svn commit: r194241 - head/lib/libdisk
ulf.lilleengen at gmail.com
Tue Jun 16 06:33:28 UTC 2009
On Tuesday 16 June 2009 03:12:58 Bruce Evans wrote:
> On Mon, 15 Jun 2009, Ulf Lilleengen wrote:
> > Marcel Moolenaar wrote:
> >> On Jun 15, 2009, at 9:18 AM, Ulf Lilleengen wrote:
> >>> Author: lulf
> >>> Date: Mon Jun 15 16:18:24 2009
> >>> New Revision: 194241
> >>> URL: http://svn.freebsd.org/changeset/base/194241
> >>> Log:
> >>> - Relax sanitazion requirements in libdisk, as a previous commit
> >>> enabling this
> >>> sanitization broke sysinstall on some disks. This was due to the
> >>> disks
> >>> reporting a geometry that was incorrectly sanitized by sysinstall.
> >>> This makes
> >>> the sanitization consistent with fdisk.
> >> *snip*
> >>> + if (disk->bios_hd > 256)
> >>> sane = 0;
> >> The number of heads cannot exceed 255. There are only 8-bits
> >> in which the number of heads is stored. This also applies to
> >> PC98, BTW.
> No, the number of heads can be 256 except in certain broken APIs and
> BIOSes. The broken APIs don't include fdisk tables, since the maximum
> number of heads isn't stored. Only the maximum head number is stored.
> Head numbers are 0-based, so their maximum is 1 less than the number
> of heads. The maximum head number of 255 fits in the 8 bits in an
> fdisk table.
> Most of the original int 0x13 BIOS interfaces are similar. E.g., AH = 8
> (read drive parameters) returns the maximum for each CHS value. C and
> H values are 0-based, so their max is one less than their number.
> S values are bogusly 1-based, so their max is the same as their total.
> AH = 2 (read sectors) and AH = 3 (write sectors) naturally pass the
> (current) head number (in an 8-bit register), so they never need to
> represent 256 in the 8-bit register.
> The only broken API that I remember the int 0x41 and 0x46
> pseudo-interrupts. These may be only de-facto (I can't find them in the
> Phoenix BIOS book copyright 1987-1991), but before about 1990 the int 0x13
> AH=8 interface was often not implemented and int 0x41 and int 0x46 worked
> better. The IDT entry for ints 0x41 and 0x46 contains a pointer to a table
> with the following data (0x41 for hard disk 0 and 0x46 for hard disk 1):
> from the header file for Minix "wini" disk drivers:
> /* BIOS table layout. */
> typedef char wn_biosdrive;
> #define WN_CYLINDERS(t) (* (unsigned short *) (&(t)[0x0]))
> #define WN_HEADS(t) (* (unsigned char *) (&(t)[0x2]))
> #define WN_PRECOMP(t) (* (unsigned short *) (&(t)[0x5]))
> #define WN_CTLBYTE(t) (* (unsigned char *) (&(t)[0x8]))
> #define WN_LANDINGZONE(t) (* (unsigned short *) (&(t)[0xc]))
> #define WN_SECTORS(t) (* (unsigned char *) (&(t)[0xe]))
> Many or most 1990's BIOSes present precisely this table for you to edit
> in their setup. They present many possibilities for obselete (even in
> 1990's) disks from arrays of this table in ROM, and at least 1 modifiable
> table per disk in CMOS.
> Since the head number is for the number of heads, and the table entry
> for it has type unsigned char, 256 heads does't fit, it becomes 0.
> Non-broken software would unwrap 0 back to 0 256 and just work, but
> having 256 (virtual) heads is not very useful so configuring it
> should be avoided.
> Apparently, some newer BIOSes know that they are non-broken here (or
> maybe for cylinder numbers), and default to larger numbers. Such
> defaults should still be avoided in case you want to move the disk
> to a machine with a BIOS that doesn't support them.
> I once tried to standardize on the maximum possible number of heads
> (256), but had to switch to 255 when 256 failed on only some machines.
> I still use 255 on all machines. With 2000's BIOSes this requires
> ignoring BIOS defaults and/or selecting so-called LBA mode in BIOS
> configurations. FreeBSD doesn't know the BIOS defaults anyway; it
> gives different defaults (except in my version) which I ignore.
> Many 2000's BIOSes produce defaults for the number of sectors that can't
> possibly work in fdisk tables since they significantly exceed 63.
> Selecting LBA mode always avoids this for me (LBA CHS = xxx/255/63).
> I don't know how misconfigurations from this are usually avoided.
> > Aha, so I should fix fdisk too then. Thanks.
> Please don't break fdisk too. There must be a way to set valid but
> dangerous values, or theoretically invalid but practically working
> values, if they are representable and are needed for compatibility.
I'm a bit confused now :)
Fdisk allowed 256 as a valid number of heads. Are you saying it should still
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