Ad:

Assembly Discussion Thread
Unsolved
Views: 2841

Dec 2nd, 2007

protected mode

Expand Post »

Now i have been able to create the gdt's and when i change to pmode, will a short JMP flush the instruciton prefetch queue? here is a sample of my code and the gdt and gdtr content,

Assembly Syntax (Toggle Plain Text)
mov ax,07c00h
mov ds,ax
lgdt [ds:descr]
;switch to pmode by setting bit 0 of CR0
;after switching to pmode
jmp pmode; when i use JMP 0:7c00h+pmode ir reboots, the 1st 0 is code gdt
pmode:
;here when i try to write to RAM, it will reboot
mov ax,8h;the data segment
mov ds,ax;it reboots here
mov ax,0f44h
mov [0h],ax
 
descr:
db 0ffh,0ffh,00h,00h,07ch,gdt
gdt:
db 0ffh,0ffh,0,0,0,8a,0cfh,0;code segment
db 0ff,0ffh,0,0,0,82,0cfh,9;data segment
mov ax,07c00h
mov ds,ax
lgdt [ds:descr]
;switch to pmode by setting bit 0 of CR0
;after switching to pmode
jmp pmode; when i use JMP 0:7c00h+pmode ir reboots, the 1st 0 is code gdt
pmode:
;here when i try to write to RAM, it will reboot
mov ax,8h;the data segment
mov ds,ax;it reboots here
mov ax,0f44h
mov [0h],ax

descr:
db 0ffh,0ffh,00h,00h,07ch,gdt
gdt:
db 0ffh,0ffh,0,0,0,8a,0cfh,0;code segment
db 0ff,0ffh,0,0,0,82,0cfh,9;data segment

whta am i suppose to do to make my pmode correct, i mean be able to use the descriptors without rebooting? writing to the RAM? pleeeeeeeeeeeeeeeeaaaaaaaaaaaase help. Sam

Last edited by Ancient Dragon; Dec 2nd, 2007 at 10:04 am. Reason: add code tags

Similar Threads

protected mode programming guide in Assembly
Protected mode reboots in Assembly
Help needed for a project concerning protected mode in Assembly
32 BIt Flat Real mode in Community Introductions
Protected Mode Memory Map in C

sgk26p10p07

Reputation Points: 10

Solved Threads: 1

Newbie Poster

Offline

19 posts
since Oct 2007

Permalink

Dec 3rd, 2007

Re: protected mode

As I recall, we sometimes needed to specify the jump to protected mode in a DB statement. Google for John Fine, he has something to get you over that problem, I think.

dan63043

Reputation Points: 12

Solved Threads: 5

Light Poster

Offline

36 posts
since Dec 2007

Permalink

Dec 3rd, 2007

Re: protected mode

Looked at Fine's stuff, good in itself, doesn't help you, though.

Here's from http://en.wikipedia.org/wiki/Protected_mode

Assembly Syntax (Toggle Plain Text)
; set PE bit
mov eax, cr0
or eax, 1
mov cr0, eax
 
; far jump (cs = selector of code segment)
jmp cs:@pm
 
@pm:
  ; Now we are in PM.
; set PE bit
mov eax, cr0
or eax, 1
mov cr0, eax

; far jump (cs = selector of code segment)
jmp cs:@pm

@pm:
  ; Now we are in PM.

The point here is to force the assembler to use a far jump. For some assemblers, unfortunately, this requires:

Assembly Syntax (Toggle Plain Text)
db 0eah ;far jump
db 0eah ;far jump

For the Pentium and later processors, clearing the prefetch queue is no longer necessary.

dan63043

Reputation Points: 12

Solved Threads: 5

Light Poster

Offline

36 posts
since Dec 2007

Permalink

Dec 4th, 2007

Re: protected mode

i know cs is for the selector but it doesnt select the right selector at all, what memory should i put at GDTR 00007c00h+gdtr or what?

sgk26p10p07

Reputation Points: 10

Solved Threads: 1

Newbie Poster

Offline

19 posts
since Oct 2007

Permalink

Dec 4th, 2007

Re: protected mode

My first thought was "Well, you're probably rebooting because of a triple-fault. Have you defined your interrupt descriptor table (IDT)?"

But now I really see the 7c00, and suppose you're writing a bootloader. It's a lot easier, nowadays, to let grub do the bootloader stuff, but if you insist, there's a lot to go wrong.

What's the nature of your project? Are you just experimenting to learn a lot?

dan63043

Reputation Points: 12

Solved Threads: 5

Light Poster

Offline

36 posts
since Dec 2007

Permalink

Dec 5th, 2007

Re: protected mode

i got it all, will post it later, now the only thig is interrrupts

sgk26p10p07

Reputation Points: 10

Solved Threads: 1

Newbie Poster

Offline

19 posts
since Oct 2007

Permalink

Mar 8th, 2008

Re: protected mode

Click to Expand / Collapse Quote originally posted by dan63043 ...

Assembly Syntax (Toggle Plain Text)
; far jump (cs = selector of code segment)
jmp cs:@pm
 
@pm:
  ; Now we are in PM.
; far jump (cs = selector of code segment) jmp cs:@pm @pm: ; Now we are in PM.
The point here is to force the assembler to use a far jump. For some assemblers, unfortunately, this requires:

Hi, I can't make it out how the far jump works out.
Sorry that I've posted a similar question on another thread here. I found this thread after I posted that.

So, how can "jmp cs: @pm" jumps to "@pm"? Isn't it supposed to jump to the physical address "cs * 16 + @pm"?

Last edited by koyi; Mar 8th, 2008 at 1:37 am.

koyi

Reputation Points: 10

Solved Threads: 0

Newbie Poster

Offline

2 posts
since Mar 2008

This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.

Previous Thread in Assembly Forum Timeline: Far jump to a label?

Next Thread in Assembly Forum Timeline: Converting this code to support Windows 98

DaniWeb Message

Cancel Changes

User Name
Password