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Asked: 2026-06-11T12:33:06+00:00

Is there any difference between them? I am using this code to jump from

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Is there any difference between them? I am using this code to jump from real mode to protected mode. the second one works.

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  1. Editorial Team

    Other than the length of the instruction (due to the longer absolute address encoded in it and any possibly required instruction prefixes) there’s no difference between the two whatsoever.

    The two versions of the instruction behave identically.

    You’re not restricted to using only the shorter version in 16-bit mode or only the longer version in 32-bit mode. As long as the address is valid (per the relevant segment descriptor in the GDT and then the underlying page tables, if any), the jump will just work.

    UPDATE: Actually (found by trial and error:), you want to request the proper operand size specifically if the offset doesn’t fit into 16 bits.

    Sample code:

    bits 16
jmp 8:0
jmp dword 8:0
jmp 8:0x12345678 ; warning: jmpabs.asm:4: warning: word data exceeds bounds
bits 32
jmp 8:0
jmp word 8:0
jmp 8:0x1234

    Assembled as nasm -f bin -l jmpabs.lst jmpabs.asm -o jmpabs.bin produces the following listing file:

     1                                  bits 16
 2 00000000 EA00000800              jmp 8:0
 3 00000005 66EA000000000800        jmp dword 8:0
 4 0000000D EA78560800              jmp 8:0x12345678
 5          ******************       warning: word data exceeds bounds
 6                                  bits 32
 7 00000012 EA000000000800          jmp 8:0
 8 00000019 66EA00000800            jmp word 8:0
 9 0000001F EA341200000800          jmp 8:0x1234
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