A high performance 32-bit ALU for programmable logic

The Arithmetic-Logic-Unit (ALU) is at the heart of a modern microprocessor, and its size and speed are often significant contributors to the overall processor's cost and performance. This paper presents the design of the ALU used in Altera's NIOS 2.0 soft processor implemented on Altera's Apex 20KE FPGA architecture. This ALU enabled the 32-bit NIOS 2.0 to consume only 1200 LEs and run at 85MHz. This is a 50% size reduction and 70% speed improvement over its predecessor, NIOS 1.1.The Logic-element (LE) is the basic building block within the Apex architecture. Making full use of the advanced features of the LE has resulted in this novel ALU design. A functional representation of the logic is used to describe how the ALU performs the core set of NIOS instructions, and an LE representation shows the amount of logic-resources needed for the implementation. The cost of additional features such as a barrel-shifter and custom instructions is also described.Likely worst-case delays for different routing and logic elements are used to estimate the ALU's speed. Further speed and size optimizations are also presented from which it is possible to create ALU ranging in speed from 87 MHz to over 100 MHz.