Fill the Low-level I/O section in grub.texi
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docs/grub.texi
522
docs/grub.texi
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@ -125,6 +125,7 @@ Implementation details
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* Low-level disk I/O:: INT 13H disk I/O interrupts.
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* MBR:: The structure of Master Boot Record.
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* Partition table:: The format of partition table.
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* Filesystem interface:: The generic interface for the fs code.
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@end detailmenu
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@end menu
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@ -1090,6 +1091,7 @@ not satisfied with this documentation.
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* Low-level disk I/O:: INT 13H disk I/O interrupts.
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* MBR:: The structure of Master Boot Record.
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* Partition table:: The format of partition table.
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* Filesystem interface:: The generic interface for the fs code.
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@end menu
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@ -1551,7 +1553,7 @@ E820h, it is present in many more systems.
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Input:
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@multitable @columnfractions 0.15 0.25 0.6
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@item @code{AX} @tab Function Code @tab E801h
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@item @code{AX} @tab Function Code @tab E801h.
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@end multitable
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Output:
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@ -1613,7 +1615,517 @@ MB.
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@node Low-level disk I/O
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@section INT 13H disk I/O interrupts
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PC_partitioning.txt
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In the PC world, living with the BIOS disk interface is definitely a
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nightmare. This section documents how awful the chaos is and how GRUB
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deals with the BIOS disks.
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@menu
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* CHS Translation:: CHS addressing and LBA addressing.
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* CHS mode disk I/O:: INT 13H, AH=0xh interrupt call.
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* LBA mode disk I/O:: INT 13H, AH=4xh interrupt call.
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@end menu
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@node CHS Translation
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@subsection CHS addressing and LBA addressing
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CHS --- Cylinder/Head/Sector --- is the traditional way to address
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sectors on a disk. There are at least two types of CHS addressing; the
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CHS that is used at the INT 13H interface and the CHS that is used at
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the ATA device interface. In the MFM/RLL/ESDI and early ATA days the CHS
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used at the INT 13H interface was the same as the CHS used at the device
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interface.
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Today we have CHS translating BIOS types that can use one CHS at the INT
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13H interface and a different CHS at the device interface. These two
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types of CHS will be called the logical CHS or @dfn{L-CHS} and the
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physical CHS or @dfn{P-CHS} in this section. L-CHS is the CHS used at
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the INT 13H interface and P-CHS is the CHS used at the device interface.
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The L-CHS used at the INT 13 interface allows up to 256 heads, up to
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1024 cylinders and up to 63 sectors. This allows support of up to 8GB
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drives. This scheme started with either ESDI or SCSI adapters many years
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ago.
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The P-CHS used at the device interface allows up to 16 heads up to 65535
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cylinders, and up to 63 sectors. This allows access to 2^28 sectors
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(136GB) on an ATA device. When a P-CHS is used at the INT 13H interface
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it is limited to 1024 cylinders, 16 heads and 63 sectors. This is where
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the old 528MB limit originated.
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LBA --- Logical Block Address --- is another way of addressing sectors
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that uses a simple numbering scheme starting with zero as the address of
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the first sector on a device. The ATA standard requires that cylinder 0,
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head 0, sector 1 address the same sector as addressed by LBA 0. LBA
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addressing can be used at the ATA interface if the ATA device supports
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it. LBA addressing is also used at the INT 13H interface by the AH=4xH
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read/write calls.
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ATA devices may also support LBA at the device interface. LBA allows
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access to approximately 2^28 sectors (137GB) on an ATA device.
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A SCSI host adapter can convert a L-CHS directly to an LBA used in the
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SCSI read/write commands. On a PC today, SCSI is also limited to 8GB
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when CHS addressing is used at the INT 13H interface.
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First, all OS's that want to be co-resident with another OS (and that is
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all of the PC based OS's that I know of) @emph{must} use INT 13H to
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determine the capacity of a hard disk. And that capacity information
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@emph{must} be determined in L-CHS mode. Why is this? Because:
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@enumerate
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@item
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FDISK and the partition tables are really L-CHS based.
