| ; ----------------------------------------------------------------------- |
| ; |
| ; 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 |
| ; =========================================================================== |