AngoLinux

80386 Programmer's Reference Manual -- Opcode RCL

RCL/RCR/ROL/ROR -- Rotate

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

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

Protected Mode Exceptions

Real Address Mode Exceptions

Virtual 8086 Mode Exceptions

[Home Page dell'ITIS "Fermi"] [80386 Programmer's Reference Manual Index] [Previous] [Next]