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@item
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MS/PC DOS uses INT 13H AH=02H and AH=03H to read and write the disk and
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these BIOS calls are L-CHS based.
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@item
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The boot processing done by the BIOS is all L-CHS based.
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@end enumerate
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During the boot processing, all of the disk read accesses are done in
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L-CHS mode via INT 13H and this includes loading the first of the OS's
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kernel code or boot manager's code.
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Second, because there can be multiple BIOS types in any one system, each
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drive may be under the control of a different type of BIOS. For example,
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drive 80H (the first hard drive) could be controlled by the original
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system BIOS, drive 81H (the second drive) could be controlled by a
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option @sc{rom} BIOS and drive 82H (the third drive) could be controlled
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by a software driver. Also, be aware that each drive could be a
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different type, for example, drive 80H could be an MFM drive, drive 81H
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could be an ATA drive, drive 82H could be a SCSI drive.
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Third, not all OS's understand or use BIOS drive numbers greater than
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81H. Even if there is INT 13H support for drives 82H or greater, the OS
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may not use that support.
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Fourth, the BIOS INT 13H configuration calls are:
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@table @asis
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@item AH=08H, Get Drive Parameters
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This call is restricted to drives up to 528MB without CHS translation
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and to drives up to 8GB with CHS translation. For older BIOS with no
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support for >1024 cylinders or >528MB, this call returns the same CHS as
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is used at the ATA interface (the P-CHS). For newer BIOS's that do
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support >1024 cylinders or >528MB, this call returns a translated CHS
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(the L-CHS). The CHS returned by this call is used by FDISK to build
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partition records.
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@item AH=41H, Get BIOS Extensions Support
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This call is used to determine if the IBM/Microsoft Extensions or if the
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Phoenix Enhanced INT 13H calls are supported for the BIOS drive number.
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@item AH=48H, Extended Get Drive Parameters
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This call is used to determine the CHS geometries, LBA information and
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other data about the BIOS drive number.
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@end table
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An ATA disk must implement both CHS and LBA addressing and must at any
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given time support only one P-CHS at the device interface. And, the
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drive must maintain a strick relationship between the sector addressing
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in CHS mode and LBA mode. Quoting @cite{the ATA-2 document}:
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@example
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@group
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LBA = ( (cylinder * heads_per_cylinder + heads )
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* sectors_per_track ) + sector - 1
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where heads_per_cylinder and sectors_per_track are the current
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translation mode values.
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@end group
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@end example
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This algorithm can also be used by a BIOS or an OS to convert a L-CHS to
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an LBA.
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This algorithm can be reversed such that an LBA can be converted to a
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CHS:
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@example
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@group
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cylinder = LBA / (heads_per_cylinder * sectors_per_track)
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temp = LBA % (heads_per_cylinder * sectors_per_track)
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head = temp / sectors_per_track
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sector = temp % sectors_per_track + 1
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@end group
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@end example
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While most OS's compute disk addresses in an LBA scheme, an OS like DOS
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must convert that LBA to a CHS in order to call INT 13H.
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The basic problem is that there is no requirement that a CHS translating
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BIOS followed these rules. There are many other algorithms that can be
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implemented to perform a similar function. Today, there are at least two
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popular implementions: the Phoenix implementation (described above) and
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the non-Phoenix implementations. Because a protected mode OS that does
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not want to use INT 13H must implement the same CHS translation
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algorithm. If it doesn't, your data gets scrambled.
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In the perfect world of tomorrow, maybe only LBA will be used. But today
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we are faced with the following problems:
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@itemize @bullet
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@item
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Some drives >528MB don't implement LBA.
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@item
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Some drives are optimized for CHS and may have lower performance when
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given commands in LBA mode. Don't forget that LBA is something new for
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the ATA disk designers who have worked very hard for many years to
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optimize CHS address handling. And not all drive designs require the use
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of LBA internally.
