[LinksOS CVS Commit]linksos/boot/x86 strap.asm,NONE,1.1

[<ke...@to...>]

Update of /cvsroot/linksos/linksos/boot/x86
In directory router-internal.tossell.net:/tmp/cvs-serv17143

Added Files:
	strap.asm 
Log Message:
Title: strap.asm
Changes: added bootstrap code to this directory (from main/)

Effects: Portability dude
a=kennyt


--- NEW FILE: strap.asm ---
;-----------------------------------------------------------------------------
;	filename:	strap.asm													
;	creation date:	unknown													
;	author:	Charles Banas
;	Edited by Kye Lewis (kye...@us...)												
;       version: 0.02														
;-----------------------------------------------------------------------------
;	revision history:														
;		12/22/2002 - 1136 MST - Charles Banas								
;			prepared for CVS commit.  added comments.

;		20021229 -   0738GMT - Kye Lewis (kye...@us...)

;			Changed so the file links and compiles on NASM correctly
;			Changed BITS 16 to BITS 32, and fixed 3 other lines.
					
;-----------------------------------------------------------------------------
;	description:															
;		this file provides the ability for the kernel to boot with a		
;		minimal bootloader that does not provide A20 or p-mode support.		
			
;-----------------------------------------------------------------------------

;[BITS 16]
[BITS 32]
[SECTION .text]
[GLOBAL _start]
	
;	this part is meant for compatibility with the standard BeFS bootloader.
;	the BeFS bootloader loads the kernel image at 1000h:0000h and jumps to
;	1000h:0200h.  the DOS-compatible bootloader i use jumps to 1000h:0000h.
	jmp		_start
	times (0x1fd) nop

;----------------------------
; _start function
;----------------------------
;   default symbol name used by the linker
_start:
	mov		ax,0x1000
	mov		ds,ax
	mov		es,ax
	mov		fs,ax
	mov		gs,ax

	cli
	mov		ax,0x9000
	mov		ss,ax
	mov		sp,ax
	sti

	mov		si,[booting]
	call	_disp_str

	call	_set_a20
;	call	_check_a20
	jnc		a20_good			; alternately, using the _check_a20 code, jz must be used.
	call	_a20_dump
a20_good:
	call	_check_mem

	xor		ebx,ebx
	mov		bx,cs
	shl		ebx,4
	mov		eax,ebx
	lea		eax,[ebx]
;	jmp		$
	mov		[gdt2 + 2],ax		; set descriptor base address=EAX
	mov		[gdt3 + 2],ax
	shr		eax,16
	mov		[gdt2 + 4],al
	mov		[gdt3 + 4],al
	mov		[gdt2 + 7],ah
	mov		[gdt3 + 7],ah
	lea		eax,[ebx + gdt]		; point gdt_ptr to the gdt
	mov		[gdt_ptr + 2],eax	; EAX=linear address of gdt
	push	dword 0				; zero EFLAGS (interrupts off,
	popfd						;  IOPL=0, NT bit=0)
	lgdt	[gdt_ptr]
	mov		eax,cr0
	or		al,1
	mov		cr0,eax
	mov		si,[pmodeswitch]
	call	_disp_str
	jmp		SYS_CODE_SEL:do_pm

;----------------------------
; _a20_dump function
;----------------------------
;	dumps because a20 is not set
_a20_dump:
	mov		si,[a20fail]
	call	_disp_str
;	xor		ax,ax
;	int		0x16
;	int		0x19
	ret							; for now.  it's a pain to test on Bochs when a20 never enables.
								; besides, using a BIOS function works on all tested machines.

;----------------------------
; _disp_str function
;----------------------------
;	input:
;		ds:si	location of string to print
;	clobbers:
;		ax, bx
;----------------------------
;	default "print string" function for use before pmode
_disp_str:
	lodsb
	or		al,al
	jz		done_str
	mov		ah,0x0E
;	mov		bx,0x7
	db		0xBB, 0x07, 0x00	; why not "mov bx, 0x7" you ask?  try it for yourself using
	int		0x10				; NASM 0.98.35.  then disassemble.  then look.  closely.
	jmp		_disp_str
done_str:
	ret

;----------------------------
; _set_a20 function
;----------------------------
;	sets the a20 line to make all memory available
;	right now, ONLY BIOS code is used.  should this start failing, i will rewrite
;	the code to try alternate methods.  *DOES NOT WORK ON BOCHS.*  in fact, none of
;	this code does.  but, the BIOS code works on most tested systems, so i have
;	rather high hopes.
;	ignore the messiness.
_set_a20:
;	push	si
;	push	ax

	mov		ax,2401h
	int		15h

;	jnc		saf3

;	in		al,92h
;	or		ah,ah
;	jne		saf1

							; BIOS bug: do NOT clear b1 if it's already clear
;	test	al,02h
;	je		saf3
;	and		al,0FDh
;	jmp		short saf2
;saf1:
							; BIOS bug: do NOT set b1 if it's already set
;	test	al,02h
;	jne		saf3
;	or		al,02h
;saf2:
;	out		92h,al
;saf3:
;	pop		ax
;	pop		si
	ret

