Sect1Ptr0_VAL equ 0xdeadbeef
Sect1Ptr1_VAL equ 0xfeedface
- 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...
-;
-
-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
-
- global SecPerClust
-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-76+8],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
-;
-harddisk:
- mov dx,[di-76-10] ; Original DS
- mov si,[di-76-12] ; Original SI
- shr si,4
- jz .no_partition ; SI == 0 -> assume no partition
- add dx,si
- cmp dx,1024 ; DS:SI < 1K (inside the IVT)?
- jb .no_partition
- cmp dx,PartInfo >> 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.
-;
- mov eax,strict dword Sect1Ptr0_VAL ; 0xdeadbeef
-Sect1Ptr0 equ $-4
- mov edx,strict dword Sect1Ptr0_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 0: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.
-;
- global kaboom
-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
- call writestr_early
-
- 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
+%include "diskboot.inc"
-;
-;
-; writestr_early: 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_early:
- pushad
-.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: popad
- ret
-
-;
-; 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 1FEh-($-$$)
-
-bootsignature dw 0xAA55
-
-;
-; ===========================================================================
-; End of boot sector
; ===========================================================================
; Start of LDLINUX.SYS
; ===========================================================================
projects / platform / upstream / syslinux.git / commitdiff