the following information is based on my understanding of the following webpage, please read the original site for detailed explanation.
http://mirror.href.com/thestarman/asm/mbr/Win2kmbr.htm
MBR is the first sector in the first cylinder, first head of the current Master hard drive active partition. It is a 512 bytes (one sector) long paragraph of data and code.
* what is inside the MBR? -- under windows, you can use any binary reader to open c:\windows\system32\dmadmin.exe, and the MBR code itself is found between offsets 34E28h through 35027h. It contains some machine code, some error message, some OS related remarks and last but not least, the partition table.
* how it is invoked? -- I am not very sure about my answer, but I do provide short answer here. remember the DOS interrupt int 13? Yes, int 13 is used in BIOS to load this particular sector into memory location 0000:7c00. The processor will execute the code from 0000:7c00, and somewhere at 0000:7c1B, the code will try to copy the rest of the sector (about 485 bytes long) to another memory location.
[why make another copy? because this copy in memory will be overwritten by boot sector of the active partition later. the remaining code has to find itself a new home. :) ]
The website listed above provide some code comment to the disassembled MBR machine code. nice work!
