ldlinux.asm

Windows上的精简Linux系统

		call getlinsec
		mov si,bs_magic
		mov di,ldlinux_magic
		mov cx,magic_len
		repe cmpsb		; Make sure that the bootsector
		jne kaboom		; matches LDLINUX.SYS
;
; Done! Jump to the entry point!
;
		jmp ldlinux_ent
;
;
; writestr: write a null-terminated string to the console
;	    This assumes we're on page 0.  This is only used for early
;           messages, so it should be OK.
;
writestr:
.loop:		lodsb
		and al,al
                jz .return
		mov ah,0Eh		; Write to screen as TTY
		mov bx,0007h		; Attribute
		int 10h
		jmp short .loop
.return:	ret

;
; disk_error: decrement the retry count and bail if zero.
;	      This gets patched once we have more space to try to
;             optimize transfer sizes on broken machines.
;
disk_error:	dec si			; SI holds the disk retry counter
		jz kaboom
		; End of patched "call" instruction!
		jmp short disk_try_again

;
; getonesec: like getlinsec, but pre-sets the count to 1
;
getonesec:
		mov bp,1
		; Fall through to getlinsec

;
; getlinsec: load a sequence of BP floppy sector given by the linear sector
;	     number in EAX into the buffer at ES:BX.  We try to optimize
;	     by loading up to a whole track at a time, but the user
;	     is responsible for not crossing a 64K boundary.
;	     (Yes, BP is weird for a count, but it was available...)
;
;	     On return, BX points to the first byte after the transferred
;	     block.
;
;	     The "stupid patch area" gets replaced by the code
;	     mov bp,1 ; nop ... (BD 01 00 90 90...) when installing with
;	     the -s option.
;
;            This routine assumes CS == DS.
;
; Stylistic note: use "xchg" instead of "mov" when the source is a register
; that is dead from that point; this saves space.  However, please keep
; the order to dst,src to keep things sane.
;
getlinsec:
		mov esi,[bxSecPerTrack]
		;
		; Dividing by sectors to get (track,sector): we may have
		; up to 2^18 tracks, so we need to use 32-bit arithmetric.
		;
		xor edx,edx		; Zero-extend LBA to 64 bits
		div esi
		xor cx,cx
		xchg cx,dx		; CX <- sector index (0-based)
					; EDX <- 0
		; eax = track #
		div dword [bxHeads]	; Convert track to head/cyl
		;
		; Now we have AX = cyl, DX = head, CX = sector (0-based),
		; BP = sectors to transfer, SI = bsSecPerTrack,
		; ES:BX = data target
		;
gls_nextchunk:	push si			; <A> bsSecPerTrack
		push bp			; <B> Sectors to transfer

		; Important - this gets patched with a call.  The call
		; assumes cx, si and bp are set up, and can modify bp
		; and destroy si.  Until we have the space to do so,
		; transfer one sector at a time.
gls_set_size:
__BEGIN_STUPID_PATCH_AREA:
		mov bp,1		; 3 bytes, same as a call insn
__END_STUPID_PATCH_AREA:

		push ax			; <C> Cylinder #
		push dx			; <D> Head #

		push cx			; <E> Sector #
		shl ah,6		; Because IBM was STOOPID
					; and thought 8 bits were enough
					; then thought 10 bits were enough...
		pop cx			; <E> Sector #
		push cx			; <E> Sector #
		inc cx			; Sector numbers are 1-based, sigh
		or cl,ah
		mov ch,al
		mov dh,dl
		mov dl,[bsDriveNumber]
		xchg ax,bp		; Sector to transfer count
					; (xchg shorter than mov)
		mov si,retry_count	; # of times to retry a disk access
;
; Do the disk transfer... save the registers in case we fail :(
;
disk_try_again: 
		pusha			; <F>
		mov ah,02h		; READ DISK
		int 13h
		popa			; <F>
		jc disk_error
;
; Disk access successful
;
		pop cx			; <E> Sector #
		mov di,ax		; Reduce sector left count
		mul word [bsBytesPerSec] ; Figure out how much to advance ptr
		add bx,ax		; Update buffer location
		pop dx			; <D> Head #
		pop ax			; <C> Cyl #
		pop bp			; <B> Sectors left to transfer
		pop si			; <A> Number of sectors/track
		sub bp,di		; Reduce with # of sectors just read
		jz writestr.return	; Done!
		add cx,di
		cmp cx,si
		jb gls_nextchunk
		inc dx			; Next track on cyl
		cmp dx,[bsHeads]	; Was this the last one?
		jb gls_nonewcyl
		inc ax			; If so, new cylinder
		xor dx,dx		; First head on new cylinder
gls_nonewcyl:	sub cx,si		; First sector on new track
		jmp short gls_nextchunk

bailmsg:	db 'Boot failed', 0Dh, 0Ah, 0

bs_checkpt	equ $			; Must be <= 7DEFh

%if 1
bs_checkpt_off	equ ($-$$)
%ifndef DEPEND
%if bs_checkpt_off > 1EFh
%error "Boot sector overflow"
%endif
%endif

