I've been searching for details about UEFI since I only have a PC with BIOS.
I have both technical and general questions:
- How large is a typical UEFI firmware? Both ROM size, and memory address
space it consume? For example, for BIOS, the typical ROM size would be 32KB
(very old PCs) to 1MB, and it uses 32KB-64KB of memory address space at
0x000F0000. For UEFI, I only know the starting memory address space, which
is 0x0000000000000000.
- Some UEFI firmwares have CSM so the they can boot older OSes such as DOS.
How well it does that? I mean, how good is its BIOS emulation?
- For UEFI firmwares that don't have CSM, is there an UEFI boot manager to
emulate CSM? (to boot older OSes).
- For above question, if there isn't any, can I assume that it's possible to
make such software?
- How does the CSM work in a little more detail? AFAIK (roughly) it would:
copy the BIOS code to the proper address; end/exit EUFI environment and
continue execution in BIOS code in real mode; (somehow) remap/unmap UEFI
firmware from memory address space; setup IVT and BIOS data; then finally
start the BIOS startup code (without actual reboot). I'm not sure about the
UEFI firmware remapping/unmapping part because I haven't seen any code that
can remap/unmap a BIOS firmware.