On the use of pseudorandom sequences for high speed resource allocators in superscalar processors

With the increasing effort towards exploiting the maximum level of instruction level parallelism, modern microprocessors are designed so simultaneously issue and execute several instructions in the same clock cycle. A number of resource identifiers and tags are used in these superscalar processors to appropriately manage various resources in the processor correctly identify and enforce data dependencies and to keep track of the instructions that are issued and completed. Structures whose delay is a function of issue window size and/or issue width are likely to become cycle time limiters and a hardware resource allocator is a potential candidate for investigation. The most straightforward technique to allocate and keep track of hardware resources in a processor is to use straight binary numbers as resource identifiers. In this paper we investigate some alternate sequences especially, a pseudo-random sequence. The pseudo-random sequence is a `maximal length sequence´ that has some key properties which enable fast sequence generation using a Linear Feedback Shift Register (LFSR). We analyze the area and timing issues of various resource allocators using models constructed in Verilog hardware description language. Based on the timing optimizations in Synopsys targeting LSI Logic´s 3.3 v G10TM-p Cell-Based 0.29 μ ASIC library we conclude that the pseudo-random sequencer can enhance the clock speed by 15-20% when compared to the traditional straight binary sequencers at the expense of 1.1 to 2.2 times more area. Considering the fact that the resource identifier allocator is required for reorder buffer entry allocation, reorder buffer tag allocation and any other internal resource allocation, and that all these units act in tandem, in reality better clock rate, and thus higher overall system performance, can be achieved by adopting the techniques presented in this paper