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@item
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The L-CHS to LBA conversion is more complex and slower than the bit
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shifting L-CHS to P-CHS conversion.
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@item
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DOS, FDISK and the MBR are still CHS based --- they use the CHS returned
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by INT 13H AH=08H. Any OS that can be installed on the same disk with
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DOS must understand CHS addressing.
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@item
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The BIOS boot processing and loading of the first OS kernel code is done
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in CHS mode --- the CHS returned by INT 13H AH=08H is used.
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@item
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Microsoft has said that their OS's will not use any disk capacity that
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can not also be accessed by INT 13H AH=0xH.
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@end itemize
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These are difficult problems to overcome in today's industry
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environment. The result: chaos.
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@node CHS mode disk I/O
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@subsection INT 13H, AH=0xh interrupt call
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Real mode only. These functions are the traditional CHS mode disk
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interface. GRUB calls them only if LBA mode is not available.
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INT 13H, AH=02h reads sectors into memory.
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Input:
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@multitable @columnfractions .15 .85
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@item @code{AH} @tab 02h
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@item @code{AL} @tab The number of sectors to read (must be non-zero).
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@item @code{CH} @tab Low 8 bits of cylinder number.
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@item @code{CL} @tab Sector number in bits 0-5, and high 2 bits of
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cylinder number in bits 6-7.
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@item @code{DH} @tab Head number.
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@item @code{DL} @tab Drive number (bit 7 set for hard disk).
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@item @code{ES:BX} @tab Data buffer.
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@end multitable
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Output:
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@multitable @columnfractions .15 .85
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@item @code{CF} @tab Set on error.
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@item @code{AH} @tab Status.
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@item @code{AL} @tab The number of sectors transferred (only valid if CF
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set for some BIOSes).
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@end multitable
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INT 13H, AH=03h writes disk sectors.
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Input:
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@multitable @columnfractions .15 .85
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@item @code{AH} @tab 03h
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@item @code{AL} @tab The number of sectors to write (must be non-zero).
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@item @code{CH} @tab Low 8 bits of cylinder number.
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@item @code{CL} @tab Sector number in bits 0-5, and high 2 bits of
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cylinder number in bits 6-7.
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@item @code{DH} @tab Head number.
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@item @code{DL} @tab Drive number (bit 7 set for hard disk).
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@item @code{ES:BX} @tab Data buffer.
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@end multitable
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Output:
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@multitable @columnfractions .15 .85
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@item @code{CF} @tab Set on error.
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@item @code{AH} @tab Status.
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@item @code{AL} @tab The number of sectors transferred (only valid if CF
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set for some BIOSes).
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@end multitable
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INT 13H, AH=08h returns drive parameters. For systems predating the IBM
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PC/AT, this call is only valid for hard disks.
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Input:
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@multitable @columnfractions .15 .85
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@item @code{AH} @tab 08h
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@item @code{DL} @tab Drive number (bit 7 set for hard disk).
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@end multitable
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Output:
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@multitable @columnfractions .15 .85
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@item @code{CF} @tab Set on error.
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@item @code{AH} @tab 0.
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@item @code{AL} @tab 0 on at least some BIOSes.
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@item @code{BL} @tab Drive type (AT/PS2 floppies only).
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@item @code{CH} @tab Low 8 bits of maximum cylinder number.
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@item @code{CL} @tab Maximum sector number in bits 0-5, and high 2 bits
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of maximum cylinder number in bits 6-7.
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@item @code{DH} @tab Maxiumum head number.
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@item @code{DL} @tab The number of drives.
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@item @code{ES:DI} @tab Drive parameter table (floppies only).
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@end multitable
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GRUB does not use this call for floppies, but attempts to read the first
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sector from the last head of the last cylinder to determine the maximum
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head number and the maximum cylinder number.
