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Code: Select all

# Link this file with "--oformat binary -Ttext 0x00007E00".

.set MAGIC,       0x1BADB002
.set AOUT_KLUDGE, 0x00010000

.set FLAGS,       AOUT_KLUDGE
.set CHECKSUM,    -(MAGIC + FLAGS)

.set newidt,      0x0632


.intel_syntax noprefix
.section .text

.code32

.global _start       # _start declared global so the linker doesn't complain
_start:              # This label is here so a 32-bit address is used


.code16

  jmp real16

 .align 4


.code32

multiboot_header:
  .long MAGIC                 # magic
  .long FLAGS                 # flags
  .long CHECKSUM              # checksum
  .long multiboot_header      # header address
  .long _start                # load address
  .long 0x00000000            # load end address
  .long 0x00000000            # bss end address
  .long multiboot_entry       # entry address

multiboot_entry:
  cli                         # disable interrupts
  mov eax, cr0
  and eax, 0x7FFFFFFE         # disable paging and protected mode
  mov cr0, eax
  xor eax, eax
  mov cr3, eax                # reset address of page directory
  ljmp 0x00:real16            # far jump to real mode


.code16

real16:
  cli
  mov sp, 0x7C00              # set up stack
  xor ax, ax                  # initialize segment registers
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  mov ss, ax
  mov ax, 0x03FF              # limit of the idt
  mov [newidt], ax
  xor ax, ax                  # base of the idt
  mov [newidt + 2], ax
  mov [newidt + 4], ax
  lidt [newidt]               # load new idt
  sti                         # enable interrupts
  mov si, message

print:
  lodsb
  or al, al
  jz hang
  mov ah, 0x0E
  int 0x10
  jmp print

hang:
  jmp hang

message:
  .ascii "Hello World!"

.att_syntax prefix

Code: Select all

;________________________________________________
;Go to 16-bit real Mode
;IN/OUT:  nothing
;
go16:
use32					;Tell assembler to generate 32-bit code
	
	cli			;Clear interrupts
	pop edx			;save return location in edx
	
	jmp 0x20:PM16		;Load CS with selector 0x20

;To go to 16-bit real mode, first we have to go to 16-bit protected mode
;

use16				;Tell assembler to generate 16-bit code
PM16:
	mov ax, 0x28		;0x28 is 16-bit protected mode selector.
	mov ss, ax	
	mov sp, 0x5000		;Stack hase base

	mov eax, cr0
	and eax, 0xfffffffe	;Clear protected enable bit in cr0
	mov cr0, eax		;Disable 32-bit mode

	jmp 0x50:realMode	;Load CS and IP

realMode:
;Load segment registers with 16-bit Values.
;
	mov ax, 0x50
	mov ds, ax
	mov ax, 0x6000
	mov ss, ax
	mov ax, 0
	mov es, ax
	mov sp, 0
	
	cli
	lidt[.idtR]		;Load real mode interrupt vector table
	sti
	
	push 0x50
	push dx			;Return to saved location in EDX
	retf			;Start real mode

;Real mode interrupt vector table
;
.idtR:	dw 0xffff		;Limit
	dd 0			;Base

Code: Select all

GDT:		dd 0, 0		;null Descriptor
.Code:		dw 0xffff	;Limit (0:15)	
		dw 0x0500	;Base (0:15)
		db 0		;Base (16:23)	
		db 10011010b	;Pr=1,Privl=00,Resr=1,ex=1,C=0,R=1,Ac=0
		db 11001111b	;Gr=1,Sz=1,Resr=00,Limit(16:19)
		db 0		;Base (24:31)


;Data Descriptor base on 500h
.Data:		dw 0xffff	;Limit (0:15)	
		dw 0x0500	;Base (0:15)
		db 0		;Base (16:23)	
		db 10010010b	;Pr=1,Privl=00,Resr=1,ex=0,D=0,W=1,Ac=0
		db 11001111b	;Gr=1,Sz=1,Resr=00,Limit(16:19)
		db 0		;Base (24:31)

;Data Descriptor base on 0h
.linear:	dw 0xffff	;Limit (0:15)	
		dw 0		;Base (0:15)
		db 0		;Base (16:23)	
		db 10010010b	;Pr=1,Privl=00,Resr=1,ex=0,D=0,W=1,Ac=0
		db 11001111b	;Gr=1,Sz=1,Resr=00,Limit(16:19)
		db 0		;Base (24:31)

;Code Descriptor for 16-bit protected mode
.codePM16:	dw 0xffff	;Limit (0:15)	
		dw 0x0500	;Base (0:15)
		db 0		;Base (16:23)	
		db 10011010b	;Pr=1,Privl=00,Resr=1,ex=1,C=0,R=1,Ac=0
		db 0		;Gr=1,Sz=1,Resr=00,Limit(16:19)
		db 0		;Base (24:31)

;Data Descriptor for 16-bit protected mode
.dataPM16:	dw 0xffff	;Limit (0:15)	
		dw 0		;Base (0:15)
		db 0		;Base (16:23)	
		db 10010010b	;Pr=1,Privl=00,Resr=1,ex=0,D=0,W=1,Ac=0
		db 0		;Gr=1,Sz=1,Resr=00,Limit(16:19)
		db 0		;Base (24:31)

GDTReg:		dw 63		;GDT Size - 1
		dd GDT+0x500	;GDT Offset (To be filled latter)