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This chapter tries to cover some of the issues, largely related to unusual forms of addressing and jump instructions, encountered when writing operating system code such as protected-mode initialisation routines, which require code that operates in mixed segment sizes, such as code in a 16-bit segment trying to modify data in a 32-bit one, or jumps between different-size segments.
The most common form of mixed-size instruction is the one used when writing a 32-bit OS: having done your setup in 16-bit mode, such as loading the kernel, you then have to boot it by switching into protected mode and jumping to the 32-bit kernel start address. In a fully 32-bit OS, this tends to be the only mixed-size instruction you need, since everything before it can be done in pure 16-bit code, and everything after it can be pure 32-bit.
This jump must specify a 48-bit far address, since the target segment is a 32-bit one. However, it must be assembled in a 16-bit segment, so just coding, for example,
jmp 0x1234:0x56789ABC ; wrong!
will not work, since the offset part of the address will be truncated to
The Linux kernel setup code gets round the inability of
jmp dword 0x1234:0x56789ABC ; right
The