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@node LBA mode disk I/O
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@subsection INT 13H, AH=4xh interrupt call
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Real mode only. These functions are IBM/MS INT 13 Extensions to support
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LBA mode. GRUB uses them if available so that it can read/write over 8GB
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area.
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INT 13, AH=41h checks if LBA is supported.
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Input:
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@multitable @columnfractions 0.15 0.85
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@item @code{AH} @tab 41h.
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@item @code{BX} @tab 55AAh.
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@item @code{DL} @tab Drive number.
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@end multitable
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Output:
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@multitable @columnfractions 0.15 0.85
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@item @code{CF} @tab Set on error.
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@item @code{AH} @tab Major version of extensions (01h for 1.x, 20h for
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2.0 / EDD-1.0, 21h for 2.1 / EDD-1.1 and 30h for EDD-3.0) if successful,
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otherwise 01h (the error code of @dfn{invalid function}).
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@item @code{BX} @tab AA55h if installed.
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@item @code{AL} @tab Internal use.
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@item @code{CX} @tab API subset support bitmap (see below).
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@item @code{DH} @tab Extension version.
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@end multitable
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The bitfields for the API subset support bitmap are:
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@multitable @columnfractions 0.15 0.85
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@item Bit(s) @tab Description
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@item 0 @tab Extended disk access functions (AH=42h-44h, 47h, 48h)
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supported.
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@item 1 @tab Removable drive controller functions (AH=45h, 46h, 48h,
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49h, INT 15H, AH=52h) supported.
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@item 2 @tab Enhanced disk drive (EDD) functions (AH=48h, 4Eh)
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supported.
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@item 3-15 @tab Reserved (0).
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@end multitable
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INT 13, AH=42h reads sectors into memory.
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Input:
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@multitable @columnfractions .15 .85
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@item @code{AH} @tab 42h.
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@item @code{DL} @tab Drive number.
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@item @code{DS:SI} @tab Disk Address Packet (see below).
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@end multitable
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Output:
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@multitable @columnfractions .15 .85
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@item @code{CF} @tab Set on error.
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@item @code{AH} @tab 0 if successful, otherwise error code.
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@end multitable
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The format of @dfn{Disk Address Packet} is:
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@multitable @columnfractions 0.15 0.15 0.7
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@item Offset (hex) @tab Size (byte) @tab Description
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@item 00 @tab 1 @tab 10h (The size of packet).
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@item 01 @tab 1 @tab Reserved (0).
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@item 02 @tab 2 @tab The number of blocks to transfer (max 007F for
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Phoenix EDD).
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@item 04 @tab 4 @tab Transfer buffer (SEGMENT:OFFSET).
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@item 08 @tab 8 @tab Starting absolute block number.
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@end multitable
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INT 13, AH=43h writes disk sectors.
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Input:
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@multitable @columnfractions 0.15 0.85
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@item @code{AH} @tab 43h.
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@item @code{AL} @tab Write flags (In version 1.0 and 2.0, bit 0 is the
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flag for @dfn{verify write} and other bits are reserved (0). In version
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2.1, 00h and 01h indicates @dfn{write without verify}, and 02h indicates
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@dfn{write with verify}.
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@item @code{DL} @tab Drive number.
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@item @code{DS:SI} @tab Disk Address Packet (see above).
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@end multitable
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Output:
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@multitable @columnfractions 0.15 0.85
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@item @code{CF} @tab Set on error.
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@item @code{AH} @tab 0 if successful, otherwise error code.
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@end multitable
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INT 13, AH=48h returns drive parameters. GRUB only makes use of the
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total number of sectors, and ignore the CHS information, because only
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L-CHS makes sense. @xref{CHS Translation}, for more information.
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Input:
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@multitable @columnfractions 0.15 0.85
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@item @code{AH} @tab 48h.
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@item @code{DL} @tab Drive number.
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@item @code{DS:SI} @tab Buffer for drive parameters (see below).
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@end multitable
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Output:
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@multitable @columnfractions 0.15 0.85
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@item @code{CF} @tab Set on error.
