Master Boot Record, with application to ETH Oberon

Summary

The next level of boot complexity is that of booting from hard disk via its MBR. Every hard disk contains an MBR but the BIOS is usually able to access only the MBR of the first hard disk (the disk with device number 80H), whilst access to the further MBRs can eventually be controlled with the machine System Setup, depending on its capabilities.

Whichever operating system is used, the MBR which comes with it is a central information repository. The Boot Loader code it contains differs with each operating system and therefore its functionality varies accordingly. The order in which multiple operating systems are installed also influences the MBR content. The Boot Loader code is also replaced by a functionally different one when a boot manager is installed. Hence, an Oberon user will in any case have to deal with and have to master all the matter around the MBR, a vast subject outside of the ETH Oberon concern. A small amount of basic information is provided here, only just enough to establish a bridge to the relevant Oberon material. More information on the subject can be found, for example, in The Boot Process, in particular in the article "Master Boot Record Tutorial".

BIOS activity during the boot sequence - follow-up part

The initial part of the description found in x86-based PC bootstrapping still applies. However, if INT 19 tries to read the MBR from the first sector on the first hard disk. If an MBR is found it is read into memory at location 0000:7C00 and the BIOS checks that the last two bytes of the sector are "AA 55". If that is correct, the routine jumps to memory location 0000:7C00. The small program in the MBR will attempt to locate a primary, active and bootable partition in its partition table from which an operating system can be started. If an Oberon partition is active, Oberon is started with the ETH Oberon boot loader in a manner similar to what happens with a boot diskette. As another example, when DOS is installed, the first record in the partition contains a DOS boot sector. If the DOS partition is active, the start code will load IO.SYS, which in turn loads DOS, and so on ...

When a boot manager is installed, the genuine MBR of a particular OS is replaced and the notion of "active" partition becomes irrelevant. Instead, any partition, primary partition or logical drive in an extended partition can be booted. Some boot managers can also be cascaded.

ETH Oberon and the MBR

ETH Oberon does not supply an MBR. It expects that the MBR, provided by third-party software, exists prior to the installation of Oberon. Attempting to install Oberon on an empty/blank first hard disk will unavoidably call for the error message "No boot sector on hard disk". Because the MBR plays such an important role, it is necessary to understand the boot mechanism part which precedes the start of Oberon in a partition on a mass storage device.

MS-DOS MBR

A DOS MBR stores the partition table and 446 bytes of Boot Loader code playing a central role when a machine is started/booted.

An MBR is located in the first sector (Cylinder 0, Head 0, Sector 1) of a hard disk and it has the structure:

Offset Bytes  Meaning 
in hex.
0000    446   Boot Loader code 
01BE     16   1. partition entry 
01CE     16   2. partition entry 
01DE     16   3. partition entry 
01EE     16   4. partition entry 
01FE      2   Signature (55H AAH)
The structure of a partition entry is:
Offset Bytes  Meaning 
in hex.
00        1   80H = active partition / 00H = not active 
01        1   begin of partition (head number) 
02        1   begin of partition (sector number) 
03        1   begin of partition (cylinder number) 
04        1   partition type - 76 = 4CH for AosFS and NatFS 
05        1   end of partition (head number) 
06        1   end of partition (sector number) 
07        1   end of partition (cylinder number) 
08        4   rel. sectors (# of sectors to partition beginning) 
0C        4   number of sectors in partition
A good description of the boot loader mechanism is found in "How it works: Master Boot Record" by Hale Landis - landis at sugs.tware.com

Replacing an MBR

Replacing an MBR is a considerable system modification which can lead to less considerable damages to all the OSes installed. Make sure beforehand that the OSes can still be started using boot diskettes prepared in advance and put aside for this eventuality. Keep also in mind that, whenever a partition is added, removed or resized on the disk, the partition table is modified and any earlier backup MBR becomes totally invalid and is a potential threat!

How to write or replace an MBR

Caution is recommended: replacing an MBR is a dangerous operation which can lead to a massive loss of data.

The MBR is created in many ways:

The list is not exhaustive and the reader is invited to consult the documentation supplied with the operating system, commercial or shareware software used.

ETH Oberon offers the facilities required to manipulate or inspect an MBR by way of a number of commands described in Introduction to ETH Oberon / PC Native alpha and beta release:

An interesting side effect is that ETH Oberon can be used to repair any MBR, including an MBR of a foreign OS, without the need to purchase further utility software.

Disaster recovery

If your machine does not boot anymore as it used to, it could be that the MBR has been damaged or overwritten. It is extremely unlikely that the data in the partitions has been affected. Bypassing the boot loader is a way to recover from the situation. Use a boot diskette, created during the installation, to get get you back into Oberon. If none is available, use the Oberon-0 diskette as universal rescue diskette.

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