dospartlibata.c

VxWorks下ata接口的文件系统实例

/* dosPartLibAta.c - show & mount ATA HDD MSDOS partitions */


/*
modification history
--------------------
01c,15sep97,jkf   removed LCHS support.  Only useful for MSDOS versions
                  before 3.3, not likely anyone needs LCHS support.  
                  Added support for big endian arches and arches without
                  dynamic bus sizing, which involved byte swapping macro
                  and reading sector data with macro offsets. Now supports
                  80x86, 68k, SPARC, MIPS, PPC, i960, etc.)  Revised docs.
01b,01sep97,jkf   fixed check for LBA support in dosPartRdLbaAta().
                  fixed stdlib.h filename case for Unix.
                  fixed args to ataDevCreate in example comments.
                  fixed cosmetics in sector and show routines.  
01a,01sep97,jkf   written.
*/


/*
DESCRIPTION:
This library is provided by Wind River Systems Customer
Support strictly as an educational example.

This code supports both mounting and displaying partition 
tables written by MSDOS FDISK.exe or by any other MSDOS 
FDISK.exe compatible partitioning software.  

The first partition table is contained within a hard drives
Master Boot Record (MBR) sector, which is defined as sector
one, cylinder zero, head zero or logical block address zero.
The mounting and displaying routines within this code will 
first parse the MBR partition tables entries (defined below)
and also recursively parse any "extended" partition tables,
which may reside within another Cylinder, Head, and Sector 
(CHS) combination further into the hard disk.   (Also known
as extended partition tables chained from the MBR's partition
table.  MSDOS file systems within extended partitions are known
to those familiar with the MSDOS FDISK.exe utility as "Logical 
drives within the extended partition".

Since MSDOS file systems have a fixed number of clusters, 64Kb,
and clusters are the minimum allocation unit at the file system
level, using partitions can help optimize cluster size for a given
hard disk,  improving performance and resource usage.  For example, 
a 1-gigabyte hard disk with a single MSDOS file system has the 
following minimum allocation unit at the dosFsLib open/read/write
level interface:

    1GB/64KB = Minimum allocation unit.

This may produce a minimum allocation unit that is larger than
what the developer desires (for cluster boundary buffer alignment,
or other buffering, ect).

One solution is to sub-divide the disk into smaller partitions
or "logical disks" on the single physical disk and format file 
systems over each of the partitions.  (Then using this code you
may mount them.)   If you used four equal sized partitions, then
this method produces:

    256MB/64KB = Minimum allocation unit.	
    256MB/64KB = Minimum allocation unit.
    256MB/64KB = Minimum allocation unit.
    256MB/64KB = Minimum allocation unit.

This produces a smaller minimum allocation unit and may be 
better suited for optimal application file system performance.
Each partition contains its own unique file system.  As far
as the file system level is concerned it is on a unique disk.

Here is a picture showing the layout of a single disk containing
multiple MSDOS file systems:

(Note, all ASCII pictures herein are best viewed with
 an 'equally sized characters' or 'fixed width' style font)

  +---------------------------------------------------------+
  |<---------------------The entire disk------------------->|
  |M                                                        |
  |B<---C:--->                                              |
  |R           /---- First extended partition--------------\|
  |           E<---D:---><-Rest of the ext part------------>|
  |P          x                                             |
  |A          t          E<---E:--->E<Rest of the ext part->|
  |R                     x          x                       |
  |T                     t          t<---------F:---------->|
  +---------------------------------------------------------+
  (Ext == extended partition sector)


A MS-DOS partition table resides within one sector on a hard
disk.  There is always one in the first sector of a hard disk
partitioned with FDISK.exe.  There first partition table may
contain references to "extended" partition tables residing on
other sectors if there are multiple partitions.  The first 
sector of the disk is the starting point.  Partition tables
are of the format:

Offset from     
the beginning 
of the sector          Description
-------------          -------------------------
   0x1be               Partition 1 table entry  (16 bytes)
   0x1ce               Partition 2 table entry  (16 bytes)
   0x1de               Partition 3 table entry  (16 bytes)
   0x1ee               Partition 4 table entry  (16 bytes)
   0x1fe               Signature  (0x55aa, 2 bytes)


Individual MSDOS partition table entries are of the format:

