Please feel free to look over this and let me know if there is anything horribly wrong.FYI currently a small kernel that occupies the second sector of the boot disk is loaded into memory. I have taken out the instruction that jumps to it because....well...I can't use my pmode descriptors to do the jump. In regards to everyone's suggestions I have tried all of them and whatever the method in the code right now is just the remnants of the last attempt to load a valid address for the GDT. Thank you to all and let the fun begin
;; pmboot.asm
;; skeleton bootloader
%define KERN_LOAD 0x500
%define STACK 0xFF00 ; Beginning of the Stack
%define STACK_P 0xFFFE ; Where the stack is (ended)
%define BASE_SEG 0x9000
%define MEM_START 0x0
org 07c00h ; start address 0000:07c00h physical
nop
skipdata:
bits 16
mov ax, 0x0600 ; BIOS-function clear/scroll window
mov cx, 0x0 ; define window to be cleared from
mov dx, 0x174F ; to 23, 79
mov bh, 0 ; fillcolor = 0
int 10h ; BIOS -clear screen- go!
mov ax, 0xB800
mov gs, ax ; point gs to video memory
mov word [gs:0],0x641 ; display brown 'A'
mov word [gs:2],0x642 ; display brown 'B'
mov ax, BASE_SEG ; set up stack
mov ss, ax ; Stack segment
mov sp, STACK_P ; and of course the stack pointer-end of stack
mov bx, KERN_LOAD ; This is where I specify where to place the
mov ax, MEM_START ; kernel I'd like it to go to 0x600 physical
mov es, ax ; but I think its obvious that I'm having some
; real mode addressing issues
read: mov ah, 2 ; function name (2)
mov al, 1 ; number of sectors to read
mov ch, 0 ; track number
mov cl, 2 ; sector number = 2 (this is the kernel)
mov dh, 0 ; head number
mov dl, 0 ; drive number 0 = floppy (?)
int 13h ; read the kernel into memory
push cs
pop ds ; load ds with cs
xor eax,eax ; This is the method that Adek and Dr. 128
mov ax,cs ; suggested but I've tried all of the others
shr eax,4 ; that people hav posted and unfortunately
add eax,gdt ; have not had that much luck with any of them
mov dword[gdtr+2],eax ; But thanks again for the help from everyone!
lgdt [ds:gdtr] ; load gdt reg
mov word [gs:4],0x643 ; display brown 'C'
;; Move into **P_MODE**
mov eax, cr0 ; read control reg
or al, 1 ; set PM bit
mov cr0, eax ; transfer it to control port
mov word [gs:6],0x644 ; display brown 'D'
; This is the last character that prints
jmp CODE_SEL:pmode ; into pmode (32-bit mode)
; I think this is where the trouble is
pmode:
[BITS 32]
;; set the segments up
mov word [gs:8],0x645 ; display brown 'E'
mov ax, DATA_SEL ; set data selector
mov ds, ax ; for the data segment
mov es, ax ; extra segment
mov fs, ax
mov gs, ax
mov ss, ax ; stack segment
mov ax, VIDEO_SEL
mov gs, ax ; point gs to video memory
mov word [gs:10],0x741 ; display white 'a'
spin:
jmp spin ; loop
[bits 16]
gdtr
dw gdt_end-gdt-1 ; length of GDT
dd gdt ; linear physical address of the GDT
gdt:
;null descriptor ; 0h = 000b
; null descriptor
dw 0
dw 0
db 0
db 0
db 0
db 0
VIDEO_SEL equ $-gdt ; 18h = 11000b
dw 0ffffh ; limit 80*25*2-1
dw 0 ; base 0xB8000
db 0
db 092h
db 0cfh ; present, ring 0, data, expand up, writable
db 0 ; byte-granular, 16-bit
code_gdt:
CODE_SEL equ $-gdt ; 8h = 1000b
; code descriptor 4 GB flat segment starting 0000:0000h
dw 0ffffh ; limit 4 GB
dw 0 ; Base 0000:0000h
db 0
db 09ah
db 0cfh
db 0h
data_gdt:
DATA_SEL equ $-gdt ; =10h = 10000b
; data descriptor 4 GB flat segment starting 0000:0000h
dw 0ffffh ; limit 4 GB
dw 0h ; Base 0000:0000h
db 0h
db 092h
db 0cfh
db 0h
gdt_end:
;; equates
lf equ 10
cr equ 13
times 510-($-$$) db 0
dw 0xaa55