Micro-threading and FPGA implementation of a RISC microprocessor : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Computer Science at Massey University, Palmerston North, New Zealand

Appendix E removed due to copyright restrictions. Articles are available in the print copy held in the libraryThis thesis is the outcome of research in two areas of computer technology: microprocessor and multi-processor architectures (specifically from the perspective of how differently they tolerate highly-latent and non-deterministic events), and hardware design of complex digital systems containing both datapath and control (particularly microprocessors). This thesis starts by pointing out that in order to achieve high processing speeds, current popular superscalar microprocessors (e.g. Intel Pentiums, Digital Alpha, etc) rely heavily on the technique of speculating the outcome of instruction flow in order to predict the behaviour of non-deterministic computing operations (as in loading operands from high-latency memory into the processor). This is fine only if the speculation is correct. But, what if it isn't? If the speculation fails, this would mean that the processor has to abandon its current decision (which now proved to be the wrong one) for the instruction flow path taken and to start all over again with the other path (the actual correct one). This is a waste of valuable processing time and hardware resources and a reduction of performance when speculation fails. Therefore, these processors can achieve high performance only when the majority of speculations are successful (being able to predict the right path). In an attempt to overcome the above shortcomings, the first part of this thesis is an investigation of the novel vector micro-threading architecture as an alternative approach to the current superscalar-based speculative microprocessor designs. Micro-threading is based on the not-so-novel multithreading technique, which avoids speculation altogether and instead, starts running a different thread of instructions while waiting for the non-determinism to be resolved. This utilizes the chip resources more efficiently without waste of any processing power. The rest of this thesis focuses on the baseline RISC processor platform, the MIPS R2000, which is reviewed first then partially synthesized from the RTL (Register Transfer Level) description using VHDL and then simulated and tested. This is conducted in order for future research to build upon and add the micro-threading architectural add-ons and modifications. Keywords: Micro-threading, Latency Tolerance, FPGA Synthesis, RISC Architecture, MIPS R2000 processor, VHDL

Improving latency tolerance of multithreading through decoupling

The increasing hardware complexity of dynamically scheduled superscalar processors may compromise the scalability of this organization to make an efficient use of future increases in transistor budget. SMT processors, designed over a superscalar core, are therefore directly concerned by this problem. The article presents and evaluates a novel processor microarchitecture which combines two paradigms: simultaneous multithreading and access/execute decoupling. Since its decoupled units issue instructions in order, this architecture is significantly less complex, in terms of critical path delays, than a centralized out-of-order design, and it is more effective for future growth in issue-width and clock speed. We investigate how both techniques complement each other. Since decoupling features an excellent memory latency hiding efficiency, the large amount of parallelism exploited by multithreading may be used to hide the latency of functional units and keep them fully utilized. The study shows that, by adding decoupling to a multithreaded architecture, fewer threads are needed to achieve maximum throughput. Therefore, in addition to the obvious hardware complexity reduction, it places lower demands on the memory system. The study also reveals that multithreading by itself exhibits little memory latency tolerance. Results suggest that most of the latency hiding effectiveness of SMT architectures comes from the dynamic scheduling. On the other hand, decoupling is very effective at hiding memory latency. An increase in the cache miss penalty from 1 to 32 cycles reduces the performance of a 4-context multithreaded decoupled processor by less than 2 percent. For the nondecoupled multithreaded processor, the loss of performance is about 23 percent.Peer ReviewedPostprint (published version

FIFTY YEARS OF MICROPROCESSOR EVOLUTION: FROM SINGLE CPU TO MULTICORE AND MANYCORE SYSTEMS

Nowadays microprocessors are among the most complex electronic systems that man has ever designed. One small silicon chip can contain the complete processor, large memory and logic needed to connect it to the input-output devices. The performance of today's processors implemented on a single chip surpasses the performance of a room-sized supercomputer from just 50 years ago, which cost over $ 10 million [1]. Even the embedded processors found in everyday devices such as mobile phones are far more powerful than computer developers once imagined. The main components of a modern microprocessor are a number of general-purpose cores, a graphics processing unit, a shared cache, memory and input-output interface and a network on a chip to interconnect all these components [2]. The speed of the microprocessor is determined by its clock frequency and cannot exceed a certain limit. Namely, as the frequency increases, the power dissipation increases too, and consequently the amount of heating becomes critical. So, silicon manufacturers decided to design new processor architecture, called multicore processors [3]. With aim to increase performance and efficiency these multiple cores execute multiple instructions simultaneously. In this way, the amount of parallel computing or parallelism is increased [4]. In spite of mentioned advantages, numerous challenges must be addressed carefully when more cores and parallelism are used.This paper presents a review of microprocessor microarchitectures, discussing their generations over the past 50 years. Then, it describes the currently used implementations of the microarchitecture of modern microprocessors, pointing out the specifics of parallel computing in heterogeneous microprocessor systems. To use efficiently the possibility of multi-core technology, software applications must be multithreaded. The program execution must be distributed among the multi-core processors so they can operate simultaneously. To use multi-threading, it is imperative for programmer to understand the basic principles of parallel computing and parallel hardware. Finally, the paper provides details how to implement hardware parallelism in multicore systems