		zb 1EFh-($-$$)
%endif
bs_magic	equ $			; From here to the magic_len equ
					; must match ldlinux_magic
ldlinux_name:	db 'LDLINUX SYS'	; Looks like this in the root dir
		dd HEXDATE		; Hopefully unique between compiles

bootsignature	dw 0AA55h
magic_len	equ $-bs_magic

;
; ===========================================================================
;  End of boot sector
; ===========================================================================
;  Start of LDLINUX.SYS
; ===========================================================================

ldlinux_sys:

syslinux_banner	db 0Dh, 0Ah
%if IS_MDSLINUX
		db 'MDSLINUX '
%else
		db 'SYSLINUX '
%endif
		db version_str, ' ', date, ' ', 0
		db 0Dh, 0Ah, 1Ah	; EOF if we "type" this in DOS

ldlinux_magic	db 'LDLINUX SYS'
		dd HEXDATE
		dw 0AA55h

;
; This area is possibly patched by the installer.  It is located
; immediately after the EOF + LDLINUX SYS + 4 bytes + 55 AA + alignment,
; so we can find it algorithmically.
;
		alignb 4, db 0
MaxTransfer	dw 00FFh		; Absolutely maximum transfer size

		align 4
ldlinux_ent:
; 
; Note that some BIOSes are buggy and run the boot sector at 07C0:0000
; instead of 0000:7C00 and the like.  We don't want to add anything
; more to the boot sector, so it is written to not assume a fixed
; value in CS, but we don't want to deal with that anymore from now
; on.
;
		jmp 0:.next
.next:

;
; Tell the user we got this far
;
		mov si,syslinux_banner
		call writestr
;
; Remember, the boot sector loaded only the first cluster of LDLINUX.SYS.
; We can really only rely on a single sector having been loaded.  Hence
; we should load the FAT into RAM and start chasing pointers...
;
		xor ax,ax
		cwd
		inc dx				; DX:AX <- 64K
		div word [bxBytesPerSec]	; sectors/64K
		mov si,ax

		push es
		mov bx,fat_seg			; Load into fat_seg:0000
		mov es,bx
		
		mov eax,[bsHidden]		; Hidden sectors
		add edx,[bxResSectors]
		add eax,edx
		mov ecx,[bxFATsecs]		; Sectors/FAT
fat_load_loop:	
		mov ebp,ecx			; Make sure high EBP = 0
		cmp bp,si
		jna fat_load
		mov bp,si			; A full 64K moby
fat_load:	
		xor bx,bx			; Offset 0 in the current ES
		call getlinsecsr
		sub cx,bp
		jz fat_load_done		; Last moby?
		add eax,ebp			; Advance sector count
		mov bx,es			; Next 64K moby
		add bx,1000h
		mov es,bx
		jmp short fat_load_loop
fat_load_done:
		pop es
;
; Fine, now we have the FAT in memory.	How big is a cluster, really?
; Also figure out how many clusters will fit in an 8K buffer, and how
; many sectors and bytes that is
;
		mov edi,[bxBytesPerSec]		; Used a lot below
		mov eax,[SecPerClust]
		mov si,ax			; Also used a lot
		mul di
		mov [ClustSize],eax		; Bytes/cluster
		mov bx,ax
		mov ax,trackbufsize		; High bit 0
		cwd
		div bx
		mov [BufSafe],ax		; # of cluster in trackbuf
		mul si
		mov [BufSafeSec],ax
		mul di
		mov [BufSafeBytes],ax
		add ax,getcbuf			; Size of getcbuf is the same
		mov [EndOfGetCBuf],ax		; as for trackbuf
;
; FAT12 or FAT16?  This computation is fscking ridiculous...
;
		mov eax,[bxSectors]
		and ax,ax
		jnz have_secs
		mov eax,[bsHugeSectors]
have_secs:	add eax,[bsHidden]		; These are not included
		sub eax,[RootDir]		; Start of root directory
		movzx ebx,word [RootDirSize]
		sub eax,ebx			; Subtract root directory size
		xor edx,edx
		div esi				; Convert to clusters
		cmp ax,4086			; FAT12 limit
		jna is_fat12
		; Patch the jump
		mov byte [nextcluster+1],nextcluster_fat16-(nextcluster+2)
is_fat12:

;
; Patch gls_set_size so we can transfer more than one sector at a time.
;
		mov byte [gls_set_size],0xe8	; E8 = CALL NEAR
		mov word [gls_set_size+1],do_gls_set_size-(gls_set_size+3)
		mov byte [disk_error],0xe8
		mov word [disk_error+1],do_disk_error-(disk_error+3)

;
; Now we read the rest of LDLINUX.SYS.	Don't bother loading the first
; cluster again, though.
;
load_rest:
		mov cx,[ClustSize]
		mov bx,ldlinux_sys
		add bx,cx
		mov si,[RunLinClust]
		call nextcluster
		xor dx,dx
		mov ax,ldlinux_len-1		; To be on the safe side
		add ax,cx
		div cx				; the number of clusters
		dec ax				; We've already read one
		jz all_read_jmp
		mov cx,ax
		call getfssec
;
; All loaded up
;
all_read_jmp:
		jmp all_read
;
; -----------------------------------------------------------------------------
; Subroutines that have to be in the first sector
; -----------------------------------------------------------------------------
;
; getfssec: Get multiple clusters from a file, given the starting cluster.
;
;	This routine makes sure the subtransfers do not cross a 64K boundary,
;	and will correct the situation if it does, UNLESS *sectors* cross
;	64K boundaries.
;
;	ES:BX	-> Buffer
;	SI	-> Starting cluster number (2-based)
;	CX	-> Cluster count (0FFFFh = until end of file)
;
;	Returns CF=1 on EOF
;
getfssec:
.getfragment:	xor ebp,ebp			; Fragment sector count
		lea eax,[si-2]			; Get 0-based sector address
		mul dword [SecPerClust]
		add eax,[DataArea]
.getseccnt:					; See if we can read > 1 clust
		add bp,[SecPerClust]
		dec cx				; Reduce clusters left to find
		lea di,[si+1]
		call nextcluster
		cmc
		jc .eof				; At EOF?
		jcxz .endfragment		; Or was it the last we wanted?
		cmp si,di			; Is file continuous?
		je .getseccnt			; Yes, we can get
.endfragment:	clc				; Not at EOF
.eof:		pushf				; Remember EOF or not
		push si
		push cx
.getchunk:
		push eax
		mov ax,es			; Check for 64K boundaries.
		shl ax,4
		add ax,bx
		xor dx,dx
		neg ax
		setz dl				; DX <- 1 if full 64K segment
		div word [bsBytesPerSec]	; How many sectors fit?
		mov si,bp
		sub si,ax			; Compute remaining sectors
		jbe .lastchunk
		mov bp,ax
		pop eax
		call getlinsecsr
		add eax,ebp			; EBP<31:16> == 0
		mov bp,si			; Remaining sector count
		jmp short .getchunk
.lastchunk:	pop eax
		call getlinsec
		pop cx
		pop si
		popf
		jcxz .return			; If we hit the count limit
		jnc .getfragment		; If we didn't hit EOF
.return:	ret

;
; getlinsecsr: save registers, call getlinsec, restore registers
;
getlinsecsr:	pushad
		call getlinsec
		popad
		ret

;
; nextcluster: Advance a cluster pointer in SI to the next cluster
;	       pointed at in the FAT tables.  CF=0 on return if end of file.
;
nextcluster:
		jmp short nextcluster_fat12	; This gets patched

nextcluster_fat12:
		push bx
		push ds
		mov bx,fat_seg
		mov ds,bx
		mov bx,si			; Multiply by 3/2
		shr bx,1			; CF now set if odd
		mov si,[si+bx]
		jnc nc_even
		shr si,4			; Needed for odd only
nc_even:
		and si,0FFFh
		cmp si,0FF0h			; Clears CF if at end of file
		pop ds
		pop bx
nc_return:	ret

;
; FAT16 decoding routine.  Note that a 16-bit FAT can be up to 128K,
; so we have to decide if we're in the "low" or the "high" 64K-segment...
;
nextcluster_fat16:
		push ax
		push ds
		mov ax,fat_seg
		shl si,1
		jnc .seg0
		mov ax,fat_seg+1000h
.seg0:		mov ds,ax
		mov si,[si]
		cmp si,0FFF0h
		pop ds
		pop ax
		ret

;
; Routine that controls how much we can transfer in one chunk.  Called
; from gls_set_size in getlinsec.
;
do_gls_set_size:
		sub si,cx		; Sectors left on track
		cmp bp,si
		jna .lastchunk
		mov bp,si		; No more than a trackful, please!
.lastchunk:
		cmp bp,[MaxTransfer]	; Absolute maximum transfer size
		jna .oktransfer
		mov bp,[MaxTransfer]
.oktransfer:	
		ret

;
; This routine captures disk errors, and tries to decide if it is
; time to reduce the transfer size.
;
do_disk_error:
		dec si			; Decrement the retry counter
		jz kaboom		; If expired, croak
		cmp si,2		; If only 2 attempts left
		ja .nodanger
		mov al,1		; Drop transfer size to 1
		jmp short .setsize
.nodanger:
		cmp si,retry_count-2
		ja .again		; First time, just try again
		shr al,1		; Otherwise, try to reduce
		adc al,0		; the max transfer size, but not to 0
.setsize:
		mov [MaxTransfer],al
.again:
		ret

;
; Debug routine
;
%ifdef debug
safedumpregs:
		cmp word [Debug_Magic],0D00Dh
		jnz nc_return
		jmp dumpregs
%endif

rl_checkpt	equ $				; Must be <= 8000h

rl_checkpt_off	equ ($-$$)
%ifndef DEPEND
%if rl_checkpt_off > 400h
%error "Sector 1 overflow"
%endif
%endif

; ----------------------------------------------------------------------------
;  End of code and data that have to be in the first sector
; ----------------------------------------------------------------------------

all_read:
;
; Let the user (and programmer!) know we got this far.  This used to be
; in Sector 1, but makes a lot more sense here.
;