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In the
x86 assembly language, the JMP
instruction performs an unconditional jump. Such an instruction transfers the flow of
execution by changing the
program counter. There are a number of different
opcodes that perform a jump; depending on whether the processor is in
real mode or
protected mode, and an override instruction is used, the instructions may take
16-bit,
32-bit, or segment:offset
pointers.
[1]
There are many different forms of jumps: relative, conditional, absolute and register-indirect jumps.
The following examples illustrate:
(Note that although the first and second jumps are relative, commonly the destination address is shown instead of the relative offset as encoded in the opcode.)
Example one: Load IP with the new value 0x89AB
, then load
CS with 0xACDC
and IP with 0x5578
.
JMP 0x89AB
JMP 0xACDC:0x5578
Example two: Load EIP with the value 0x56789AB1
, only in
protected mode or
unreal mode.
JMP 0x56789AB1
Example three: Jump to the value stored in the EAX register, only in protected mode.
JMP EAX