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RCL/RCR/ROL/ROR -- Rotate

Opcode       Instruction       Clocks  Description

D0  /2       RCL r/m8,1        9/10    Rotate 9 bits (CF,r/m byte) left
                                       once
D2  /2       RCL r/m8,CL       9/10    Rotate 9 bits (CF,r/m byte) left CL
                                       times
C0  /2 ib    RCL r/m8,imm8     9/10    Rotate 9 bits (CF,r/m byte) left
                                       imm8 times
D1  /2       RCL r/m16,1       9/10    Rotate 17 bits (CF,r/m word) left
                                       once
D3  /2       RCL r/m16,CL      9/10    Rotate 17 bits (CF,r/m word) left
                                       CL times
C1  /2 ib    RCL r/m16,imm8    9/10    Rotate 17 bits (CF,r/m word) left
                                       imm8 times
D1  /2       RCL r/m32,1       9/10    Rotate 33 bits (CF,r/m dword) left
                                       once
D3  /2       RCL r/m32,CL      9/10    Rotate 33 bits (CF,r/m dword) left
                                       CL times
C1  /2 ib    RCL r/m32,imm8    9/10    Rotate 33 bits (CF,r/m dword) left
                                       imm8 times
D0  /3       RCR r/m8,1        9/10    Rotate 9 bits (CF,r/m byte) right
                                       once
D2  /3       RCR r/m8,CL       9/10    Rotate 9 bits (CF,r/m byte) right
                                       CL times
C0  /3 ib    RCR r/m8,imm8     9/10    Rotate 9 bits (CF,r/m byte) right
                                       imm8 times
D1  /3       RCR r/m16,1       9/10    Rotate 17 bits (CF,r/m word) right
                                       once
D3  /3       RCR r/m16,CL      9/10    Rotate 17 bits (CF,r/m word) right
                                       CL times
C1  /3 ib    RCR r/m16,imm8    9/10    Rotate 17 bits (CF,r/m word) right
                                       imm8 times
D1  /3       RCR r/m32,1       9/10    Rotate 33 bits (CF,r/m dword) right
                                       once
D3  /3       RCR r/m32,CL      9/10    Rotate 33 bits (CF,r/m dword) right
                                       CL times
C1  /3 ib    RCR r/m32,imm8    9/10    Rotate 33 bits (CF,r/m dword) right
                                       imm8 times
D0  /0       ROL r/m8,1        3/7     Rotate 8 bits r/m byte left once
D2  /0       ROL r/m8,CL       3/7     Rotate 8 bits r/m byte left CL
                                       times
C0  /0 ib    ROL r/m8,imm8     3/7     Rotate 8 bits r/m byte left imm8
                                       times
D1  /0       ROL r/m16,1       3/7     Rotate 16 bits r/m word left once
D3  /0       ROL r/m16,CL      3/7     Rotate 16 bits r/m word left CL
                                       times
C1  /0 ib    ROL r/m16,imm8    3/7     Rotate 16 bits r/m word left imm8
                                       times
D1  /0       ROL r/m32,1       3/7     Rotate 32 bits r/m dword left once
D3  /0       ROL r/m32,CL      3/7     Rotate 32 bits r/m dword left CL
                                       times
C1  /0 ib    ROL r/m32,imm8    3/7     Rotate 32 bits r/m dword left imm8
                                       times
D0  /1       ROR r/m8,1        3/7     Rotate 8 bits r/m byte right once
D2  /1       ROR r/m8,CL       3/7     Rotate 8 bits r/m byte right CL
                                       times
C0  /1 ib    ROR r/m8,imm8     3/7     Rotate 8 bits r/m word right imm8
                                       times
D1  /1       ROR r/m16,1       3/7     Rotate 16 bits r/m word right once
D3  /1       ROR r/m16,CL      3/7     Rotate 16 bits r/m word right CL
                                       times
C1  /1 ib    ROR r/m16,imm8    3/7     Rotate 16 bits r/m word right imm8
                                       times
D1  /1       ROR r/m32,1       3/7     Rotate 32 bits r/m dword right once
D3  /1       ROR r/m32,CL      3/7     Rotate 32 bits r/m dword right CL
                                       times
C1  /1 ib    ROR r/m32,imm8    3/7     Rotate 32 bits r/m dword right imm8
                                       times

Operation

(* ROL - Rotate Left *)
temp := COUNT;
WHILE (temp <> 0)
DO
   tmpcf := high-order bit of (r/m);
   r/m := r/m * 2 + (tmpcf);
   temp := temp - 1;
OD;
IF COUNT = 1
THEN
   IF high-order bit of r/m <> CF
   THEN OF := 1;
   ELSE OF := 0;
   FI;
ELSE OF := undefined;
FI;
(* ROR - Rotate Right *)
temp := COUNT;
WHILE (temp <> 0 )
DO
   tmpcf := low-order bit of (r/m);
   r/m := r/m / 2 + (tmpcf * 2^(width(r/m)));
   temp := temp - 1;
DO;
IF COUNT = 1
THEN
   IF (high-order bit of r/m) <> (bit next to high-order bit of r/m)
   THEN OF := 1;
   ELSE OF := 0;
   FI;
ELSE OF := undefined;
FI;

Description

Each rotate instruction shifts the bits of the register or memory operand given. The left rotate instructions shift all the bits upward, except for the top bit, which is returned to the bottom. The right rotate instructions do the reverse: the bits shift downward until the bottom bit arrives at the top.

For the RCL and RCR instructions, the carry flag is part of the rotated quantity. RCL shifts the carry flag into the bottom bit and shifts the top bit into the carry flag; RCR shifts the carry flag into the top bit and shifts the bottom bit into the carry flag. For the ROL and ROR instructions, the original value of the carry flag is not a part of the result, but the carry flag receives a copy of the bit that was shifted from one end to the other.

The rotate is repeated the number of times indicated by the second operand, which is either an immediate number or the contents of the CL register. To reduce the maximum instruction execution time, the 80386 does not allow rotation counts greater than 31. If a rotation count greater than 31 is attempted, only the bottom five bits of the rotation are used. The 8086 does not mask rotation counts. The 80386 in Virtual 8086 Mode does mask rotation counts.

The overflow flag is defined only for the single-rotate forms of the instructions (second operand := 1). It is undefined in all other cases. For left shifts/rotates, the CF bit after the shift is XORed with the high-order result bit. For right shifts/rotates, the high-order two bits of the result are XORed to get OF.

Flags Affected

OF only for single rotates; OF is undefined for multi-bit rotates; CF as described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

up: Chapter 17 -- 80386 Instruction Set
prev: PUSHF/PUSHFD Push Flags Register onto the Stack
next: REP/REPE/REPZ/REPNE/REPNZ Repeat Following String Operation