loader.s~

简单的操作系统源代码

#include "pm.h"
.code16
.text
    jmp LABEL_BEGIN

LABEL_GDT: Descriptor          0,                        0, 0
LABEL_DESC_CODE32: Descriptor  0,       (SegCode32Len - 1), (DA_C + DA_32)
LABEL_DESC_VIDEO: Descriptor  0xB800,               0xffff, DA_DRW

.set GdtLen, (.-LABEL_GDT)

Gdtptr :.2byte (GdtLen - 1)
        .4byte 0


.set SelectorCode32, (LABEL_DESC_CODE32 - LABEL_GDT)
.set SelectorVideo , (LABEL_DESC_VIDEO  - LABEL_GDT)


LABEL_BEGIN:
       mov %cs,%ax
       mov %ax,%ds
       mov %ax,%es
       mov %ax,%ss
       mov $0x100, %sp
	xor %eax,%eax
	mov %cs,%ax
	shl $4,%eax
	addl $(LABEL_SEG_CODE32),%eax
	movw %ax,(LABEL_DESC_CODE32 + 2)
	shr $16,%eax
	movb %al,(LABEL_DESC_CODE32 + 4)
	movb %ah,(LABEL_DESC_CODE32 + 7)


	xor %eax,%eax
	mov %ds, %ax
	shl $4,%eax
	add $(LABEL_GDT), %eax
	movl %eax,(GdtPtr + 2)


	lgdtw GdtPtr

	cli
	
	inb $0x92,%al
	orb $0b00000010,%al
	outb %al,$0x92


	movl %cr0,%eax
	orl $1,%eax
	movl %eax,%cr0
	
	ljmpl $SelectorCode32,$0
LABEL_SEG_CODE32:
	.code32
	mov $(SelectorVideo),%ax
	mov %ax,%gs
	movl $((80*10 + 0)*2),%edi
	movb $0xC,%ah
	movb $'K',%al
	mov %ax,%gs:(%edi)
	jmp .
	
	.set Seg_code32Len, .-LABEL_SEG_CODE32