Does the 512 byte limit for a boot sector exist in UEFI too? I don't know much about UEFI so if someone could educate me about how the boot sector and its size limit differs in UEFI, I'd love to know.
Before the ESP is accessed, there is no standardized way to customize the boot process. You could put these kinds of sectorX toys into the firmware directly, which would come with more constraints, but it would be vendor-specific.
There is a platform-independent VM running a special EFI byte code that is part of the EFI specification, which allows you to extend the UEFI system with things like additional drivers, but those are also loaded from the ESP.
1. IIUC PE executables are Windows executables. So a Linux system that targets UEFI ends up writing a PE executable to the EFI System Partition?
2. I know that some UEFIs (or is it all?) support BIOS boot sector as backward compatibility feature? How does that work? If I write a "hello world" program in pure machine code in the 1st sector of the boot disk, would UEFI read that and execute that? How would it even know whether what's in the first sector is valid code or garbage? By checking the magic 0x55 0xaa at the end of the boot sector?
Short answer: Yes.
Longer answer: Yes, but it is not exactly a classic Windows desktop application. A PE file itself is at its core just a way to store data in a somewhat structured way. It has metadata (like imports/exports and the target architecture) and actual contents (like data and code). I think it is a good idea to reuse an already existing, common executable file format for this use case. That the choice was PE instead of ELF or Mach-O is, IMO, the result of Microsoft being in the UEFI consortium. They are a big player and likely pushed for this. Whether or not ELF would have been a better choice, I can not say, but at this point I don't think it matters.
More info on that topic at OSDev Wiki: https://wiki.osdev.org/UEFI#Binary_Format
File output on my GRUB install:
/boot/EFI/GRUB/grubx64.efi: PE32+ executable (EFI application) x86-64 (stripped to external PDB), for MS Windows, 4 sectionsSimilarly how many file formats are actually just pkzip files with standardzied names and contents.
2. Boot sector support requires UEFI CSM support, which most PCs have (but ARM based devices and Intel Macs don't). With the deprecation of 32-bit architectures in a lot of operating systems, some vendors also started dropping CSM support in some devices, but most devices still ship with CSM either enabled by default or as an option in the settings.
Still, that would be 4GB per file, and UEFI provides a file system access interface, so you could easily load in more.