This is admittedly a bit of esoterica.....

As we know, the map command is required when a Windows install is anywhere except the first drive. The typical explanatory statement is the "Windows wants to be on the first drive."

Trying to research this leads me to conclude that the map instructions cause parameters to be passed to NTLDR when grub does the handoff with "chainloader". Further speculation says that--somewhere in NTLDR someone hard-coded a drive reference--or did some similarly sloppy programming.* **

Can anyone confirm or further explain what is going on?



* Who in the computer world would DO such a thing?...

**Long ago, I found something like this in an application running on a 128K Mac with the hot new "HyperDrive" (All 10 MEGAbytes of it!!) I offered the solution to the vendor and requested compensation. They refused and we both went on with our lives.

AFAIK the map command is simply a way of swapping master/slave drive configurations without opening the case. I believe it simply fools the BIOS in some way. Perhaps the 'info grub' section on map may give a better explanation...

Because it is typically only used when chainloading to boot Windows, it must have something to with how NTLDR calls the harddrive. So I infer that GRUB map does one of two things:
--Modifies BIOS settings as you said
--Passes parameters to NTLDR

If it modifying BIOS settings--then how? I did find something on where the BIOS stores key parameters in memory, but nothing relevant popped out at me.