140 lines
5.9 KiB
HTML
140 lines
5.9 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>Large Disk HOWTO: Disk Access</TITLE>
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<LINK HREF="Large-Disk-HOWTO-4.html" REL=next>
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<LINK HREF="Large-Disk-HOWTO-2.html" REL=previous>
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<LINK HREF="Large-Disk-HOWTO.html#toc3" REL=contents>
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<BODY>
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<A HREF="Large-Disk-HOWTO-4.html">Next</A>
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<A HREF="Large-Disk-HOWTO-2.html">Previous</A>
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<A HREF="Large-Disk-HOWTO.html#toc3">Contents</A>
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<HR>
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<H2><A NAME="s3">3. Disk Access</A></H2>
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<P>Disk access is done in units called <I>sectors</I>.
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In order to read or write something from or to the disk, we have
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to specify the position on the disk, for example by giving the
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sector number.
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If the disk is a SCSI disk, then this sector number goes directly
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into the SCSI command and is understood by the disk.
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If the disk is an IDE disk using LBA, then precisely the same holds.
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But if the disk is old, RLL or MFM or IDE from before the LBA times,
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then the disk hardware expects a triple (cylinder,head,sector) to
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designate the desired spot on the disk.
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<P>
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<H2><A NAME="ss3.1">3.1 Cylinders, heads and sectors</A>
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</H2>
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<P>A disk has sectors numbered 0, 1, 2, ...
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This is called <I>LBA addressing</I>.
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<P>In ancient times, before the advent of IDE disks,
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disks had a <I>geometry</I> described by three constants
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C, H, S: the number of cylinders, the number of heads,
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the number of sectors per track.
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The address of a sector was given by three numbers:
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<I>c</I>, <I>h</I>, <I>s</I>: the cylinder number
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(between 0 and C-1), the head number (between 0 and H-1),
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and the sector number within the track (between 1 and S), where
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for some mysterious reason <I>c</I> and <I>h</I> count from 0,
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but <I>s</I> counts from 1. This is called <I>CHS addressing</I>.
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<P>No disk manufactured less than ten years ago has a geometry, but
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this ancient 3D sector addressing is still used by the INT13
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BIOS interface (with fantasy numbers C, H, S
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unrelated to any physical reality).
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<P>The correspondence between the linear numbering and this 3D notation
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is as follows: for a disk with C cylinders, H heads and S sectors/track
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position (<I>c</I>,<I>h</I>,<I>s</I>) in 3D or CHS notation
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is the same as position
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<I>c</I><CODE>*</CODE>H<CODE>*</CODE>S + <I>h</I><CODE>*</CODE>S + (<I>s</I>-1)
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in linear or LBA notation.
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<P>Consequently, in order to access a very old non-SCSI disk, we need to know
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its <EM>geometry</EM>, that is, the values of C, H and S.
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(And if you don't know, there is a lot of good information on
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<A HREF="http://www.thetechpage.com/cgi-bin/default.cgi">www.thetechpage.com</A>.)
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<P>
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<H2><A NAME="ss3.2">3.2 Sectorsize</A>
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</H2>
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<P>
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<!--
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disk!sectorsize
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-->
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In the present text a sector has 512 bytes. This is almost always
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true, but for example certain MO disks use a sectorsize of 2048 bytes,
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and all capacities given below must be multiplied by four.
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(When using <CODE>fdisk</CODE> on such disks, make sure you have version
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2.9i or later, and give the `-b 2048' option.)
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<P>
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<H2><A NAME="ss3.3">3.3 Disksize</A>
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</H2>
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<P>
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<!--
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disk!disksize
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-->
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A disk with C cylinders, H heads and S sectors per track
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has C<CODE>*</CODE>H<CODE>*</CODE>S sectors in all, and can store
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C<CODE>*</CODE>H<CODE>*</CODE>S<CODE>*</CODE>512 bytes.
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For example, if the disk label says C/H/S=4092/16/63
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then the disk has 4092<CODE>*</CODE>16<CODE>*</CODE>63=4124736 sectors, and can hold
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4124736<CODE>*</CODE>512=2111864832 bytes (2.11 GB).
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There is an industry convention to give C/H/S=16383/16/63
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for disks larger than 8.4 GB, and the disk size can no longer
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be read off from the C/H/S values reported by the disk.
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<P>
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<H2><A NAME="ss3.4">3.4 The 1024 cylinder and 8.5 GB limits</A>
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</H2>
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<P>The old INT13 BIOS interface to disk I/O uses 24 bits to address
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a sector: 10 bits for the cylinder, 8 bits for the head, and 6 bits
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for the sector number within the track (counting from 1).
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This means that this interface cannot address more than
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1024*256*63 sectors, which is 8.5 GB (with 512-byte sectors).
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And if the (fantasy) geometry specified for the disk has fewer
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than 1024 cylinders, or 256 heads, or 63 sectors per track,
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then this limit will be less.
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<P>(More precisely: with INT 13, AH selects the function to perform,
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CH is the low 8 bits of the cylinder number, CL
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has in bits 7-6 the high two bits of the cylinder number
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and in bits 5-0 the sector number, <CODE>DH</CODE> is the head number,
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and DL is the drive number (80h or 81h).
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This explains part of the layout of the partition table.)
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<P>This state of affairs was rectified when the so-called Extended INT13
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functions were introduced. A modern BIOS has no problems accessing
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large disks.
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<P>(More precisely: DS:SI points at a 16-byte Disk Address Packet
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that contains an 8-byte starting absolute block number.)
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<P>Linux does not use the BIOS, so does (and did) not have this problem.
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<P>However, this geometry stuff plays a role in the interpretation
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of partition tables, so if Linux shares a disk with for example DOS,
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then it needs to know what geometry DOS will think the disk has.
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It also plays a role at boot time, where the BIOS has to load
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a boot loader, and the boot loader has to load the operating system.
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<P>
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<H2><A NAME="ss3.5">3.5 The 137 GB limit</A>
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</H2>
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<P>The old ATA standard describes how to address a sector on an IDE disk
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using 28 bits (8 bits for the sector, 4 for the head, 16 for the cylinder).
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This means that an IDE disk can have at most 2^28 addressable sectors
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With 512-byte sectors this is 2^37 bytes, that is, 137.4 GB.
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<P>The ATA-6 standard includes a specification how to address
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past this 2^28 sector boundary. The new standard allows addressing
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of 2^48 sectors. There is support in recent Linux kernels that
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have incorporated Andre Hedrick's IDE patch, for example
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2.4.18-pre7-ac3 and 2.5.3.
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<P>Maxtor sells 160 GB IDE disks since Fall 2001.
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An old kernel will treat such disks as 137.4 GB disks.
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<P>
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<HR>
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<A HREF="Large-Disk-HOWTO-4.html">Next</A>
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<A HREF="Large-Disk-HOWTO-2.html">Previous</A>
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<A HREF="Large-Disk-HOWTO.html#toc3">Contents</A>
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</HTML>
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