Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!philmtl!philabs!linus!gwr
From: gwr@linus.UUCP (Gordon W. Ross)
Newsgroups: comp.os.minix
Subject: How to get hard disk parameters at boot time
Message-ID: <83311@linus.UUCP>
Date: 15 Dec 89 20:17:55 GMT
References: <26317@sequent.UUCP> <901@crash.cts.com> <832@ocsmd.ocs.com>
Reply-To: gwr@linus.mitre.org
Organization: The MITRE Corporation, Bedford, MA.
Lines: 72

I have noticed that recent discussions about the wini driver and its
adaptation to differing hard disk geometries have not included any
mention of the PC BIOS hard disk parameter vectors.

The best way to determine the hard disk geometry at boot time (on a
PC/AT compatible) is to examine the BIOS hard disk parameter tables.
All PC/AT compatible (and 386/AT) systems have boot-time
initialization code in the system BIOS that sets two interrupt
vectors (0x41, 0x46) to point at parameter tables for the first and
second hard disk, respectively.  The content of these tables is
described below.

These tables are the best source of information about hard disk
geometry for two reasons:

First, many hard disk controllers include a controller BIOS with a
boot-time routine that installs correct disk geometry parameters in
some area of RAM and then points the parameter vectors at those new
parameters.  Adaptec and Western Digital controllers for RLL and ESDI
drives are usually configured to do this "parameter fixing."
Note that when the controller changes the hard disk parameters, the
values indicated by the CMOS RAM are no longer valid.

Second, these hard disk parameter tables use a 16-bit word to store
any cylinder number quantities and are therefore not limited to a
10-bit cylinder number as is BIOS int 0x13 sub-function 8.
Though MS-DOS can't deal with cylinder numbers  greater than 1024,
some other systems that want to know the true disk geometry use
these parameter tables to determine the disk geometry.
(AT&T Sys V/386 Rel 2 does it this way.)

Unfortunately, I haven't had time to work on the at-wini driver.
I am posting this information in hopes that someone will use it to
make the at-wini driver figure out the disk parameters at boot time.

Gordon W. Ross (E025)	ARPA:  gwr@linus.mitre.org
The MITRE Corporation	UUCP:  {decvax,philabs}!linus!gwr
Burlington Road, Bedford, MA 01730	(617) 271-3205

Attached is a description of the PC/AT BIOS hard disk parameters.

/****************************************************************
 * The structure declaration below represents the layout of the
 * PC/AT BIOS hard disk parameter tables.  Interrupt vectors
 * 0x41 and 0x46 point to parameter structures for the first and
 * second hard disks, respectively.  This information was found
 * in the "IBM PC/AT Hardware Technical Reference Manual."
 *
 * Note that the number of cylinders is not limited to a
 * ten-bit quantity in this structure, so it is possible to
 * have num_cyl > 1024 (even though MS-LOSS can't handle it).
 ****************************************************************/

typedef unsigned short word;	/* a 16-bit quantity */
typedef unsigned char byte;	/* an 8-bit quantity */

struct BIOSHDParms {
  word	num_cyl;	/* NUMber of CYLinders (arm positions) */
  byte	heads;		/* tracks per cylinder */
  word	xt3;		/* PC/XT (unknown use) */
  word	wpc_cyl;	/* Write PreCompensation CYL */
  byte	xt7;		/* PC/XT (unknown use) */
  byte	control;	/* control bits for retry (ignore) */
  byte  xt9, xtA, xtB;	/* PC/XT (unknown use) */
  word	off_cyl;	/* power-OFF Landing CYLinder */
  byte	sectors;	/* sectors per track */
  byte	reserved;	/* IBM says "for future use" */
}
-- 
Gordon W. Ross (E025)	ARPA:  gwr@linus.mitre.org
The MITRE Corporation	UUCP:  {decvax,philabs}!linus!gwr
Burlington Road, Bedford, MA 01730	(617) 271-3205