Application specific low power alu design

1. Application specific low power alu design Yu Zhou and HuiGuo By Nathan Windels

2. Outline • Review Sources of Power Consumption • Review ALU Structures • Chain Structure Design and Proposed ALU Customization • The Test Setup • Results

3. Power Consumption • Power consumption is a critical design issue in embedded processor designs • Two Types of Power Consumption: • Dynamic: Reduce switching capacitance, switching frequency, or supply voltage • Static: reduce circuit size, operating temperature, increase transistor threshold voltage • Areas of Power Consumption • Semiconductor Chip Design Level (transistor sizing, threshold voltage scaling) • Register Transfer Level (clock gating, power gating) • System Level (dynamic voltage scaling) • Modify individual functional components of the processor (ALU customization)

4. ALU Structures • The top is a tree structure (faster, larger area) • The bottom is a chain structure (slower, smaller area) • In a lot of applications, the ALU is not in the critical path of the processor, so the chain structure is often used • ASIPMeister uses the chain structure to save area.

5. The idea of this paper is to customize the ALU by repositioning the elements in the chain structure • Swapping the ‘add’ and ‘or’ components may favour some applications and can save a considerable amount of ALU power. • This approach to power reduction is almost cost free and is extremely simple to implement.

6. Proposed Chain Structure • There are n functional components and they are concatenated by 2-to-1 multiplexers. • Oi is the operational activity of component i • Omuxj is the operational activity of the multiplexor j • Change of component positions with not effect Oi, but it will effect Omuxj

7. ALU Customization • We can customize the ALU design by identifying frequent functional components and placing them close to the output. • In application specific designs, the frequency of a functional component is obtained from instruction frequencies. • We can therefore partition the instruction set into ALU and non-ALU instructions. The ALU instructions can then be grouped according to the functional component they actually use. • Different weights of power consumption can be assigned to different functional components. • Design in such a way that high weight and high frequency components are placed closer to the output.

8. ALU Test Setup • Created a simple ALU in VHDL • The design was synthesized with the Synopsis Design Compiler based on the ts11fs120 library • The Power consumption was estimated by Synopsis PrimePower.

9. ALU Test Setup (2) • A reduced ALU of 4 functions was used for exploring designs of all possible placements in order to verify the effectiveness of this approach. • The adder is the longest component, therefore when it is positioned next to the output in the chain, the overall delay is reduced. • The power always reaches a minimum level when the related functional component is placed closest to the output.

10. Full Processor Test Setup • The processor design for a given application was automatically generated. • The target processor instruction set was Portable Instruction Set Architecture. • The VHDL model was automatically generated by ASIPMeister. • Simplescalar was used to compile the application program and to profile the program execution.

11. ALU Operation Frequency • We can see from this graph that addition has the highest frequency for all designs, so the adder is placed closest to the output. • This table was obtained using the Simplescalar profiler.

12. ALU Test Results

13. ALU Test Results (2) • The CPU clock time remains unchanged throughout all the designs, demonstrating that the ALU is not on the critical path.

14. Conclusion • The order of functional components in the chain effects the power consumption, therefore, the frequently operating component should be placed close to the output. • This change is easy to make. All you have to do is swap the order of ALU operations in the if-then-else statement in the HDL code. • This approach may be applicable to other designs with similar chain structure (floating-point ALU’s).