What defines the signature of a bootsector?

Question

I've attempted to simplify the question, by getting at the gist of the problem:

What exactly makes the "first sector" of a device the "bootsector?"

• Is the bootsector a logical sector, a physical sector, or both?
• If logical sector and physical sector sizes can differ between devices, does this influence the size of the bootsector?

Parts from the old question below: Part I: What are the differences between READING from the bootsector of 512-byte and non-512-byte sector devices?

For the following devices, which sector is considered the "first sector" or "bootsector"? How are the bootsectors identified in 512-byte sector devices and non-512-byte sector devices? What are their signatures?

• Floppy disk (512-byte sector)
• Traditional HDD (512-byte sector)
• CD-ROM (2048-byte sector)
• DVD-ROM (?-byte sector)
• BD-ROM (?-byte sector)
• Flash Memory Device (Are "sectors" even applicable here?)
• SSDs (Are "sectors" even applicable here?)
• Advanced format HDD with 512-byte emulation (4096-byte sector)
• Advanced format HDD without emulation (4096-byte sector)

For CDs, DVDs, and BDs, what is the physical and logical sector size? CDs are 2048-byte sized sectors; I'm not sure about the DVDs and BDs. For CDs you follow the El-Torito standard to reading and writing bootsectors. For DVDs and BDs, I do not know what standard to follow. The El-Torito standard allows you to either emulate 512-byte sectors (like a floppy or HDD) or you can boot at the native size.

Part II: What are the differences between WRITING to the bootsector of 512-byte and non-512-byte sector devices?

• Specifically, how do you write to the bootsector of the above devices such that you are able to read from them in Part I?
• I see dd can be used, but I also see one can simply write a simple C program to open() the device, lseek() to a specified byte offset, and write() directly to the first 512 bytes of the device. (i.e. write starting at index 0)
• For non-512-byte sized devices, and for OSes that are not aware of non-512-byte sized sector devices, is this even possible? Or is this completely irrelevant?
• Which portions of the physical sector should be written to for devices that support: (1) 512-byte emulation and (2) no emulation?

Answer 1

That is a bit too much for one question. It depends on the computer what media it can boot from and what the physical layout/interfaces/addressing it supports for that media.

In general older interfaces and addressing modes are emulated/translated to support newer devices to maintain a large window of compatibility (new drives work in older computers, a new device type works on an existing interface). The actual physical sector size really only matters for performance - aligning data and sizing structures to such boundaries can be faster than not doing so.

One CDROM standard, El Tortio, for example supports putting a floppy disk image on the CD which is then read by a computer's BIOS and emulated as a floppy. This means a bootable floppy disk could be used to bootstrap a bootable CDROM.