Offset   Size      Description
------   ----      ------------------------------
 0x0     8 bits    boot type
 0x1     8 bits    beginning sector head value
 0x2     8 bits    beginning sector (2 high bits of cylinder#)
 0x3     8 bits    beginning cylinder# (low order bits of cylinder#)
 0x4     8 bits    system indicator
 0x5     8 bits    ending sector head value
 0x6     8 bits    ending sector (2 high bits of cylinder#)
 0x7     8 bits    ending cylinder# (low order bits of cylinder#)
 0x8    32 bits    number of sectors preceding the partition
 0xc    32 bits    number of sectors in the partition
  
In the partition table entry, the Sector/Cylinder (16bits)
(offset 0x2 & 0x3) fields data are stored in the format:

 |7|6|5|4|3|2|1|0|  The first 8 bits of offset 0x2.
  | | `------------ 6 least significant bit's contain the sector 
  | |               offset within cylinder.  sector == 6 bits.
  | |
  `-`-------------- Two most significant (high order) bits of the 
                    cylinder value.

 |7|6|5|4|3|2|1|0|  2nd 8 bits  (cylinder 0x3)
  `---------------- Eight least significant (low order) bits of the
                    cylinder value.

This format restricts partition entries to 10 cylinder bits & 6 sector
bits and 8 head bits.  This is also the format used by the PC-AT BIOS
Int13h functions, which is used by FDISK.exe 

dosPartLibAta/Scsi.c uses the following method of parsing the CHS values
stored in the 0x2 & 0x3 partition table offsets into distinct values:

Cyl = ((((dospt_startSec) & 0x00c0) << 2) |
       (((dospt_startSec) & 0xff00) >> 8));

Hd  = pPartTbl->dospt_startHead;

Sec = (pPartTbl->dospt_startSec & 0x003f); 

(see DOS_PART_TBL struct typedef in dosFsLib.h)

This produces individual CHS value from the values that
are contained within the MSDOS partition table entry.  

Definitions of CHS terms:

CHS - Cylinder/Head/Sector. 

L-CHS - Logical - CHS (used by INT 13 AxH/FDISK.exe) 
        256 heads, 1024 cylinders, 63 sectors. max.
        LCHS allow ~8.4 GB addressable space.

P-CHS - Physical CHS (used by physical ATA device) 
        16 heads, 65535 cylinders, 63 sectors.
        PCHS allow ~137GB addressable space.
        (VxWorks ataRawio() uses PCHS)

LBA - Logical Block Address. ATA/SCSI absolute sector
      numbering scheme.  LBA sector 0 == addresses the
      first sector on a device by definition.  ATA devices
      may also support LBA at the device interface. ATA
      standard says that cylinder 0, head 0, sector 1 == LBA 0.
      LBA allows (~137GB) on an ATA device. 

The  MSDOS "logical CHS" (LCHS) format combined with the 
Physical IDE/ATA CHS interface (PCHS) gives MSDOS the 
following addressing limitation:

                  +------------+----------+----------------+   
                  | BIOS INT13 | IDE/ATA  | Combined Limit |
+-----------------+------------+----------+----------------+   
|Max Sectors/Track|  63        |    255   |     63         |
+-----------------+------------+----------+----------------+
|Max Heads        |  256       |     16   |     16         |
+-----------------+------------+----------+----------------+
|Max Cylinders    |  1024      |  65536   |    1024        |
+-----------------+------------+----------+----------------+
|Capacity         |  8.4GB     |  136.9GB |   528 MB       |
+-----------------+------------+----------+----------------+

1024 x 16 x 63 X 512 bytes/sector = 528.48 MB limit.

Cylinder bits : 10 = 1024
Head bits     :  4 = 16
Sector bits   :  6 (-1) = 63 

(by convention, CHS sector numbering begins at
 #1, not #0; LBA begins at zero, which is CHS#1) 

To overcome the limitation. PC BIOS/OS vendors perform what is often
referred to as geometric or "drive" CHS translation to address more 
than 528MB, and to "address" cylinders above 1024.

BIOS vendors offer CHS translation extensions that are based
around the format of the PC-AT BIOS call "INT 0x13 function 0x8"
aka "Get Current Drive Parameters"  There are also two parameter
tables EDPT and FDPT set by the BIOS interfaces.  EDPT is sometimes
not supported depending on the system BIOS.  The FDPT is always 
supported on PC-AT compatible BIOS.  Code is available for the x86
VxWorks BSP to call INT13h F8h romInit.s BIOS call, which returns
the FDPT.  This is not needed when using LBA on newer systems.
It is possible in the pc486/pc386 VxWorks BSP's to make an
BIOS (INT13h F8H) call in romInit.s before the switch to 
protected mode.  Sample romInit.s code demonstrating this 
will be provided upon request, contact [e-mail].
requesting "Int13h romInit.s x86 code".