;----------------------------
; _check_a20 function
;----------------------------
;	output:
;		ZF	1 if a20 off, 0 if on.
;----------------------------
;	verifies a20 status
;_check_a20:
;	mov		ax,2402h
;	int		15h
;	jnc		check_done
;
;	push	ax
;	push	ds
;	push	es
;	xor		ax,ax
;	mov		ds,ax
;	dec		ax
;	mov		es,ax
;
;	mov		ax,[es:10h]		; read word at FFFF:0010 (1 meg)
;	not		ax				; 1's complement
;	push	word [0]		; save word at 0000:0000 (0)
;	mov		[0],ax			; word at 0 = ~(word at 1 meg)
;	mov		ax,[0]			; read it back
;	cmp		ax,[es:10h]		; fail if word at 0 == word at 1 meg
;	pop		word [0]
;	pop		es
;	pop		ds
;	pop		ax
;
;check_done:
;	ret						; if ZF=1, the A20 gate is NOT enabled

;----------------------------
; _check_mem function
;----------------------------
;	uses BIOS calls to get memory size - falls back to manual scans if needbe.
;	i've elected to not use INT15h, AH=88h because of its severe limitations.
;	i'll implement a manual scan only if the need arises.  i don't expect this
;	to be executed on ancient machines, so the need for antiquated methods may
;	not even arise.  we'll see.  i need to print the contents of "_availmem"
;	very soon, though.  in the kernel init maybe?
_check_mem:
mem1:	;-------	INT 15h AX=E801h
	xor		eax,eax
	mov		ax,0xe801
	int		15h
	jc		mem2
	cmp		ax,0xe801
	je		mem2
	shl		ebx,8
	add		eax,ebx
	mov		[_availmem],eax
	jmp		mem_done
mem2:	;-------	INT 15h AH=8Ah
	xor		eax,eax
	xor		edx,edx
	mov		ah,0x8A
	int		15h
	cmp		eax,0x0
	jz		mem3
	shl		edx,16
	or		eax,edx
	mov		[_availmem],eax
	jmp		mem_done
mem3:	;-------	alternate method
mem_done:
	ret

[SECTION .data]

; strings
booting:
	db		'Initializing LinksOS kernel...',0xd,0xa,0x0
a20fail:
	db		'Failed setting A20!',0xd,0xa,0x0
pmodeswitch:
	db		'Switching to protected mode...',0xd,0xa,0x0
pmodefail:
	db		'P-mode failed!',0xd,0xa,0x0

_availmem:
	dd		0				; to be filled with amount of memory available in K
[GLOBAL _availmem]

; null descriptor
gdt:
	dw		0				; limit 15:0
	dw		0				; base 15:0
	db		0				; base 23:16
	db		0				; type
	db		0				; limit 19:16, flags
	db		0				; base 31:24
; linear data segment descriptor
LINEAR_SEL	equ	$-gdt
	dw		0xFFFF			; limit 0xFFFFF (1 meg? 4 gig?)
	dw		0				; base for this one is always 0
	db		0
	db		0x92			; present,ring 0,data,expand-up,writable
	db		0xCF			; page-granular (4 gig limit), 32-bit
	db		0
; code segment descriptor
SYS_CODE_SEL	equ	$-gdt
gdt2:
	dw		0xFFFF
	dw		0				; (base gets set above)
	db		0
	db		0x9A			; present,ring 0,code,non-conforming,readable
	db		0xCF
	db		0
; data segment descriptor
SYS_DATA_SEL	equ	$-gdt
gdt3:
	dw		0xFFFF
	dw		0				; (base gets set above)
	db		0
	db		0x92			; present,ring 0,data,expand-up,writable
	db		0xCF
	db		0
gdt_end:

gdt_ptr:
	dw		gdt_end - gdt - 1	; GDT limit
	dd		gdt				; linear, physical address of GDT


;---------------
; Protected-mode segment
;---------------

do_pm:
	mov		ax,SYS_DATA_SEL		; now in 32-bit pmode
	mov		ds,eax				; EAX works, one byte smaller :)
	mov		ss,eax
	nop
	mov		es,eax
	mov		fs,eax
	mov		gs,eax
	xor		eax,eax				; zero top 16 bits of ESP
	mov		ax,sp
	mov		esp,eax
;[EXTERN _main]
;	call	_main				; call C code
	jmp		$					; freeze

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;	time to link to C code now!		;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;