Ignore:
Timestamp:
Apr 8, 2017, 12:28:09 AM (8 years ago)
Author:
Ben Rietbroek
Message:

Ignore LARGE FLOPPY removables -- NOT BULLET PROOF YET! [v1.1.1-testing]

A LARGE FLOPPY will have a BPB instead of an MBR and chances are that
the MBR boot-flag locations contain values other than 00h or 80h.
The method implemented here needs to be improved but may catch most
LARGE FLOPPY formatted disks. Especially those with text in the PT area,
which is where AiR-BOOT choked on. LARGE FLOPPIES with a BPB but ZEROS
in the PT area should be OK for AiR-BOOT.

CAUTION:
This is a testbuild !
AirBoot uses the BIOS to access disks and a small coding error can trash
partition tables or other vital disk structures. You are advised to make
backups of TRACK0 and EBRs before using this testbuild. More info at:
​https://rousseaux.github.io/netlabs.air-boot/pdf/AirBoot-v1.1.0-manual.pdf

File:
1 edited

Legend:

Unmodified
Added
Removed

  • trunk/bootcode/regular/partscan.asm

    r164 r171  
    3131
    3232IFDEF   AUX_DEBUG
    33         IF 0
     33        IF 1
    3434        DBG_TEXT_OUT_AUX    'PARTSCAN_ScanForPartitions:'
    3535        PUSHRF
    … …  
    4848        mov     [NewPartitions], al
    4949
    50         mov     byte ptr [CurIO_Scanning], 1             ; Set flag due scanning partitions
    51         mov     dl, 80h                       ; is first harddisc
     50        mov     byte ptr [CurIO_Scanning], 1    ; Set flag due scanning partitions
     51        mov     dl, 80h                         ; Is first harddisc
    5252    PSSFP_HarddiscLoop:
    5353
     54
     55
    5456; ========================================================= [ Scan Partitions ]
    5557
     58
     59        ; Save BIOS disk number
    5660        push    dx
     61
     62            ;!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     63            ;!! THIS IS WHERE WE TRY TO DETECT LARGE FLOPPIES !
     64            ;!! ---------------------------------------------------------------
     65            ;!! This is a quick hack to ignore LARGE FLOPPY removable media.
     66            ;!! With LARGE FLOPPY, the boot record will be a BPB instead of
     67            ;!! an MBR. To distinguish, it is assumed that the MBR bootable
     68            ;!! flag is either 00h or 80h. LARGE FLOPPIES will often have
     69            ;!! other values there. So, the block below tests if all 4 entries
     70            ;!! result in 0 when first ored together and then anded with 7fh.
     71            ;!!
     72            ;!! THIS IS NOT BULLET PROOF AND WILL BE ENHANCED IN THE FUTURE
     73            ;!!
     74            ;!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     75
     76            ; Push the whole shebang
     77            pusha
     78
     79            ; Check if this is a removable drive
     80            call    DriveIO_CalcDiskInfoPointer     ; Pointer to DISKINFO
     81            mov     ax, [bx+LocDISKINFO_I13X_Flags] ; Get INT13X flags
     82            test    ax, 04h                         ; Bit 3=1 if removable
     83            clc                                     ; Assume fixed
     84
     85            ; Fixed drive, so skip further checking for LARGE FLOPPY.
     86            ; We *DO* halt on broken partition tables !
     87            ; This is needed because AiR-BOOT cannot continue with broken
     88            ; tables on truly partitioned disks.
     89            je      @F
     90
     91
     92            ; !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     93            ; !! THE DISK IS A REMOVABLE !
     94            ; !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     95
     96
     97IFDEF   AUX_DEBUG
     98        IF 0
     99        DBG_TEXT_OUT_AUX    'test bigflop'
     100        PUSHRF
     101            ;~ call    DEBUG_DumpRegisters
     102            ;~ call    AuxIO_DumpParagraph
     103            ;~ call    AuxIO_TeletypeNL
     104        POPRF
     105        ENDIF
     106ENDIF
     107
     108            ; If it has LVM info, it must be partitioned and thus OK
     109            mov     al, [bx+LocDISKINFO_LVM_Secs]   ; Get LVM_SPT
     110            test    al, al                          ; See if 0
     111            clc                                     ; Assume OK
     112            jne     @F                              ; LVM_SPT present, OK
     113
     114            ; Now load its boot sector and see if AiR-BOOT is there, if so, OK
     115            mov     si, offset [Scratch]        ; Pointer to sector buffer
     116            call    DriveIO_LoadMBR             ; Load it
     117            test    al, 08h                     ; AiR-BOOT signature found ?
     118            clc                                 ; Assume OK
     119            jne     @F                          ; AB found, must be partitioned
     120
     121            ; A zero sector is also OK -- ready to be partitioned by OS/2 ;)
     122            call    IsSectorBufferZero          ; check if sector is all zeros
     123            clc                                 ; Assume OK
     124            je      @F                          ; Zero sector, OK
     125
     126            ; !! NOW WE TEST MBR BOOT FLAGS.
     127            ; !! MOST LARGE FLOPPIES WILL HAVE DATA THERE.
     128            ; !! NOT BULLET PROOF, NEEDS TO BE IMPROVED.
     129            xor     ax, ax          ; Clear for oring and anding
     130            add     si, 01beh       ; Advance to partition table
     131            mov     cx, 4           ; Four entries to test
     132            cld                     ; Load upwards
     133        __nxt:
     134            lodsb                   ; Load Boot Flag (00h OR 80h)
     135            or      ah, al          ; Merge all bits to AH
     136            add     si, 16          ; Point to next entry
     137            loop    __nxt           ; Again if still entries to test
     138
     139            ; A LARGE FLOPPY with 00h or 80h bytes at these 4 locations
     140            ; will PASS !!
     141            and     ah, 7fh         ; Strip of potential Boot Flag
     142            clc                     ; Assume OK
     143            jz      @F              ; Good indication this is NOT a BPB, but...
     144
     145
     146            ;!! The Boot Flag bytes were NOT either 00h or 80h, so this is
     147            ;!! definintly not a partitioned disk.
     148            ;!! SET CY TO INDICATE SKIPPING 'PARTSCAN_ScanDriveForPartitions'
     149            stc
     150
     151        ; End of LARGE FLOPPY test
     152        @@:
     153
     154            ; Restore the whole shebang
     155            popa
     156
     157        ; CARRY IS SET, LARGE FLOPPY OR WHAT -- SKIP IT !!
     158        jc      @F
     159
     160        ; This disk should be partitioned, so scan it.
     161        ; Broken partitions *DOS* halt AiR-BOOT !
    57162        call    PARTSCAN_ScanDriveForPartitions
     163
     164    @@:
     165
     166
    58167        pop     dx
    59168        inc     dl
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