OSDev.org

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[BITS 16]
[ORG 0x7C00]
main: ; entry point
	mov [bootdisk], dl
	mov si, booted
	call println

	call loadimage
	shr ax, 8
	or al, al
	jz .loaded

.bailout:
	mov si, failed
	call println
	xor ax, ax
	int 0x16
	int 0x19
	cli
	jmp $

.loaded:
	mov dl, [bootdisk]
	jmp 0x7C0:0x200
	mov si, failed
	call println
	cli
	jmp $

;
; Image loader
;

%include 'boot/x86/stage1/loadimage.asm'

;
; Print routines
;

%include 'boot/x86/println.asm'

;
; Since flat binary is one big heap of code without sections, is the code below some sort of data section.
;

	bootdisk db 0
	booted db 'GEBL has been loaded by the bios! Executing...', 0x0
	failed db '(0x0) Failed to load the next stage.. ready to reboot. Press any key.', 0x0

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

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loadimage:
	
.checkextensions:
	mov ah, 0x41	; check ext
	mov dl, [bootdisk]	; HDD0
	mov bx, 0x55AA
	int 0x13
	jc .checkextensions

.extread:
	mov ah,0x42
	mov dl,[bootdisk]
	lea si,[lbar]        
	int 0x13
	jnc .return

.oldway:
	xor ah, ah ; function 0 = reset
	mov dl, [bootdisk]
	int 0x13
	jc .oldway

.oldload:
	mov bx, 0x7C0	; segment
	mov es, bx
	mov bx, 0x200	; offset

	mov ah, 0x2					; function 2
	mov al, 0x1					; read 1 sector
	xor ch, ch					; track
	mov cl, 0x2					; sector to read
	xor dh, dh					; head number
	mov dl, [bootdisk]					; drive number
	int 0x13					; call BIOS - Read the sector

.return:
	ret
;
; LBA register
;
; Loading a sector with this seg:off will place it right after the mbr
;

lbar:
	db 0x10      	; register size
	db 0      	; reserved, must be 0
	dw 0x1      	; sectors to read
	dw 0x200   	; memory offset
	dw 0x7C0   	; memory segment
	dq 0x1		; starting sector (sector to read)

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main:
	mov [diskid], dl

	call enable_A20
	jnc  .loadstage2
	mov si, a20fail

.bailout:
	call println
	xor ah, ah
	int 0x16
	int 0x19
	cli
	jmp $

.loadstage2:
	mov si, a20ok
	call println
	
	call getmemorymap
	mov si, a20fail
	jc .bailout

	mov ax, word [mmr+4]
	or ax, ax
	jz .bailout		; we are not here to bully our user with al zero-length memory map.

	call dynamicloader
	shr ax, 8
	or al, al
	mov si, nostage2
	jnz .bailout

	jmp 0x7E0:0x400

	jmp .bailout

;
; Dynamic disk reader
;

%include 'boot/x86/stage1/stage1.5/dynamicloader.asm'

;
; Memory map
;

%include 'boot/x86/stage1/stage1.5/getmemorymap.asm'

;
; A20 Gate
;

%include 'boot/x86/stage1/stage1.5/enable_A20.asm'

;
; Print routines
;

%include 'boot/x86/println.asm'
	
	diskid db 0
	a20ok db 'The A20 line has been enabled.', 0x0
	a20fail db '(0x1) The A20 gate couldn`t be opened. Press a key to reboot.', 0x0
	nostage2 db '(0x2) Failed to load the second stage.. Press a key to reboot.', 0x0

times 1024 - ($ - $$) db 0

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SECTIONS
{	
	.boot 0x7E00:
	{
		*(.stage1)
		*(.stage2)
		*(.pmode)
	}
	.text :
	{
		*(.text)
	}
	.rodata :
	{
		*(.rodata)
	}
	.data :
	{
		*(.data)
	}
	.bss :
	{
		*(.bss)
	}
	.end ALIGN(2) :
	{
		*(.end)
	}
}

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xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x7BFE
mov bp, sp

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main:
jmp 0000:main1
main1:
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x7BFE
mov bp, sp
mov [bootdisk],dl

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cli

; initialize segment registers
xor ax, ax
mov ds, ax
mov es, ax

mov ss, ax
mov sp, 0x7c00

sti

mov [bootdisk], dl
; do stuff
; load from drive [bootdisk]

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call println

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;
; Print routines
;

%include 'boot/x86/println.asm'

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println:
	lodsb
	or al, al
	jz .return	; null byte found
	mov ah, 0x0E
	xor bh, bh	; page 0
	int 0x10
	jmp println

.return:
	mov al, 0x0A 	; new line
	mov ah, 0x0E
	xor bh, bh
	int 0x10

	mov al, 0x0D	; carrage return
	mov ah, 0x0E
	xor bh, bh
	int 0x10
	ret

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   jmp 0x7C0:0x200

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[ORG 0x7C00]
[BITS 16]

_start: ; entry point
	cli
	jmp 0x0:.flush
.flush:
	mov ax, cs
	mov ds, ax
	mov es, ax
	mov ss, ax
	mov sp, 0x7BFE
	mov [bootdisk], dl
	sti


.reset:
	xor ax, ax
	mov dl, [bootdisk]
	int 0x13
	jc .reset

.read:
	mov ah, 0x2
	mov al, 1 ; read 1 sector
	xor cx, cx ; track 0
	mov cl, 2 ; read sector 2
	xor dx, dx ; head 0
	mov dl, [bootdisk]
	
	; setup buffer
	xor bx, bx
	mov es, bx
	mov bx, 8000h ; offset 8000 hex
	int 0x13
	jc .read
	or ah, ah
	jnz .reset

	; print a character
	mov al, 65
	mov ah, 0x0E
	xor bh, bh	; page 0
	int 0x10
	jmp 0x0:8000h

	bootdisk db 0

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

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[ORG 0x8000]
[BITS 16]

	mov al, 66
	mov ah, 0x0E
	xor bh, bh	; page 0
	int 0x10

	jmp $

times 512 - ($ - $$) db 0