	mov al,65
	int 0x10      
  _halt:
      hlt
 
[SECTION .bss]
     resb STACKSZ
    _kernel_stack:

but my linker doesn't like these lines

Code: Select all

  mov   bx,[_RealModeCS]  
  lea   bx,[_do_rm]

error :

[LINK] kernel.bin
boot.o:boot.o:(.text+0x3d): relocation truncated to fit: 16 against `.text'
boot.o:boot.o:(.text+0x42): relocation truncated to fit: 16 against `.text'
make.exe: *** [kernel.bin] Error 1

My .ld file

Code: Select all

OUTPUT_FORMAT("coff-go32")
ENTRY(_etry)

SECTIONS
{
    _linkt = 0x00100000;
    .text 0x00100000 :
    {
        *(.text)
        . = ALIGN(4096);
    }
    .data . :
    {
        *(.data)
        . = ALIGN(4096);
    }
    _linkd = .;
    .bss . :
    {
        *(.bss)
        . = ALIGN(4096);
    }
    _linkb = .;
}

Thx in advance For ur hrlp

EDIT : Cannot I ask grub to just leave it in real mode ??

[Combuster]

The first one needs some help (force an address size prefix)

I'm more concerned about do_pm being inside the kernel, and thus above 1MB. Hence, it won't ever fit into a 16-bit register (limited to 64k). Either you must relocate the code to below 1MB, or you have to push some bits to the segment part of CS (and use 0xFFFF:do_rm-0xffff0 as the cs:ip pair and hope that the address is low enough)

[packet50071]

The first one needs some help (force an address size prefix)

I'm more concerned about do_pm being inside the kernel, and thus above 1MB. Hence, it won't ever fit into a 16-bit register (limited to 64k). Either you must relocate the code to below 1MB, or you have to push some bits to the segment part of CS (and use 0xFFFF:do_rm-0xffff0 as the cs:ip pair and hope that the address is low enough)

sry But i don't get a thing U said

I think you must have mistyped do_rm to do_pm

This is what i understood . "The do_rm's address is above 1mb so it needs to relocated ( I have no clue how to do that ) ." the second part I don't get it at all . but my guess would be that you want me to do a far jmp to that addr ?

I am soo clue less

[Dex]

I think you may find it hard to convert this code to grubs, as it does a lot of stuff in realmode before moving to pmode.
Eg: The code make addressing identical in real mode and protected mode, by setting the base of the code and data descriptors to DS * 16, this is computed at run-time.
We also save the contents of the CS register. To simplify the return to real mode, we also store the return-to-real-mode address, do_rm.

This also meens it need to be loaded below 1 MB as pointed out by Combuster.
Your best bet is to use bootprog to load your kernel
http://alexfru.chat.ru/epm.html#bootprog

As the demo does.

[packet50071]

Screw real mode and GUI [sry for wasting your time ).

what manuals has this kind of information [i.e. switching modes, Implementing Paging , ETC)
thx in advance.

[btw Isn't there a way to change the vmode from PM mode!. ] Feel free to ignore this question