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rhyno_dagreat



Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau
rhyno_dagreat
This is more of a question of how the GDT works, than if anything else... I recall in one of my earlier threads I was having trouble with the GDT, and I think it was Artlav that told me I needed to go up two bytes(I think it was two bites) from the GDT_Descriptor, and add 600 to the value of it, which didn't make much sense to me.
Also, he told me to put in a second org into my code that was at memory address 600. I'm sorry if I'm making myself seem stupid to y'all with all these questions, I just still don't understand how all of this is fitting together.

Here's the old code in case anyone wants to take a gander at it:

Rhynload.asm

Code:
;BOOTSTART
org 7C00h

jmp start

message db 'Loading RhynOS...',13,10,0

start:
        xor ax, ax
        mov ds, ax

        mov si, message
        call bios_print

load_os:
        mov ax, 0060h
        mov es, ax
        mov bx, 0000h
        mov ah, 02h
        mov al, 5
        mov ch, 0
        mov cl, 2
        mov dh, 0
        ;mov dl, 0  ; Not sure if needed, been told that dl already contains the drive that was booted on
        int 13h
        jmp 0060h:0000h

bios_print:
        lodsb
        or al, al
        jz done
        mov ah, 0x0E
        int 0x10
        jmp bios_print

done:
        call load_os

;BOOTEND
        times 510-($-$$) db 0
        dw 0xAA55                
    



RhynOS.asm

Code:
org 0000h
use16
cli

mov ax, 0060h
mov ds, ax
add dword [gdt_descriptor+2], 600h

lgdt [gdt_descriptor]

mov eax, cr0
or eax, 1
mov cr0, eax

jmp 08h:clear_pipe

gdt:
dd 0, 0
gdt_code:
db 0FFh, 0FFh, 0, 0, 0, 10011010b, 11001111b, 0
gdt_data:
db 0FFh, 0FFh, 0, 0, 0, 10010010b, 11001111b, 0
gdt_end:

gdt_descriptor:
    dw gdt_end - gdt - 1
    dd gdt

org 0600h+$
use32
clear_pipe:
mov ax, 10h
mov ds, ax
mov ss, ax

mov esp, 090000h

;ACTUAL KERNEL CODE STARTS HERE
mov si, WelMsg
call PrintFunc

call Reroute_PICs
lidt [idtp]
sti

GetInput:
        int 0x21
        cmp al, 02
        je DoClrScrn
        jmp GetInput

DoClrScrn:
        call ClrScrn

mov si, CmdStart
call PrintFunc

;KERNEL CODE ENDS HERE
infinite_loop:
        jmp infinite_loop

times 668-($-$$) db 0
include 'interrupts.inc'
WelMsg db "Welcome to RhynOS! Press 1 To Continue.", 0    
    


Thank you all again for your time.
Post 29 Sep 2006, 17:30
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rhyno_dagreat



Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau
rhyno_dagreat
Also, I was wondering how the stack works in this all and why I keep needing to change it's place and the place of the stack pointer?
Post 29 Sep 2006, 17:31
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Dex4u



Joined: 08 Feb 2005
Posts: 1601
Location: web
Dex4u
This may help
Quote:

; How to get into protected mode? First, you need a GLOBAL DESCRIPTOR
; TABLE (GDT). There is one at the end of this file, at address "gdt:"
; The GDT contains 8-byte DESCRIPTORS for each protected-mode segment.
; Each descriptor contains a 32-bit segment base address, a 20-bit segment
; limit, and 12 bits describing the segment type. The descriptors look
; like this:
;
; MSB bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 LSB
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 0 | bit 7<---------------- segment limit------------------->bit 0 |
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 1 |bit 15<---------------- segment limit------------------->bit 8 |
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 2 | bit 7<---------------- segment base-------------------->bit 0 |
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 3 |bit 15<---------------- segment base-------------------->bit 8 |
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 4 |bit 23<---------------- segment base-------------------->bit 16|
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 5 | P | DPL | <----------- segment type ----------> |
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; P is the Segment Present bit. It should always be 1.
;
; DPL is the DESCRIPTOR PRIVILEGE LEVEL. For simple code like this, these
; two bits should always be zeroes.
;
; Segment Type (again, for simple code like this) is hex 12 for data
; segments, hex 1A for code segments.
;
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 6 | G | B | 0 | avail | bit 19<-- seg limit--->bit 16 |
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; G is the Limit Granularity. If zero, the segment limit is in bytes
; (0 to 1M, in 1-byte increments). If one, the segment limit is in 4K PAGES
; (0 to 4G, in 4K increments). For simple code, set this bit to 1, and
; set the segment limit to its highest value (FFFFF hex). You now have
; segments that are 4G in size! The Intel CPUs can address no more than
; 4G of memory, so this is like having no segments at all. No wonder
; protected mode is popular.
;
; B is the Big bit; also called the D (Default) bit. For code segments,
; all instructions will use 32-bit operands and addresses by default
; (BITS 32, in NASM syntax, USE32 in Microsoft syntax) if this bit is set.
; 16-bit protected mode is not very interesting, so set this bit to 1.
;
; None of these notes apply to the NULL descriptor. All of its bytes
; should be set to zero.
;
; +-------+-------+-------+-------+-------+-------+-------+-------+
;byte 7 |bit 31<---------------- segment base------------------->bit 24 |
; +-------+-------+-------+-------+-------+-------+-------+-------+
;
; Build a simple GDT with four descriptors: NULL (all zeroes), linear data
; (lets you use 32-bit addresses), code, and data/stack. (An extra
; descriptor or two is needed to return to real mode.) For simplicity,
; the limits of all descriptors (except NULL and the real-mode descriptors)
; are FFFFF hex, the largest possible limit.
;
Post 30 Sep 2006, 00:14
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rhyno_dagreat



Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau
rhyno_dagreat
Thanks again for the help Dex. Did you get it out of that one link to that one zip you gave me? (I believe it was you)
Post 30 Sep 2006, 02:13
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Dex4u



Joined: 08 Feb 2005
Posts: 1601
Location: web
Dex4u
That the veryone Wink, but i forgot i give it you.
Post 30 Sep 2006, 03:41
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rhyno_dagreat



Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau
rhyno_dagreat
Thought so! I knew I recognized it from somewhere. That was from the first tutorial, right?
Post 30 Sep 2006, 03:50
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