A.5.332 XBTS: Extract Bit String
XBTS reg16,r/m16 ; o16 0F A6 /r [386,UNDOC]
XBTS reg32,r/m32 ; o32 0F A6 /r [386,UNDOC]
The implied operation of this instruction is:
XBTS r/m16,reg16,AX,CL
XBTS r/m32,reg32,EAX,CL
Writes a bit string from the source operand to the destination. CL
indicates the number of bits to be copied, and (E)AX indicates the low
order bit offset in the source. The bits are written to the low order
bits of the destination register. For example, if CL is set to 4 and AX
(for 16-bit code) is set to 5, bits 5-8 of "src" will be copied to bits
0-3 of "dst". This instruction is very poorly documented, and I have
been unable to find any official source of documentation on it.
XBTS 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 IBTS (see section A.5.116).