Rotate Right Through Carry ALU Function
Original Publication Date: 1984-May-01
Included in the Prior Art Database: 2005-Feb-03
A Rotate Right Through Carry instruction for an arithmetic and logic unit (ALU) is implemented with no additional control lines, hardware external to the ALU, or ALU propagation delay. This Rotate Right Through Carry implementation uses a redundant ALU function (i.e., flush side A) to overcome disadvantages of additional external hardware, additional control lines and degraded performance attendant with implementations external to the ALU. Fig. 1 shows a logic block diagram of the ALU in the logical mode only. The least significant bit (LSB) 0 and the most significant bit (MSB) 7 as well as the carry latch are shown only. Fig. 2 is a diagram of the rotation direction and path. Logic elements 1, 2 and 3 (Fig. 1) form a decode of the rotate function from ALU control lines C1, C2, C3 and C4. The "A" side of the ALU is rotated.