memdisk16.asm

linux内核

;; -*- fundamental -*-
;; $Id$
;; -----------------------------------------------------------------------
;;   
;;   Copyright 1994-2004 H. Peter Anvin - All Rights Reserved
;;
;;   This program is free software; you can redistribute it and/or modify
;;   it under the terms of the GNU General Public License as published by
;;   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
;;   Boston MA 02111-1307, USA; either version 2 of the License, or
;;   (at your option) any later version; incorporated herein by reference.
;;
;; -----------------------------------------------------------------------

;;
;; init16.asm
;;
;; Routine to initialize and to trampoline into 32-bit
;; protected memory.  This code is derived from bcopy32.inc and
;; com32.inc in the main SYSLINUX distribution.
;;

MY_CS		equ 0x0800		; Segment address to use
CS_BASE		equ (MY_CS << 4)	; Corresponding address

; Low memory bounce buffer
BOUNCE_SEG	equ (MY_CS+0x1000)

%define DO_WBINVD 0

%define STACK_HEAP_SIZE	(128*1024)

		section .rodata align=16
		section .data   align=16
		section .bss    align=16

;; -----------------------------------------------------------------------
;;  Kernel image header
;; -----------------------------------------------------------------------

		section .text		; Must be first in image
		bits 16

cmdline		times 497 db 0		; We put the command line here
setup_sects	db 0
root_flags	dw 0
syssize		dw 0
swap_dev	dw 0
ram_size	dw 0
vid_mode	dw 0
root_dev	dw 0
boot_flag	dw 0xAA55

_start:		jmp short start

		db "HdrS"		; Header signature
		dw 0x0203		; Header version number

realmode_swtch	dw 0, 0			; default_switch, SETUPSEG
start_sys_seg	dw 0x1000		; obsolete
version_ptr	dw memdisk_version-0x200	; version string ptr
type_of_loader	db 0			; Filled in by boot loader
loadflags	db 1			; Please load high
setup_move_size	dw 0			; Unused
code32_start	dd 0x100000		; 32-bit start address
ramdisk_image	dd 0			; Loaded ramdisk image address
ramdisk_size	dd 0			; Size of loaded ramdisk
bootsect_kludge	dw 0, 0
heap_end_ptr	dw 0
pad1		dw 0
cmd_line_ptr	dd 0			; Command line
ramdisk_max	dd 0xffffffff		; Highest allowed ramdisk address

		section .rodata
memdisk_version:
		db "MEMDISK ", VERSION, " ", DATE, 0

;; -----------------------------------------------------------------------
;;  End kernel image header
;; -----------------------------------------------------------------------

;
; Move ourselves down into memory to reduce the risk of conflicts;
; then canonicalize CS to match the other segments.
;
		section .text
		bits 16
start:
		mov ax,MY_CS
		mov es,ax
		movzx cx,byte [setup_sects]
		inc cx			; Add one for the boot sector
		shl cx,7		; Convert to dwords
		xor si,si
		xor di,di
		mov fs,si		; fs <- 0
		cld
		rep movsd
		mov ds,ax
		mov ss,ax
		xor esp,esp		; Stack at top of 64K segment
		jmp MY_CS:.next
.next:

;
; Copy the command line, if there is one
;
copy_cmdline:
		xor di,di		; Bottom of our own segment (= "boot sector")
		mov eax,[cmd_line_ptr]
		and eax,eax
		jz .endcmd		; No command line
		mov si,ax
		shr eax,4		; Convert to segment
		and si,0x000F		; Starting offset only
		mov gs,ax
		mov cx,496		; Max number of bytes
.copycmd:
		gs lodsb
		and al,al
		jz .endcmd
		stosb
		loop .copycmd
.endcmd:
		xor al,al
		stosb

;
; Now jump to 32-bit code
;
		sti
		call init32
;
; When init32 returns, we have been set up, the new boot sector loaded,
; and we should go and and run the newly loaded boot sector
;
; The setup function returns (in AL) the drive number which should be
; put into DL
;
		mov dx,ax

		cli
		xor esi,esi		; No partition table involved
		mov ds,si		; Make all the segments consistent
		mov es,si
		mov fs,si
		mov gs,si
		mov ss,si
		mov esp,0x7C00		; Good place for SP to start out
		jmp 0:0x7C00

;
; We enter protected mode, set up a flat 32-bit environment, run rep movsd
; and then exit.  IMPORTANT: This code assumes cs == MY_CS.
;
; This code is probably excessively anal-retentive in its handling of
; segments, but this stuff is painful enough as it is without having to rely
; on everything happening "as it ought to."
;
		section .rodata

	; desc base, limit, flags
%macro	desc 3
	dd (%2 & 0xffff) | ((%1 & 0xffff) << 16)
	dd (%1 & 0xff000000) | (%2 & 0xf0000) | ((%3 & 0xf0ff) << 8) | ((%1 & 0x00ff0000) >> 16)
%endmacro
	
		align 8, db 0
call32_gdt:	dw call32_gdt_size-1	; Null descriptor - contains GDT
.adj1:		dd call32_gdt+CS_BASE	; pointer for LGDT instruction
		dw 0
		
		; 0008: Code segment, use16, readable, dpl 0, base CS_BASE, 64K
		desc CS_BASE, 0xffff, 0x009b

		; 0010: Data segment, use16, read/write, dpl 0, base CS_BASE, 64K
		desc CS_BASE, 0xffff, 0x0093

