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(nasm.info)Section B.4.116

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B.4.116. `IBTS': Insert Bit String
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     IBTS r/m16,reg16              ; o16 0F A7 /r         [386,UNDOC]
     IBTS r/m32,reg32              ; o32 0F A7 /r         [386,UNDOC]

   The implied operation of this instruction is:

     IBTS r/m16,AX,CL,reg16
     IBTS r/m32,EAX,CL,reg32

   Writes a bit string from the source operand to the destination. `CL'
indicates the number of bits to be copied, from the low bits of the
source.  `(E)AX' indicates the low order bit offset in the destination
that is written to. For example, if `CL' is set to 4 and `AX' (for
16-bit code) is set to 5, bits 0-3 of `src' will be copied to bits 5-8
of `dst'. This instruction is very poorly documented, and I have been
unable to find any official source of documentation on it.

   `IBTS' is supported only on the early Intel 386s, and conflicts with
the opcodes for `CMPXCHG486' (on early Intel 486s). NASM supports it
only for completeness. Its counterpart is `XBTS' (see *Note Section
B.4.332::).