How does the IPL know to load Stage1_5?
 

I’ve been searching high & low, but cannot find a definitive answer as to how GRUB Stage_1 (IPL of MBR) knows how to load Stage1_5.

I’ve read the GNU Grub manual and various other forums but since I don’t know C, I can’t reverse engineer or figure out the source code to determine whether Stage1_5 is loaded or not.

I understand that, as part of the OS install procedure, GRUB writes Stage_1 (as the IPL) into the MBR, and that this then calls either Stage1_5 or Stage_2, depending on whether Stage1_5 is installed.

I also believe that Stage1_5 is usually installed into the sectors following the MBR into the so-called DOS compatibility region (the next 62 sectors within the first cylinder before the first partition).

However, assuming Stage1_5 is installed, is there a routine within Stage_1 that checks to see if Stage1_5 is installed and if so jumps & runs to it?

Or, if Stage1_5 is not installed would the same routine then jump to & run Stage_2?

Or, does the GRUB installer intelligently install specific Stage_1 code dependent upon whether Stage1_5 was installed or not? i.e. If the GRUB installer was able to load Stage1_5 then the Stage_1 code it installs would have the routine to jump to & run Stage1_5. Or, if the GRUB installer did not install Stage1_5, it would install different Stage_1 code to jump to & run Stage_2

I appreciate this may be moot, what with GRUB 2 but I’d like to have a better understanding of this process.

TIA – Derrick

This is my understanding. The stage1 code simply (always) jumps out - whether to stage 1.5 or 2 it neither knows nor cares. It is simply a sector number that the install procedure writes into the stage1 code as it's written into the MBR or (partition) boot sector.

The GRUB installer writes the jump instruction in the stage 1 code, directing the CPU where to pick up the next set of instructions. The installer keeps track of if stage 1_5 is installed, and writes the jump instruction to point to it if it is installed. If stage1_5 is mot installed, the installer sets the jump instruction to point to the first instruction in stage_2. If anything changes (e.g.: a GRUB update) that may relocate or eliminate stage 1_5, or relocate stage _2, the boot loader in the MBR is rewritten with a new jump instruction.

HTH,

Understood.

I had a feeling that might be the case, but needed positive confirmation instead of my (hopefully) 'educated guesses'!

Thanks guys :)