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@item @code{AH} @tab 0 if successful, otherwise error code.
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@end multitable
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The format of drive parameters is:
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@multitable @columnfractions 0.25 0.15 0.6
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@item Offset (hex) @tab Size (byte) @tab Description
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@item 00 @tab 2 @tab The size of buffer. Before calling this function,
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set to the maximum buffer size, at least 1Ah. The size actually filled
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is returned (1Ah for version 1.0, 1Eh for 2.x and 42h for 3.0).
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@item 02 @tab 2 @tab Information flags (see below).
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|
||||
@item 04 @tab 4 @tab The number of physical cylinders.
|
||||
|
||||
@item 08 @tab 4 @tab The number of physical heads.
|
||||
|
||||
@item 0C @tab 4 @tab The number of physical sectors per track.
|
||||
|
||||
@item 10 @tab 8 @tab The total number of sectors.
|
||||
|
||||
@item 18 @tab 2 @tab The bytes per sector.
|
||||
|
||||
@comment Add an empty row for readability...
|
||||
@item @tab @tab
|
||||
|
||||
@item @strong{v2.0 and later} @tab @tab
|
||||
|
||||
@item 1A @tab 4 @tab EDD configuration parameters.
|
||||
|
||||
@comment Add an empty row for readability...
|
||||
@item @tab @tab
|
||||
|
||||
@item @strong{v3.0} @tab @tab
|
||||
|
||||
@item 1E @tab 2 @tab Signature BEDD to indicate presence of Device Path
|
||||
information.
|
||||
|
||||
@item 20 @tab 1 @tab The length of Device Path information, including
|
||||
signature and this byte (24h for version 3.0).
|
||||
|
||||
@item 21 @tab 3 @tab Reserved (0).
|
||||
|
||||
@item 24 @tab 4 @tab ASCIZ name of host bus (@samp{ISA} or @samp{PCI}).
|
||||
|
||||
@item 28 @tab 8 @tab ASCIZ name of interface type (@samp{ATA},
|
||||
@samp{ATAPI}, @samp{SCSI}, @samp{USB}, @samp{1394} or @samp{FIBRE}).
|
||||
|
||||
@item 30 @tab 8 @tab Interface Path.
|
||||
|
||||
@item 38 @tab 8 @tab Device Path.
|
||||
|
||||
@item 40 @tab 1 @tab Reserved (0).
|
||||
|
||||
@item 41 @tab 1 @tab Checksum of bytes 1Eh-40h (2's complement of sum,
|
||||
which makes the 8 bit sum of bytes 1Eh-41h equal to 00h).
|
||||
@end multitable
|
||||
|
||||
The information flags are:
|
||||
|
||||
@multitable @columnfractions 0.15 0.85
|
||||
@item Bit(s) @tab Description
|
||||
|
||||
@item 0 @tab DMA boundary errors handles transparently.
|
||||
|
||||
@item 1 @tab CHS information is valid.
|
||||
|
||||
@item 2 @tab Removable drive.
|
||||
|
||||
@item 3 @tab Write with verify supported.
|
||||
|
||||
@item 4 @tab Drive has change-line support (required if drive is
|
||||
removable).
|
||||
|
||||
@item 5 @tab Drive can be locked (required if drive is removable).
|
||||
|
||||
@item 6 @tab CHS information set to maximum supported values, not
|
||||
current media.
|
||||
|
||||
@item 7-15 @tab Reserved (0).
|
||||
@end multitable
|
||||
|
||||
|
||||
@node MBR
|
||||
|
|
@ -1628,6 +2140,12 @@ PC_partitioning.txt
|
|||
PC_partitioning.txt
|
||||
|
||||
|
||||
@node Filesystem interface
|
||||
@section The generic interface for the fs code
|
||||
|
||||
filesystem.txt
|
||||
|
||||
|
||||
@node Index
|
||||
@unnumbered Index
|
||||
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue