Browse Prior Art Database

Rotate Right Through Carry ALU Function

IP.com Disclosure Number: IPCOM000042404D
Original Publication Date: 1984-May-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 2 page(s) / 55K

Publishing Venue

IBM

Related People

Kriss, FJ: AUTHOR [+4]

Abstract

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.

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Rotate Right Through Carry ALU Function

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. Both a Hex '8' and '9' will result in the "A" side being propagated or "flushed" through the ALU in logical mode. Hex '9' was decoded to provide the rotate right with no loss of other ALU function. Logic elements 4 and 5 were changed from invert logic functions to "AND-OR" functions and select the next most significant bit, or "carry" in the case of Bit 7, whenever rotate is decoded. Logic element 6 selects the normal carry function or the least significant bit 0 to be loaded into the carry latch for rotate. Logic element 7 is added to force the ALU into logical mode when rotate is decoded. This allows t...