Q87135: Large Drive Translation Under Microsoft OS/2 1.3
Article: Q87135
Product(s): Microsoft LAN Manager
Version(s):
Operating System(s):
Keyword(s):
Last Modified: 30-JUL-2001
SUMMARY
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This article discusses large drive support for Microsoft OS/2 version 1.3.
MORE INFORMATION
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1. Q. What is a large drive?
A. A large drive is any disk drive with more than 1024 cylinders, 256 heads,
or 63 sectors per track. "Large" does not refer to the capacity of the drive,
but rather these parameters. The combination of cylinders, heads, and sectors
per track is referred to as the actual geometry of the disk.
These particular capacities were chosen to offer compatibility with IBM ROM
BIOS and MS-DOS. Because OS/2 machines can be dual booted, or booted from an
MS-DOS floppy disk, it is necessary to maintain the same use of partition
tables as that employed by MS-DOS. Because these limits exist in MS-DOS, they
are inherited in OS/2.
2. Q. Why does this affect booting OS/2?
A. OS/2 employs the ROM BIOS during boot. Until the OS/2 kernel, loader, and
disk drivers have loaded, OS/2 must go through the ROM BIOS. After these have
loaded, OS/2 goes into protect mode disk operation, eliminating the need to
interface with the ROM BIOS. However, as the kernel, loader, and disk drivers
are loaded, it is imperative that these files lie within the boundaries the
ROM BIOS can access. If any of them lies outside these boundaries, that file
cannot be accessed as OS/2 boots, and therefore the system will hang.
3. Q. How is disk capacity computed?
A. Disk drive capacity is computed by the following formula:
cylinders * heads * sectors per track * sector size = disk capacity
The IBM ROM BIOS standard dictates that the cylinder addressing is limited to
10 bits (maximum of 1024), head addressing to 8 bits (maximum of 256), and
sector addressing to 6 bits (maximum of 63, because sector addressing is
1-based). OS/2 uses a sector size of 512 bytes.
4. Q. What can we do with drives that exceed the maximum geometry?
A. Two choices are available when a drive exceeds any of the above parameters:
truncation or translation.
If the truncation method is chosen, the offending parameter will simply be
truncated to the maximum allowable for that field. For example, suppose the
drive in question has 1740 cylinders (c), 14 heads (h), and 54 sectors per
track (spt). If truncation were employed, OS/2 would use an apparent geometry
of 1024 cylinders, 14 heads, and 54 sectors per track. The other 716
cylinders would be lost. In this case, the truncated drive would lose about
41 percent of the available drive space.
If translation were used, no drive space would be lost. Translation requires
decreasing the offending parameter until it is within acceptable bounds, and
increasing another parameter a corresponding amount. Using the above example,
the drive could be translated to have 870 cylinders, 28 heads, and 54 sectors
per track. In this way, no drive space is lost.
5. Q. Which method does Microsoft OS/2 version 1.3 employ?
A. Microsoft OS/2 1.3 employs both methods under different circumstances.
In the case of a boot drive, truncation is used. This is because of the
limitations stated above (the OS/2 kernel, loader, and disk driver can be
located on a disk partition that is accessible by the ROM BIOS). If
translation were done, one or more of these files could reside on a disk
location outside of the addressable geometry, resulting in an unbootable
system. Therefore, for boot drives, disk truncation is used. This may result
in some available drive space being lost; however, it does ensure that the
machine will be bootable.
For non-boot drives, however, translation is employed. This can be done safely
because the operating system is already loaded. Therefore, translation can be
done safely and no disk space should be lost.
How Translation Is Done
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Translation can be done in one of several ways: via hardware, hardware with
software support, or software only. For hardware incapable of doing translation
at a hardware level, MS OS/2 1.3 supports software translation. Because all disk
activity goes through the disk drivers as provided by the LADDR architecture,
translation can be done at this level. For hardware with software support
translation, the bus interface driver (BID) is responsible. For software-only
support, the DISK.TSD (type specific driver) does all of the work.
The DISK.TSD does the translation in the following manner:
If there are more than 1024 cylinders, and if this is the boot drive, set the
apparent geometry to have 1024 cylinders, and the actual number of heads and
sectors per track. If it is not the boot drive, halve the number of cylinders,
and double either the heads, or sectors per track. Repeat the last step until
the number of cylinders is less than 1024, or until the heads and sectors per
track no longer can be doubled. Sectors per track are increased before the
number of heads.
For example, a Seagate ST 4769E (691 MB ESDI drive) will generate the following
translation table:
Actual Physical Translation If Translation If
Drive Parameters Boot Drive Not Boot Drive
---------------- -------------- ---------------
1552 cylinders 1024 cylinders 776 cylinders
15 heads 15 heads 30 heads
53 sectors per track 53 sectors per track 53 sectors per track
To determine which method has been employed, use the LOGSHOW.EXE utility, which
is shipped with MS OS/2 1.3. This utility reports what the operating system
determines your actual and apparent geometry to be.
If you are Running LOGSHOW from an OS/2 command prompt, press F1 to examine the
existing log files, and then select the latest log file from the list. A screen
showing a variety of information will be displayed. The area of interest is the
DISK01 header, approximately midpoint of the screen. For example:
DISK01 ffff 04/10/92 14:12:47 - Informational: configure physical disk
AOK
Apparent geometry: 00512 000391200 000815 0015 0032 3700
Actual geometry: 00000 000000000 000815 0015 0032 65535
The fields are the same for the both the Actual and Apparent geometry, with the
exception of the last field in each. The fields are as follows (from left to
right):
- Sector size (in BYTES)
- Number of sectors on disk
- Number of cylinders on the disk
- Number of heads on the disk
- Number of sectors per track
- For the apparent geometry record, the last field is a set of flags that
detail how the geometry was determined.
- For the actual geometry record, the last field is the write precompensation
cylinder
Using this information, it is easy to see what, if any, translation or truncation
has been performed on the disk drive.
Known Problems with MS OS/2 1.3 Drive Translation
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There are two problems with disk translation and the released version of MS OS/2
1.3. These problems deal with doing the translation incorrectly, and affect the
following files:
DISK.TSD
ESDI-506.BID
The ESDI-506.BID is needed for any ESDI/IDE/ST-506 (MFM, RLL) drives for which
translation is required, while DISK.TSD is required for all drives doing
translation. These corrections are available from Microsoft Product Support
Services. Note that these corrections only affect systems doing translation.
Additional query words: 2.10 1.30 disk.tsd esdi.bid translate hard disk drive
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