technical info on hard drives-4

[abeetha.m]

CMOS RAM


The BIOS stores some values in an area called CMOS RAM.
These are the values entered by the user in the BIOS setup program, often
called CMOS setup. This includes CHS values and other information on installed
hard drives. CMOS RAM is normally 64 bytes but may be larger.


FDPT, EDPT, INT41, INT 46


FDPT is an acronym for Fixed Disk Parameter Table. This is
a table that a BIOS creates to store the CHS values for a hard drive. Newer
BIOSes may create a newer type of FDPT which may be called an Enhanced FDPT
(EDPT). An Enhanced FDPT will contain both the translated and untranslated CHS
values for a hard drive. The EDPT will also contain other info on the hard
drive, such as whether or not it supports LBA addressing.


The FDPT contains a list of parameters used by 47 different
drive types, usually with 2 extra entries for user-defineable drive types.
Although the drive types in this table are now obsolete, this structure is
normally maintained for backwards compatibility.


INT41 and INT46 are unusual in that they do not represent
Device Service Routines. INT41 points to the entry for drive 80 in its FDPT,
INT46 points to the entry for drive 81 in its FDPT. Although a BIOS will create
a FDPT for any drive, there is no standard way to access the FDPT for any
drives beyond the first two.


INT 13 Extensions


Due to limitations in the BIOS INT13 services, a new
standard has been developed to add new services. These new INT13 services are
known as INT13 Extensions.

---

Some Sources for More Information


The
Indispensible Hardware Book - Second Edition - Hans-Peter Messmer




Addison Wesley - 1995 (widely available
paperback)





Upgrading and
Repairing PCs - Sixth Edition - Scott Mueller




Que - 1996 (widely available paperback)





System BIOS for
IBM PCs, Compatibles, and EISA Computers - Second




Edition - 1991 - Phoenix Technologies,
Ltd. - published by




Addison Wesley (now out of date but
full of basic info)





How It Works
series of documents by Hale Landis - these are available




as a ZIP file from Western Digital's
ftp site at this URL:





ftp://ftp.fission.dt.wdc.com/pub/otherdocs/pc_systems/how_it_works/ALLHIW.ZIP




Also the current collection can be
obtained in a ZIP file by sending




an email message to hiw@sugs.tware.com
- this is an automatic




system, the subject and body of message
are ignored.





EIDE FAQ -
authored by John Wehman and Peter den Haan




home page is
http://thef-nym.sci.kun.nl/~pieterh/storage.html





Set of articles
on booting at PC Lube and Tune on the web:




http://pclt.cis.yale.edu/pclt/BOOT





Document on
using NT Boot Loader and multiple OSes:





http://www.windows-nt.com/multiboot/directboot.html





For more
detailed information on how newer BIOSes handle large drives,


see
specification document available on WDC's ftp site:




Enhanced BIOS Services For Disk Drives
- Working Draft - Revision 6





ftp://fission.dt.wdc.com/x3t13/incoming/project/d1226r6.pdf





Technical
documents on Micro Firmware's web site and FTP site:







32GBBUG - discusses a bug in some BIOSes
limiting HDs to 3.2GB




CLUSTER.TXT - discusses cluster size
and slack space




HDMATH.TXT - lots of numbers




LBA.TXT - discusses LBA - Logical Block
Addressing




MEGABYTE.TXT - discusses difference in
binary and decimal megabytes




OVER2GB.TXT - discusses BIOS limitation
of 2GB hard drives




OVER4GB.TXT - discusses BIOS/DOS 4.2GB
drive size limitation




WIN95PARTN.TXT - discusses bug in WIN95
and new partition types





Utilities on
Micro Firmware's FTP site:




PARTITN - This program shows the information
contained in all of the




partition tables on all
drives in the system.







ATAID - Shows info returned by ATA
Identify Device command.