source: bootcd/isolinux/syslinux-6.03/core/diskboot.inc

; -----------------------------------------------------------------------
;
;   Copyright 1994-2009 H. Peter Anvin - All Rights Reserved
;   Copyright 2009-2011 Intel Corporation; author: H. Peter Anvin
;
;   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., 51 Franklin St, Fifth Floor,
;   Boston MA 02110-1301, USA; either version 2 of the License, or
;   (at your option) any later version; incorporated herein by reference.
;
; -----------------------------------------------------------------------

;
; diskboot.inc
;
; Common boot sector code for harddisk-based Syslinux derivatives.
;
; Requires macros z[bwd], labels ldlinux_ent, ldlinux_magic, ldlinux_sys
; and constants BS_MAGIC_VER, LDLINUX_MAGIC, retry_count, Sect1Ptr[01]_VAL,
; STACK_TOP
;

                section .init
;
; Some of the things that have to be saved very early are saved
; "close" to the initial stack pointer offset, in order to
; reduce the code size...
;

                global StackBuf, PartInfo, Hidden, OrigESDI, DriveNumber
                global OrigFDCTabPtr
StackBuf        equ STACK_TOP-44-92     ; Start the stack here (grow down - 4K)
PartInfo        equ StackBuf
.mbr            equ PartInfo
.gptlen         equ PartInfo+16
.gpt            equ PartInfo+20
FloppyTable     equ PartInfo+76
; Total size of PartInfo + FloppyTable == 76+16 = 92 bytes
Hidden          equ StackBuf-24         ; Partition offset (qword)
OrigFDCTabPtr   equ StackBuf-16         ; Original FDC table
OrigDSSI        equ StackBuf-12         ; DS:SI -> partinfo
OrigESDI        equ StackBuf-8          ; ES:DI -> $PnP structure
DriveNumber     equ StackBuf-4          ; Drive number
StackHome       equ Hidden              ; The start of the canonical stack

;
; Primary entry point.  Tempting as though it may be, we can't put the
; initial "cli" here; the jmp opcode in the first byte is part of the
; "magic number" (using the term very loosely) for the DOS superblock.
;
bootsec         equ $
_start:         jmp short start         ; 2 bytes
                nop                     ; 1 byte
;
; "Superblock" follows -- it's in the boot sector, so it's already
; loaded and ready for us
;
bsOemName       db MY_NAME              ; The SYS command sets this, so...
                zb 8-($-bsOemName)

;
; These are the fields we actually care about.  We end up expanding them
; all to dword size early in the code, so generate labels for both
; the expanded and unexpanded versions.
;
%macro          superb 1
bx %+ %1        equ SuperInfo+($-superblock)*8+4
bs %+ %1        equ $
                zb 1
%endmacro
%macro          superw 1
bx %+ %1        equ SuperInfo+($-superblock)*8
bs %+ %1        equ $
                zw 1
%endmacro
%macro          superd 1
bx %+ %1        equ $                   ; no expansion for dwords
bs %+ %1        equ $
                zd 1
%endmacro
superblock      equ $
                superw BytesPerSec
                superb SecPerClust
                superw ResSectors
                superb FATs
                superw RootDirEnts
                superw Sectors
                superb Media
                superw FATsecs
                superw SecPerTrack
                superw Heads
superinfo_size  equ ($-superblock)-1    ; How much to expand
                superd Hidden
                superd HugeSectors
                ;
                ; This is as far as FAT12/16 and FAT32 are consistent
                ;
                ; FAT12/16 need 26 more bytes,
                ; FAT32 need 54 more bytes
                ;
superblock_len_fat16    equ $-superblock+26
superblock_len_fat32    equ $-superblock+54
                zb 54                   ; Maximum needed size
superblock_max  equ $-superblock

SecPerClust     equ bxSecPerClust

;
; Note we don't check the constraints above now; we did that at install
; time (we hope!)
;
start:
                cli                     ; No interrupts yet, please
                cld                     ; Copy upwards
;
; Set up the stack
;
                xor cx,cx
                mov ss,cx
                mov sp,StackBuf-2       ; Just below BSS (-2 for alignment)
                push dx                 ; Save drive number (in DL)
                push es                 ; Save initial ES:DI -> $PnP pointer
                push di
                push ds                 ; Save original DS:SI -> partinfo
                push si
                mov es,cx

;
; DS:SI may contain a partition table entry and possibly a GPT entry.
; Preserve it for us.  This saves 56 bytes of the GPT entry, which is
; currently the maximum we care about.  Total is 76 bytes.
;
                mov cl,(16+4+56)/2      ; Save partition info
                mov di,PartInfo
                rep movsw               ; This puts CX back to zero

                mov ds,cx               ; Now we can initialize DS...

