Improving Floating-Point Performance in Less Area: Fractured Floating Point Units (FFPUs)

Embedded systems designers often use fixed-point instead of floating-point due to the performance and area overhead of floating-point units. If the range of floating-point representation is required, the system may use a software-based floating-point library on an integer-only processor to save area—at the cost of much lower performance. Instead, we propose a Fractured Floating Point Unit (FFPU)—a hybrid solution that uses a set of custom hardware instructions to accelerate software-based floating-point emulation. An FFPU is intended as a compromise between software libraries and full FPUs in terms of both area and performance. We present four potential 32-bit FFPU designs for a Nios II soft processor. We compare their performance and area to the baseline Nios II, as well as a Nios II with a complete FPU. We show that an FFPU can improve various floating-point operations, including improving addition and subtraction performance by 24 to 52 percent over the baseline. This performance comes at a resource cost of only an 11 to 29 percent ALM increase, and no increase in DSP blocks.