head.s

计算机启动例子代码

;***********************************************************
;
; 重设 32 bit 环境,以便内核能够访问idt gdt tss
;
;*************************************************************
global Start_OS

extern c

Start_OS:
        mov	ax,cs		; right, forgot this at first. didn't work :-)
	mov	ds,ax
	lidt	[idt_descr]		; reload idt with kernel idt
	lgdt	[gdt_descr]		; reload gdt with kernel gdt


        mov     ax,0x10
        mov     ds,ax
        mov     es,ax
        mov     ss,ax
        mov     esp,0x1000
        mov     ax,0
      
        call c
;*********************全局变量****************************************         
;
;
align 2
   dw 0
idt_descr:
	dw  256*8-1	; idt contains 256 entries
	dd _idt
align 2
   dw 0
gdt_descr:
	dw 256*8-1		; so does gdt (not that that's any
	dd _gdt		; magic number, but it works for me :^)

	align 3
_idt:	times 256*8 db 0		; idt is uninitialized

_gdt:	dw	0,0,0,0		; dummy

	dw	0x07FF		; 8Mb - limit=2047 (2048*4096=8Mb)
	dw	0x0000		; base address=0
	dw	0x9A00		; code read/exec
	dw	0x00C0		; granularity=4096, 386

	dw	0x07FF		; 8Mb - limit=2047 (2048*4096=8Mb)
	dw	0x0000		; base address=0
	dw	0x9200		; data read/write
	dw	0x00C0		; granularity=4096, 386

	dw      0,0,0,0 	;/* TEMPORARY - don't use */
	times 252*8 db 0 ;/* space for LDT's and TSS's etc */

	
;*******************************Sub*********************************
; This routine checks that the keyboard command queue is empty
; No timeout is used - if this hangs there is something wrong with
; the machine, and we probably couldn't proceed anyway.