The Starting Sector and Ending Sector fields are each six bits long, which limits the range of these fields to 0— However, the enumeration of sectors starts at 1 not 0, as for other fields , so the maximum number of sectors per track is Maximum disk capacity Because all hard disks are low-level formatted with a standard byte sector, the maximum disk capacity described by the partition table is calculated as follows:. The calculation results in a maximum capacity of slightly less than 8 gigabytes GB.
Before logical block addressing LBA were introduced, the active, primary partition could not exceed 7. Marketing Advertising Analytics Email. Contact [email protected] Privacy Policy Status. Data Storage and Manipulation Computer Architecture 14 pages. The master boot record MBR is the first sector of a hard disk which is always located at sector 1 of cylinder 0, head 0 The MBR is the most important data structure on the disk and is created when the disk is partitioned.
Any additional partitions must be logical partitions that are linked to or are part of one of the primary partitions. One of these partitions is marked as active, indicating that it is the one that the computer should used to continue the boot process. This code, when fully executed, transfers control to the boot program stored on the boot active partition to load the operating system. Obviously, due to the great importance of the information stored in the master boot record, should it become damaged or corrupted in some way, serious data loss often occurs.
The master boot code is the first program executed when you turn on your PC, and is often the target of a virus. In order to understand the Master Boot Record and the Master Boot Code buried within it, it might be helpful to understand the sequence of events that occur when your computer starts.
Although often overlooked as a diagnostic tool, the Boot Sequence performed by your computer can often tell you what may be wrong with the hardware itself should you be experiencing problems, even moderate ones.
The following will take you through the System Boot sequence step by step. If you notice any thing unusual through each sequence of events, then you only need to look at the step where the delay appears to occur or back one step.
The system BIOS, a small bit of code inserted in the BIOS prom chip located on your computers motherboard, is what starts the computer running when you turn it on. Below we have outlined the typical sequence of events that occur during your computers startup process, although they will vary by the manufacturer of your hardware, BIOS, and the peripherals you have in the PC.
Again, this is a typical sequence of events that occur when you turn on your computers power switch: The internal power supply turns on, initializes and then takes a few moments to generate reliable power for the rest of the computer.
If the power received by the motherboard's chipset, and subsequently the processor, is not within expected parameters, the chipset will generate a reset signal to the processor in the same fashion as if you were to touch the reset button. This will continue until the motherboard receives a Power Good signal from the power supply or you turn the system off because of a failed power supply. After a Power Good signal is received, or after the reset button is released and there is confirmation of reliable power, the processor will be ready to start executing.
When the processor first starts, it really has no idea what to do next as there is nothing at all in the memory to execute. Of course the processor designers are aware of this, so they pre-program the processor to always look at the same place in the system, the BIOS ROM, for the small bit of startup code to begin the boot process. This is typically located at memory location FFFF0h, or right at the end of the system memory.
Developers locate it there in the event the size of the ROM has to be changed so as to prevent compatibility problems. If you look in the code section below, starting at offset h instruction: "MOV AL,[0 7B7 ]" , you'll see these three bytes are used to reference the offset in Memory of the first byte of each Error Message that can be displayed on screen at boot up: 0 h, 0 77B h and 0 79A h.
Since the code portion above the messages will always be the same, the first offset 0 h will never change no matter what languages and string lengths are used. Now that you know what the bytes at offsets 1B5 h through 1B7 h are used for, you could change these error messages to display whatever you wish as long as they all fit into the space between offsets h and 1B4 h by counting their character lengths and using a disk editor on the MBR sector to make the appropriate changes.
But this code must first copy itself into another area of Memory. This is necessary because the code must also load the Boot Sector of the Active Partition into the same area of Memory that it occupies just after being loaded!
You can learn a great deal about the instructions used here by obtaining the x86 Opcode Windows Help file and Ralf Brown's Interrupt List from our Intro to Assembly page. These pages refer to the Bochs Enhanced Debugger as the tool chosen to provide these illustrative steps We would appreciate any email comments or questions you might have concerning these pages.
Here's a Listing of the disassembled code ; with comments after first being loaded into Memory at :7C00 by the BIOS all Memory locations listed below are in Segment Note: If you compare this code to that of the Windows Vista MBR , you'll find there's only a slight variation due to changing two jump instructions from bit to bit at Memory locations 06A2 and 06C9 ; allowing for the insertion of a CLI instruction immediately after it, and an STI instruction at 06E1.
Lastly, they also decided to add a HLT Halt instruction at Due to these changes, which added 12 more bytes to the Windows 7 MBR code, various offset bytes also needed to be adjusted. The next adjustment doesn't occur until E ; just a few bytes before all the same code bytes become shifted by 1 to 4 bytes compared to Vista's MBR.
If you Ignore it, the NTVDM program might warn you one more time about accessing the hard disk, but it will eventually freeze or go off into oblivion if you Ignore it again; and you'll have to use Task Manager to close the DOS Prompt window gracefully. If E JNZ ; not, jump to offset Remember, the Partition ; Table Preceding Sectors entry can only be a max. In either ; case, DAP's block count field is set to number of blocks actually transferred.
The following code is missing some comments, but all the instructions are here for you to study. This is how it would be seen in a disk editor that can interpret Partition Table data:. The MBR consists of or more bytes located in the first sector of the drive.
It may contain one or more of: A partition table describing the partitions of a storage device. In this context the boot sector may also be called a partition sector. The master boot record MBR is a small program that is executed when a computer is booting i. A sector is a segment of a track on a magnetic disk i. Repalce it with the your disk name. This command will scan for the partition table present on the disk and display it inside the terminal.
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