There exists also some 3rd party software based disk overlay
programs (Such as OnTrack's Disk Manager,TM.) which uses a
semi-proprietary strategy to support drives over 528MB.  

In most cases, the translated LCHS geometry is applied such that
all sectors are addressed at the same physical location as when
the drive is used within an untranslated environment.  However,
not in all cases, and then there is a breakdown.  Alas, there 
is not any standard for defining the altered geometry translation
scheme, so compatibility issues will arise when moving drives from
one BIOS, Operating System, driver, and/or controller to another.

VxWorks SCSI and ATA drivers communicate directly with the
controller and do not use the BIOS.  The VxWorks ATA driver
may address the full 136.9GB offered by ATA, and the full range
offered by SCSI.  So this is not an issue for VxWorks.  It should
be noted however, since all the partition table data will be stored 
in LCHS format.

Here is an example of a typical PCHS to LCHS translation.

(Note, all ASCII pictures herein are best viewed with
 an 'equally sized characters'/'fixed width' style font)

+----------------+------------+----------------+-------------+--------+
|Actual Cylinders|Actual Heads|Altered Cylinder|Altered Heads|Max Size|
+----------------+------------+----------------+-------------+--------+
|  1<C<=1024     | 1<H<=16    |     C=C        |    H=H      |528 MB  |
+----------------+------------+----------------+-------------+--------+
| 1024<C<=2048   | 1<H<=16    |     C=C/2      |    H=H*2    |  1GB   |
+----------------+------------+----------------+-------------+--------+
| 2048<C<=4096   | 1<H<=16    |     C=C/4      |    H=H*4    | 2.1GB  |
+----------------+------------+----------------+-------------+--------+
| 4096<C<=8192   | 1<H<=16    |     C=C/8      |    H=H*8    | 4.2GB  |
+----------------+------------+----------------+-------------+--------+
| 8192<C<=16384  | 1<H<=16    |     C=C/16     |    H=H*16   | 8.4GB  |
+----------------+------------+----------------+-------------+--------+
| 16384<C<=32768 | 1<H<=8     |     C=C/32     |    H=H*32   | 8.4GB  |
+----------------+------------+----------------+-------------+--------+
| 32768<C<=65536 | 1<H<=4     |     C=C/64     |    H=H*64   | 8.4GB  |
+----------------+------------+----------------+-------------+--------+


Another type of translation always sets the sectors to 63, 
this produces different geometry than the above.

+----------------+----------+---------+--------------+
|Range           | Sectors  | Heads   | Cylinders    |
+----------------+----------+---------+--------------+
|  1<X<=528MB    |   63     |  16     | X/(63*16*512)|
+----------------+----------+---------+--------------+
| 528MB<X<=1GB   |   63     |  32     | X/(63*32*512)|
+----------------+----------+---------+--------------+
| 1GB<X<=2.1GB   |   63     |  64     | X/(63*64*512)|
+----------------+----------+---------+--------------+
| 2.1GB<X<=4.2GB |   63     |  128    |X/(63*128*512)|
+----------------+----------+---------+--------------+
| 4.2GB<X<=8.4GB |   63     |  256    |X/(63*255*512)|
+----------------+----------+---------+--------------+

There are several MSDOS utilities (from Phoenix, and Western 
Digital, and more, such as wdtblchk.exe, and chkbios.exe) which
can be used on PC's to check translations.  These utilities
use BIOS calls to read the EPDT and FPDT to determine the PCHS
and LCHS and LBA values for a given system.  

TRANSLATION ALGORITHMS:
----------------------

The algorithm used in this code for LBA to PCHS translation 
(in dosPartRdLba[Ata/Scsi]) is:
        
PCHS_cylinder = LBA / (heads_per_cylinder * sectors_per_track)
         temp = LBA % (heads_per_cylinder * sectors_per_track)
PCHS_head     = temp / sectors_per_track
PCHS_sector   = temp % sectors_per_track + 1


The reverse, PCHS to LBA, is not implement but shown here
should anyone need to implement it:

LBA = (((cylinder * heads_per_cylinder + heads )
         * sectors_per_track ) + sector - 1)


Both of these can also be used with either LCHS or PCHS.