Question

I am learning how Linux works. I have encountered a strange assembly language instruction, jmpi. I can find some explanation at various websites, but strangely I can’t find it in assembly language books, including the Intel® 64 and IA-32 Architectures Software Developer’s Manual. I have searched the book, but it doesn’t contain the instruction jmpi. I think the Intel manual should be the the most authoritative book on Intel assembly language. So it is very strange.

My question is: is there some book or authoritative files which document this instruction?

Answer 1

There are several JMP instructions in the Intel processor instruction set. They have the same intended result but differ by the type of the operand they take: an 8-bit constant, a 16-bit constant, a 32-bit constant, a value specified indirectly; also is it a relative or an absolute jump. See the table from p.854 of the Architecture manual

Opcode Instruction  Description
------+------------+----------------------------
EB cb  JMP rel8     Jump short
E9 cw  JMP rel16    Jump near, relative
E9 cd  JMP rel32    Jump near, relative
FF /4  JMP r/m16    Jump near, absolute indirect
FF /4  JMP r/m32    Jump near, absolute indirect
FF /4  JMP r/m64    Jump near, absolute indirect
EA cd  JMP ptr16:16 Jump far, absolute
...

They are treated differently as a legacy of different memory models supported at different stages of the architecture development.

Many assemblers introduce additional mnemonics to make the assembly code more readable. So in as86 the JMPI instruction refers to a near JMP it can be either absolute, or relative, but should always stay within a code segment, the intrasegment jump. The as86 is the only authoritative reference.