I actually have an experiment in assembly that I've started and that is to look at the old legacy boot process from quite a low level. The way I see it is this, I know sector0 of the bootable disk is the MBR and wikipedia has plenty of juicy information about what that record contains and how it is used, so I am going to get hold of one ane take a good look at it.

Step1. getting hold of the MBR. a brief summary about the MBR is:

        The master boot record is the first 512 bytes (a single sector) at the 
        start of a disk.  Inside the MBR there's a section at the start which is
         interpreted as code, this code is 16-bit old x86 instructions.

        For backward compatibility the mbr code is loaded at address 7C00

        The tail end of the MBR lists 4 possible partitions where booting can 
        resume.
        

            ## MBR construction

+-------------------------------------------------------+
| 0x0000  MBR code area (446 bytes)                     |
+-------------------------------------------------------+
| 0x01BE  Partition entry #1  (16 bytes)                |
+-------------------------------------------------------+
| 0x01CE  Partition entry #2  (16 bytes)                |
+-------------------------------------------------------+
| 0x01DE  Partition entry #3  (16 bytes)                |
+-------------------------------------------------------+
| 0x01EE  Partition entry #4  (16 bytes)                |
+--------------------+----------------------------------+
| 0x01FE  0x55       |                                  |
+--------------------|  Boot Signature                  |
| 0x01FF  0xAA       |                                  |
+--------------------+----------------------------------+

Partition Entries::
+-------------------------------------------------------+
| 0x00 (1 byte ) status or physical drive bit-7 set for |
|                active or bootable drive               |
+-------------------------------------------------------+
| 0x01 (3 bytes) CHS address of first absolute sector of|
|                partition                              |
|                byte-1 is HEAD                         |
|                byte-2 low 6 bits are sector           |
|                       high 2 bits are high bits of the|
|                       cylinder                        |
|                byte-3 the rest of the cylinder bits   |
+-------------------------------------------------------+
| 0x04 (1 byte ) partition type                         |
+-------------------------------------------------------+
| 0x05 (3 bytes) CHS of last absolute sector in         |
|                partition, bytes follow same definition|
|                as CHS address given above             |
+-------------------------------------------------------+
| 0x08 (4 bytes) LBA of first absolute sector           |
+-------------------------------------------------------+
| 0x0C (4 bytes) number of sectors in partition         |
+-------------------------------------------------------+