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Diffstat (limited to 'core/diskboot.inc')
| -rw-r--r-- | core/diskboot.inc | 444 |
1 files changed, 444 insertions, 0 deletions
diff --git a/core/diskboot.inc b/core/diskboot.inc new file mode 100644 index 0000000..9dea6f9 --- /dev/null +++ b/core/diskboot.inc @@ -0,0 +1,444 @@ +; ----------------------------------------------------------------------- +; +; 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 +; =========================================================================== |