		; 0018: Data segment, use16, read/write, dpl 0, base 0, 4G
		desc 0, 0xfffff, 0x809b

		; 0020: Code segment, use32, read/write, dpl 0, base 0, 4G
		desc 0, 0xfffff, 0xc09b

		; 0028: Data segment, use32, read/write, dpl 0, base 0, 4G
		desc 0, 0xfffff, 0xc093
	
call32_gdt_size:	equ $-call32_gdt

err_a20:	db 'ERROR: A20 gate not responding!',13,10,0
	
		section .bss
		alignb 4
SavedSSSP	resd 1			; Place to save SS:SP
Return		resd 1			; Return value
A20Test		resw 1			; Space to test A20
A20Tries	resb 1
		
		section .data
		alignb 4
Target		dd 0			; Target address
Target_Seg	dw 20h			; Target CS

A20Type		dw 0			; Default = unknown
		
		section .text
		bits 16
;
; Routines to enable and disable (yuck) A20.  These routines are gathered
; from tips from a couple of sources, including the Linux kernel and
; http://www.x86.org/.  The need for the delay to be as large as given here
; is indicated by Donnie Barnes of RedHat, the problematic system being an
; IBM ThinkPad 760EL.
;
; We typically toggle A20 twice for every 64K transferred.
; 
%define	io_delay	call _io_delay
%define IO_DELAY_PORT	80h		; Invalid port (we hope!)
%define disable_wait 	32		; How long to wait for a disable

%define A20_DUNNO	0		; A20 type unknown
%define A20_NONE	1		; A20 always on?
%define A20_BIOS	2		; A20 BIOS enable
%define A20_KBC		3		; A20 through KBC
%define A20_FAST	4		; A20 through port 92h

		align 2, db 0
A20List		dw a20_dunno, a20_none, a20_bios, a20_kbc, a20_fast
A20DList	dw a20d_dunno, a20d_none, a20d_bios, a20d_kbc, a20d_fast
a20_adjust_cnt	equ ($-A20List)/2

slow_out:	out dx, al		; Fall through

_io_delay:	out IO_DELAY_PORT,al
		out IO_DELAY_PORT,al
		ret

enable_a20:
		pushad
		mov byte [A20Tries],255 ; Times to try to make this work

try_enable_a20:

;
; Flush the caches
;
%if DO_WBINVD
		call try_wbinvd
%endif

;
; If the A20 type is known, jump straight to type
;
		mov bp,[A20Type]
		add bp,bp			; Convert to word offset
.adj4:		jmp word [bp+A20List]

;
; First, see if we are on a system with no A20 gate
;
a20_dunno:
a20_none:
		mov byte [A20Type], A20_NONE
		call a20_test
		jnz a20_done

;
; Next, try the BIOS (INT 15h AX=2401h)
;
a20_bios:
		mov byte [A20Type], A20_BIOS
		mov ax,2401h
		pushf				; Some BIOSes muck with IF
		int 15h
		popf

		call a20_test
		jnz a20_done

;
; Enable the keyboard controller A20 gate
;
a20_kbc:
		mov dl, 1			; Allow early exit
		call empty_8042
		jnz a20_done			; A20 live, no need to use KBC

		mov byte [A20Type], A20_KBC	; Starting KBC command sequence

		mov al,0D1h			; Command write
		out 064h, al
		call empty_8042_uncond

		mov al,0DFh			; A20 on
		out 060h, al
		call empty_8042_uncond

		; Verify that A20 actually is enabled.  Do that by
		; observing a word in low memory and the same word in
		; the HMA until they are no longer coherent.  Note that
		; we don't do the same check in the disable case, because
		; we don't want to *require* A20 masking (SYSLINUX should
		; work fine without it, if the BIOS does.)
.kbc_wait:	push cx
		xor cx,cx
.kbc_wait_loop:
		call a20_test
		jnz a20_done_pop
		loop .kbc_wait_loop

		pop cx
;
; Running out of options here.  Final attempt: enable the "fast A20 gate"
;
a20_fast:
		mov byte [A20Type], A20_FAST	; Haven't used the KBC yet
		in al, 092h
		or al,02h
		and al,~01h			; Don't accidentally reset the machine!
		out 092h, al

.fast_wait:	push cx
		xor cx,cx
.fast_wait_loop:
		call a20_test
		jnz a20_done_pop
		loop .fast_wait_loop

		pop cx

;
; Oh bugger.  A20 is not responding.  Try frobbing it again; eventually give up
; and report failure to the user.
;

		dec byte [A20Tries]
		jnz try_enable_a20


		; Error message time
		mov si,err_a20
print_err:
		lodsb
		and al,al
		jz die
		mov bx,7
		mov ah,0xe
		int 10h
		jmp print_err


die:
		sti
.hlt:		hlt
		jmp short .hlt

;
; A20 unmasked, proceed...
;
a20_done_pop:	pop cx
a20_done:	popad
		ret

;
; This routine tests if A20 is enabled (ZF = 0).  This routine
; must not destroy any register contents.
;
a20_test:
		push es
		push cx
		push ax
		mov cx,0FFFFh		; HMA = segment 0FFFFh
		mov es,cx
		mov cx,32		; Loop count
		mov ax,[A20Test]
.a20_wait:	inc ax
		mov [A20Test],ax
		io_delay		; Serialize, and fix delay
		cmp ax,[es:A20Test+CS_BASE+10h]
		loopz .a20_wait
.a20_done:	pop ax
		pop cx
		pop es
		ret