;
; Now sautee the BIOS floppy info block to that it will support decent-
; size transfers; the floppy block is 11 bytes and is stored in the
; INT 1Eh vector (brilliant waste of resources, eh?)
;
; Of course, if BIOSes had been properly programmed, we wouldn't have
; had to waste precious space with this code.
;
                mov bx,fdctab
                lfs si,[bx]             ; FS:SI -> original fdctab
                push fs                 ; Save on stack in case we need to bail
                push si

                ; Save the old fdctab even if hard disk so the stack layout
                ; is the same.  The instructions above do not change the flags
                and dl,dl               ; If floppy disk (00-7F), assume no
                                        ; partition table
                js harddisk

floppy:
                xor ax,ax
                mov cl,6                ; 12 bytes (CX == 0)
                ; es:di -> FloppyTable already
                ; This should be safe to do now, interrupts are off...
                mov [bx],di             ; FloppyTable
                mov [bx+2],ax           ; Segment 0
                fs rep movsw            ; Faster to move words
                mov cl,[bsSecPerTrack]  ; Patch the sector count
                mov [di-12+4],cl

                push ax                 ; Partition offset == 0
                push ax
                push ax
                push ax

                int 13h                 ; Some BIOSes need this
                        ; Using xint13 costs +1B
                jmp short not_harddisk
;
; The drive number and possibly partition information was passed to us
; by the BIOS or previous boot loader (MBR).  Current "best practice" is to
; trust that rather than what the superblock contains.
;
; Note: di points to beyond the end of PartInfo
; Note: false negatives might slip through the handover area's sanity checks,
;       if the region is very close (less than a paragraph) to
;       PartInfo ; no false positives are possible though
;
harddisk:
                mov dx,[di-76-10]       ; Original DS
                mov si,[di-76-12]       ; Original SI
                shr si,4
                add dx,si
                cmp dx,4fh              ; DS:SI < 50h:0 (BDA or IVT) ?
                jbe .no_partition
                cmp dx,(PartInfo-75)>>4 ; DS:SI in overwritten memory?
                jae .no_partition
                test byte [di-76],7Fh   ; Sanity check: "active flag" should
                jnz .no_partition       ; be 00 or 80
                cmp [di-76+4],cl        ; Sanity check: partition type != 0
                je .no_partition
                cmp eax,'!GPT'          ; !GPT signature?
                jne .mbr
                cmp byte [di-76+4],0EDh ; Synthetic GPT partition entry?
                jne .mbr
.gpt:                                   ; GPT-style partition info
                push dword [di-76+20+36]
                push dword [di-76+20+32]
                jmp .gotoffs
.mbr:                                   ; MBR-style partition info
                push cx                 ; Upper half partition offset == 0
                push cx
                push dword [di-76+8]    ; Partition offset (dword)
                jmp .gotoffs
.no_partition:
;
; No partition table given... assume that the Hidden field in the boot sector
; tells the truth (in particular, is zero if this is an unpartitioned disk.)
;
                push cx
                push cx
                push dword [bsHidden]
.gotoffs:
;
; Get disk drive parameters (don't trust the superblock.)  Don't do this for
; floppy drives -- INT 13:08 on floppy drives will (may?) return info about
; what the *drive* supports, not about the *media*.  Fortunately floppy disks
; tend to have a fixed, well-defined geometry which is stored in the superblock.
;
                ; DL == drive # still
                mov ah,08h
                call xint13
                jc no_driveparm
                and ah,ah
                jnz no_driveparm
                shr dx,8
                inc dx                  ; Contains # of heads - 1
                mov [bsHeads],dx
                and cx,3fh
                mov [bsSecPerTrack],cx
no_driveparm:
not_harddisk:
;
; Ready to enable interrupts, captain
;
                sti

;
; Do we have EBIOS (EDD)?
;
eddcheck:
                mov bx,55AAh
                mov ah,41h              ; EDD existence query
                call xint13
                jc .noedd
                cmp bx,0AA55h
                jne .noedd
                test cl,1               ; Extended disk access functionality set
                jz .noedd
                ;
                ; We have EDD support...
                ;
                mov byte [getonesec.jmp+1],(getonesec_ebios-(getonesec.jmp+2))
.noedd:

;
; Load the first sector of LDLINUX.SYS; this used to be all proper
; with parsing the superblock and root directory; it doesn't fit
; together with EBIOS support, unfortunately.
;
Sect1Load:
                mov eax,strict dword Sect1Ptr0_VAL      ; 0xdeadbeef
Sect1Ptr0       equ $-4
                mov edx,strict dword Sect1Ptr1_VAL      ; 0xfeedface
Sect1Ptr1       equ $-4
                mov bx,ldlinux_sys      ; Where to load it
                call getonesec

