In brief, control is passed from BIOS to the loaded MBR code at 07C0:0000. The code relocates itself to 07E0:0000, checks for a Boot Manager partition, and checks for a bootable partition. Once a bootable partition is found, the partition boot record or more correctly the BIOS parameter block (BPB) is loaded at 07C0:0000 and control is passed to it at 07C0:0000.

Remember, as I understand the MBR is created by the LVM utility (ex: LVM /NEWMBR). At this point the system is using INT13 calls to interface with the drive. The errors that can be encountered during this phase are:

• SYS01462
• SYS01463
• SYS01464

More detailed information can be found here:
LVM MBR Disassembled