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Join Date: Oct 2007
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protected mode 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,
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
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
Last edited by Ancient Dragon : Dec 2nd, 2007 at 10:04 am. Reason: add code tags
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Join Date: Dec 2007
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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.
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Looked at Fine's stuff, good in itself, doesn't help you, though.
Here's from http://en.wikipedia.org/wiki/Protected_mode
The point here is to force the assembler to use a far jump. For some assemblers, unfortunately, this requires:
For the Pentium and later processors, clearing the prefetch queue is no longer necessary.
Here's from http://en.wikipedia.org/wiki/Protected_mode
; 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:
db 0eah ;far jump
For the Pentium and later processors, clearing the prefetch queue is no longer necessary.
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Join Date: Oct 2007
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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?
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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?
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?
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i got it all, will post it later, now the only thig is interrrupts
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Originally Posted by dan63043 
; 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.
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