Removed remaining questions about MIPS Big Endian. Now rounding up the count
of cylinders. Described MBR DOS-style partition table format.
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@ -251,6 +251,7 @@ Byte Range | Value | Meaning
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Sources:
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http://en.wikipedia.org/wiki/Master_boot_record
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Mailing list conversations with H. Peter Anvin and Vladimir Serbinenko.
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The candidates for MBR booting will normally use El Torito rather than MBR
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@ -259,9 +260,86 @@ The eventual MBR comes into effect it the image is on a media that is
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interpreted by the BIOS as some kind of hard disk. Usually real hard disks,
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floppy disks, USB sticks, memory cards.
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>>>
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An important part of an MBR is the DOS style partition table. It describes up
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to four primary partitions. There are two formats used for block address:
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Cylinder/Head/Sector (C/H/S) and Logical Block Address (LBA). Both are based
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on units of 512 bytes. So MBR_LBA = ISO_LBA * 4.
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>>> iso_write_opts_set_part_offset()
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For C/H/S, the sector address is broken up into whole cylinders, remaining
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heads, and remaining sectors. The nomenclature seems to stem from antique
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drum storage.
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C/H/S counting starts with 0/0/1, not 0/0/0.
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There are two parameters, sectors_per_head and heads_per_cylinder which are not
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stored in the MBR. So it is more or less arbitray how to convert a LBA into
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a C/H/S address and vice versa. For maximum range of C/H/S addresses one
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may use sectors_per_head = 63 , heads_per_cylinder = 255.
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Words are composed little-endian style.
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Byte Range | Value | Meaning
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---------- | ---------- | ----------------------------------------------------
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0 - 439 | = opaque = | Code Area filled with bytes for some boot system,
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| | typically machine code.
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| |
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440 - 443 | disk_sgntr | Disc signature, an individual disk id of obscure
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| | usability.
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| | (The Code Area might extend up to this field.)
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| |
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444 - 445 | 0 | "usually nulls"
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| | (The Code Area might extend up to this field.)
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446 - 461 | ========== | Partition Table Entry for partition 1
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| |
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446 - 446 | status | Governs bootability:
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| | 0x80 = bootable/active , 0x00 non-bootable/inactive
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447 - 449 | ========== | C/H/S address of partition start
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447 - 447 | start_head | Heads part of start address.
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448 - 448 | start_c_s | Bits 0 to 5 : Sectors part of start address.
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| | Bits 6 to 7 : Bits 8 to 9 of cylinders part.
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449 - 449 | start_cyl | Lower 8 bits of cylinders part of start address
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| |
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450 - 450 | part_type | Partition type indicates the purpose or kind of
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| | filesystem in the partition.
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| |
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451 - 453 | ========== | C/H/S address of last absolute sector in partition
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451 - 451 | end_head | Heads part of end address.
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452 - 452 | end_c_s | Bits 0 to 5 : Sectors part of end address.
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| | Bits 6 to 7 : Bits 8 to 9 of cylinders part.
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453 - 453 | end_cyl | Lower 8 bits of cylinders part of end address
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| |
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454 - 457 | start_lba | LBA of first absolute sector in partiton.
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| | Block size is 512. Counting starts at 0.
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458 - 461 | num_blocks | Number of sectors in partition.
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462 - 477 | ========== | Partition Table Entry for partition 2
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| part_entr2 | 16 bytes. Format as with partition 1.
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| | All 0 means that partition is unused/undefined.
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| |
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478 - 493 | ========== | Partition Table Entry for partition 3
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| part_entr3 | 16 bytes. See above.
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| |
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494 - 509 | ========== | Partition Table Entry for partition 4
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| part_entr4 | 16 bytes. See above.
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| |
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510 - 510 | 0x55 | MBR signature
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511 - 511 | 0xaa | MBR signature
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---------- | ---------- | ----------------------------------------------------
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By tradition the MBR itself and possibly more blocks are not claimed by any
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partition. But starting the first partition at a non-zero block address causes
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on Linux a partition device file (e.g. /dev/sdb1) which cannot be used to mount
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the ISO filesystem.
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libisofs is able to produce a second set of trees and meta data which is
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suitable for being mounted at start block 16 (ISO) resp. 64 (MBR).
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See <libisofs/libisofs.h> for call iso_write_opts_set_part_offset()
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and http://libburnia-project.org/wiki/PartitionOffset for examples with
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program xorriso.
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------------------------------------------------------------------------------
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@ -274,7 +352,7 @@ Sources:
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Mailing list conversations with hpa.
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An isohybrid MBR directs the booting BIOS to an Isolinux boot image which
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An isohybrid MBR directs the booting BIOS to an ISOLINUX boot image which
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is also the target of an El Torito boot catalog entry.
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For that purpose one has to take an MBR template and has to set a few bytes
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to values which sufficiently describe the ISO image and the boot image file.
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@ -284,14 +362,18 @@ Words are composed little-endian style.
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Byte Range | Value | Meaning
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---------- | ---------- | ----------------------------------------------------
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0 - 431 | = opaque = | Syslinux machine code provided by MBR template
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432 - 439 | hd_bootlba | Address of the ISOLINUX boot image file in the
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| | ISO image. Counted in 512 byte blocks.
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440 - 443 | mbr_id | Random number
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444 - 445 | 0 | Padding
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446 - 509 | ========== | Partition table
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446 - 461 | part_entry | Partition table entry 1 describing ISO image size
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| | starting at LBA 0. I.e. contrary to tradition.
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| | See above for partition table entry format.
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| |
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462 - 509 | 0 | Unused partition entries 2 to 4
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510 - 511 | 0xaa55 | MBR signature
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@ -331,6 +413,7 @@ Byte Range | Value | Meaning
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446 - 461 | part_entry | Partition table entry 1 describing ISO image size
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| | Peculiar is the start offset of 1 block.
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| | This prevents mounting of the partition.
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| | See above for partition table entry format.
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462 - 509 | 0 | Unused partition entries 2 to 4
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510 - 511 | 0xaa55 | MBR signature
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@ -277,8 +277,7 @@ static int make_mips_volume_header(Ecma119Image *t, uint8_t *buf, int flag)
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iso_msb(buf, 0x0be5a941, 4);
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/* 28 - 29 | num_cyl_l | Number of usable cylinder, lower two bytes */
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/* >>> Shall i rather orund up ? */
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num_cyl = image_size / (bps * spt);
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num_cyl = (image_size + (bps * spt) - 1) / (bps * spt);
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iso_msb(buf + 28, num_cyl & 0xffff, 2);
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/* 32 - 33 | 1 | Number of tracks per cylinder */
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@ -361,7 +360,7 @@ static int make_mips_volume_header(Ecma119Image *t, uint8_t *buf, int flag)
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stream = iso_file_get_stream((IsoFile *) node);
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file_size = iso_stream_get_size(stream);
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/* >>> shall i really round up to 2048 ? */
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/* Shall i really round up to 2048 ? Steve says yes.*/
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iso_msb(buf + (72 + 16 * idx) + 12,
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((file_size + 2047) / 2048 ) * 2048, 4);
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