How can I tell what sector, or the offset of the partition, I was loaded from? Where's some documentation on what can be depended on after being chainloaded(ie is BDA overwritten by grub?)?

Tyler,

GRUB itself expects ebx to point to struct grub_multiboot_info if at startup eax==0x2BADB002. You may consult Multiboot Specification and GRUB source as well.

Is it that way for chainloading? I want to make my own simple bootloader and chainload it, not using grub as the actual boot loader, just a means of booting my os without having to write it to a cd(no floppy drive) or the mbr. I've set aside a ~20mb partition to install my bootloader to, to boot from.

Tyler,

In general case you get boot drive number in dl, according to BIOS Boot Specification Version 1.01 [PDF] (it seems that leftovers from int13/02 call partially came to the spec; most BIOSes jump to loaded sector right after 0xAA55 check; dump your BIOS and look at the boot sequence to be sure). Legacy bootsectors then follow standard search sequence to find active partition which was used to read them.

GRUB can load arbitrary data, thus the need for extra specs (like mentioned Multiboot).

How do you suppose to boot from that partition? GRUB/NTLDR/whatever chainload? Active partition flag? It depends.