RCL rotates left through the carry flag. The operand and CF together form a ring of 9 bits (for a byte) or 17 bits (for a word). Each step, every bit moves one place left, the old CF enters bit 0, and the old top bit becomes the new CF.

  +-- CF <- b7 b6 b5 b4 b3 b2 b1 b0 <-+
  |                                    |
  +------------------------------------+
 
        CF=1  0100 0000
RCL 1   CF=0  1000 0001

Forms and flags

Register or memory operand; count 1 or CL on the 8086. Only CF and OF change; SF, ZF, PF and AF keep their values. With a count of 1, OF is set when the new CF and the new top bit differ. For other counts OF is undefined. Because the old CF goes into the value, set it on purpose first: CLC for a 0, STC for a 1.

Multi-word shifts

This is what RCL is for. To shift a value wider than one register left by one bit, shift the lowest part with SHL, then RCL each higher part: the bit leaving one part travels through CF into the next. For a 32-bit number in DX:AX:

SHL     AX, 1  ; low word: top bit into CF
RCL     DX, 1  ; high word: CF into bit 0

Repeat the pair to shift further; a CL count does not work here, because the carry must pass between the words after every single bit. The same pattern collects bits one at a time: anything that leaves a result in CF (a compare, a shift of another register) can be followed by RCL BL, 1 to add that bit at the bottom of BL.

BH:BL shifted left as one 16-bit value. Prints 00000011 10000110
MOV     BH, 01h       ; high byte 0000 0001
MOV     BL, 0C3h      ; low byte  1100 0011
SHL     BL, 1         ; low byte first: bit 7 -> CF
RCL     BH, 1         ; CF enters the high byte
MOV     AL, BH
CALL    SHOWBIN       ; 0000 0011
MOV     AL, BL
CALL    SHOWBIN       ; 1000 0110
 
SHOWBIN PROC    NEAR  ; print AL as 8 binary digits and a space
PUSHF                 ; keep the caller's flags
PUSH    AX
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BL, AL
MOV     CX, 8
NEXTBIT:
SHL     BL, 1         ; the top bit falls into CF
MOV     DL, "0"
ADC     DL, 0         ; "0" + CF = "0" or "1"
MOV     AH, 02h
INT     21h
LOOP    NEXTBIT
MOV     DL, " "
MOV     AH, 02h
INT     21h
POP     DX
POP     CX
POP     BX
POP     AX
POPF
RET
SHOWBIN ENDP

Later CPUs

The 8086 takes the whole of CL as the count. The 80186 and later mask it to 5 bits and accept an immediate count, as in RCL AX, 2.