User's Manual

BASIC INSTRUCTION SET
3.4.2.2 ROTATE INSTRUCTIONS
Rotate instructions allow bits in bytes and words to be rotated. Bits rotated out of an operand are not
lost
as in a shift, but are "circled" back into the other "end" of the operand.
Rotates affect only the carry and overflow flags.
CF may act as an extension of the operand in two of
the rotate instructions, allowing a bit
to be isolated and then tested by a conditional jump instruction
(JC or 1NC).
CF always contains the value of the last bit rotated out, even if the instruction does not
use this bit
as an extension of the rotated operand.
In single-bit rotates,
OF is set if the operation changes the high-order (sign) bit of the destination
operand.
If the sign bit retains its original value, OF is cleared. On multibit rotates, the value of OF is
always undefined.
ROL (Rotate Left) rotates the byte or word destination operand left by one or by the number of bits
specified
in the count operand (an immediate value or the value contained in CL). For each rotation
specified, the high-order bit that exists from the left of the operand returns at the right
to become the
new low-order bit of the operand.
See figure 3-9.
Example: ROL AL, 8. Rotates the contents of AL left by 8 bits. This rotate instruction returns AL
to its original state but isolates the low-order bit
in CF for testing by a 1C or 1NC
instruction.
ROR (Rotate Right) rotates the byte or word destination operand right by one or by the number of
bits specified
in the count operand (an immediate value or the value contained in CL). For each rotation
specified, the low-order bit that exits from the right of the operand returns at the left
to become the
new high-order bit of the operand. See figure 3-10.
1
,0
1
0
01, ,1,101, 0101,1, 01
0
1
0
1
BEFORE ROL
ROL shills the bits In the memory or register operand to the lell by the specified number 01 bit positions. It copies the bit shilled out
01 the lell 01 the operand Into the right 01 the operand. The last bit shilled Into the least slgnilicant bit 01 the operand also appears In
CF. This Instruction also operates on byte operands.
G3010B
Figure 3-9. ROL
3-13