                ; Some modicum of integrity checking
                cmp dword [ldlinux_magic+4],LDLINUX_MAGIC^HEXDATE
                jne kaboom

                ; Go for it!
                jmp ldlinux_ent

;
; getonesec: load a single disk linear sector EDX:EAX into the buffer
;            at ES:BX.
;
;            This routine assumes CS == DS == SS, and trashes most registers.
;
; 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.
;
getonesec:
                add eax,[Hidden]                ; Add partition offset
                adc edx,[Hidden+4]
                mov cx,retry_count
.jmp:           jmp strict short getonesec_cbios

;
; getonesec_ebios:
;
; getonesec implementation for EBIOS (EDD)
;
getonesec_ebios:
.retry:
                ; Form DAPA on stack
                push edx
                push eax
                push es
                push bx
                push word 1
                push word 16
                mov si,sp
                pushad
                mov ah,42h                      ; Extended Read
                call xint13
                popad
                lea sp,[si+16]                  ; Remove DAPA
                jc .error
                ret

.error:
                ; Some systems seem to get "stuck" in an error state when
                ; using EBIOS.  Doesn't happen when using CBIOS, which is
                ; good, since some other systems get timeout failures
                ; waiting for the floppy disk to spin up.

                pushad                          ; Try resetting the device
                xor ax,ax
                call xint13
                popad
                loop .retry                     ; CX-- and jump if not zero

                ; Total failure.  Try falling back to CBIOS.
                mov byte [getonesec.jmp+1],(getonesec_cbios-(getonesec.jmp+2))

;
; getonesec_cbios:
;
; getlinsec implementation for legacy CBIOS
;
getonesec_cbios:
.retry:
                pushad

                movzx esi,word [bsSecPerTrack]
                movzx edi,word [bsHeads]
                ;
                ; Dividing by sectors to get (track,sector): we may have
                ; up to 2^18 tracks, so we need to use 32-bit arithmetric.
                ;
                div esi
                xor cx,cx
                xchg cx,dx              ; CX <- sector index (0-based)
                                        ; EDX <- 0
                ; eax = track #
                div edi                 ; Convert track to head/cyl

                cmp eax,1023            ; Outside the CHS range?
                ja kaboom

                ;
                ; Now we have AX = cyl, DX = head, CX = sector (0-based),
                ; SI = bsSecPerTrack, ES:BX = data target
                ;
                shl ah,6                ; Because IBM was STOOPID
                                        ; and thought 8 bits were enough
                                        ; then thought 10 bits were enough...
                inc cx                  ; Sector numbers are 1-based, sigh
                or cl,ah
                mov ch,al
                mov dh,dl
                mov ax,0201h            ; Read one sector
                call xint13
                popad
                jc .error
                ret

.error:
                loop .retry
                ; Fall through to disk_error

;
; kaboom: write a message and bail out.
;
%ifdef BINFMT
                global kaboom
%else
                global kaboom:function hidden
%endif
disk_error:
kaboom:
                xor si,si
                mov ss,si
                mov sp,OrigFDCTabPtr    ; Reset stack
                mov ds,si               ; Reset data segment
                pop dword [fdctab]      ; Restore FDC table
.patch:                                 ; When we have full code, intercept here
                mov si,bailmsg
.loop:          lodsb
                and al,al
                jz .done
                mov ah,0Eh              ; Write to screen as TTY
                mov bx,0007h            ; Attribute
                int 10h
                jmp short .loop

.done:
                xor ax,ax
.again:         int 16h                 ; Wait for keypress
                                        ; NB: replaced by int 18h if
                                        ; chosen at install time..
                int 19h                 ; And try once more to boot...
.norge:         hlt                     ; If int 19h returned; this is the end
                jmp short .norge

;
; INT 13h wrapper function
;
xint13:
                mov dl,[DriveNumber]
                push es         ; ES destroyed by INT 13h AH 08h
                int 13h
                pop es
                ret

;
; Error message on failure
;
bailmsg:        db 'Boot error', 0Dh, 0Ah, 0

                ; This fails if the boot sector overflowsg
                zb 1F8h-($-$$)

bs_magic        dd LDLINUX_MAGIC
bs_link         dw (Sect1Load - bootsec) | BS_MAGIC_VER
bootsignature   dw 0xAA55

;
; ===========================================================================
;  End of boot sector
; ===========================================================================