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(nasm.info)Section B.4.267

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B.4.267. `RCL', `RCR': Bitwise Rotate through Carry Bit
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     RCL r/m8,1                    ; D0 /2                [8086]
     RCL r/m8,CL                   ; D2 /2                [8086]
     RCL r/m8,imm8                 ; C0 /2 ib             [186]
     RCL r/m16,1                   ; o16 D1 /2            [8086]
     RCL r/m16,CL                  ; o16 D3 /2            [8086]
     RCL r/m16,imm8                ; o16 C1 /2 ib         [186]
     RCL r/m32,1                   ; o32 D1 /2            [386]
     RCL r/m32,CL                  ; o32 D3 /2            [386]
     RCL r/m32,imm8                ; o32 C1 /2 ib         [386]

     RCR r/m8,1                    ; D0 /3                [8086]
     RCR r/m8,CL                   ; D2 /3                [8086]
     RCR r/m8,imm8                 ; C0 /3 ib             [186]
     RCR r/m16,1                   ; o16 D1 /3            [8086]
     RCR r/m16,CL                  ; o16 D3 /3            [8086]
     RCR r/m16,imm8                ; o16 C1 /3 ib         [186]
     RCR r/m32,1                   ; o32 D1 /3            [386]
     RCR r/m32,CL                  ; o32 D3 /3            [386]
     RCR r/m32,imm8                ; o32 C1 /3 ib         [386]

   `RCL' and `RCR' perform a 9-bit, 17-bit or 33-bit bitwise rotation
operation, involving the given source/destination (first) operand and
the carry bit. Thus, for example, in the operation `RCL AL,1', a 9-bit
rotation is performed in which `AL' is shifted left by 1, the top bit
of `AL' moves into the carry flag, and the original value of the carry
flag is placed in the low bit of `AL'.

   The number of bits to rotate by is given by the second operand. Only
the bottom five bits of the rotation count are considered by processors
above the 8086.

   You can force the longer (286 and upwards, beginning with a `C1'
byte) form of `RCL foo,1' by using a `BYTE' prefix: `RCL foo,BYTE 1'.
Similarly with `RCR'.