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(nasm.info)Section 9.1

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9.1. Mixed-Size Jumps
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   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 `0x9ABC' and the jump will be an ordinary 16-bit far one.

   The Linux kernel setup code gets round the inability of `as86' to
generate the required instruction by coding it manually, using `DB'
instructions. NASM can go one better than that, by actually generating
the right instruction itself. Here's how to do it right:

             jmp     dword 0x1234:0x56789ABC         ; right

   The `DWORD' prefix (strictly speaking, it should come _after_ the
colon, since it is declaring the _offset_ field to be a doubleword; but
NASM will accept either form, since both are unambiguous) forces the
offset part to be treated as far, in the assumption that you are
deliberately writing a jump from a 16-bit segment to a 32-bit one.

   You can do the reverse operation, jumping from a 32-bit segment to a
16-bit one, by means of the `WORD' prefix:

             jmp     word 0x8765:0x4321      ; 32 to 16 bit

   If the `WORD' prefix is specified in 16-bit mode, or the `DWORD'
prefix in 32-bit mode, they will be ignored, since each is explicitly
forcing NASM into a mode it was in anyway.