Late allocation and early release of physical registers

The register file is one of the critical components of current processors in terms of access time and power consumption. Among other things, the potential to exploit instruction-level parallelism is closely related to the size and number of ports of the register file. In conventional register renaming schemes, both register allocation and releasing are conservatively done, the former at the rename stage, before registers are loaded with values, and the latter at the commit stage of the instruction redefining the same register, once registers are not used any more. We introduce VP-LAER, a renaming scheme that allocates registers later and releases them earlier than conventional schemes. Specifically, physical registers are allocated at the end of the execution stage and released as soon as the processor realizes that there will be no further use of them. VP-LAER enhances register utilization, that is, the fraction of allocated registers having a value to be read in the future. Detailed cycle-level simulations show either a significant speedup for a given register file size or a reduction in the register file size for a given performance level, especially for floating-point codes, where the register file pressure is usually high.Peer ReviewedPostprint (published version

Frontend frequency-voltage adaptation for optimal energy-delay/sup 2/

In this paper, we present a clustered, multiple-clock domain (CMCD) microarchitecture that combines the benefits of both clustering and globally asynchronous locally synchronous (GALS) designs. We also present a mechanism for dynamically adapting the frequency and voltage of the frontend of the CMCD with the goal to optimize the energy-delay/sup 2/ product (ED2P). Our mechanism has minimal hardware cost, is entirely self-adjustable, does not depend on any thresholds, and achieves results close to optimal. We evaluate it on 16 SPEC 2000 applications and report 17.5% ED2P reduction on average (80% of the upper bound).Peer ReviewedPostprint (published version

Hybrid analytical modeling of pending cache hits, data prefetching, and MSHRs

As the number of transistors integrated on a chip continues to increase, a growing challenge is accurately modeling per-formance in the early stages of processor design. Analytical models have been employed to rapidly search for higher performance designs, and can provide insights that detailed simulators may not. This paper proposes techniques to predict the impact of pending cache hits, hardware prefetching, and realistic miss status holding register (MSHR) resources on superscalar performance in the presence of long latency memory systems when employing hybrid analytical models that apply instruction trace analysis. Pending cache hits are secondary references to a cache block for which a request has already been initiated but has not yet completed. We find pending hits resulting from spatial locality and the fine-grained selection of instruction profile window blocks used for analysis both have non-negligible influences on the accuracy of hybrid analytical models and subsequently propose techniques to account for their effects. We then introduce techniques to estimate the performance impact of data prefetching by modeling the timeliness of prefetches and to account for a limited number of MSHRs by restricting the size of profile window blocks. As with earlier hybrid analytical models, our approach is roughly two orders of magnitude faster than detailed simulations. When modeling pending hits for a processor with unlimited outstanding misses we improve the accuracy of our baseline by a factor of 3.9, decreasing average error from 39.7 % to 10.3%. When modeling a processor with data prefetching, a limited number of MSHRs, or both, the techniques result in an average error of 13.8%, 9.5 % and 17.8%, respectively. 1

A cost-effective clustered architecture

In current superscalar processors, all floating-point resources are idle during the execution of integer programs. As previous works show, this problem can be alleviated if the floating-point cluster is extended to execute simple integer instructions. With minor hardware modifications to a conventional superscalar processor, the issue width can potentially be doubled without increasing the hardware complexity. In fact, the result is a clustered architecture with two heterogeneous clusters. We propose to extend this architecture with a dynamic steering logic that sends the instructions to either cluster. The performance of clustered architectures depends on the inter-cluster communication overhead and the workload balance. We present a scheme that uses run-time information to optimise the trade-off between these figures. The evaluation shows that this scheme can achieve an average speed-up of 35% over a conventional 8-way issue (4 int+4 fp) machine and that it outperforms the previously proposed one.Peer ReviewedPostprint (published version

High performance extendable instruction set computing

In this paper, a new architecture called the extendable instruction set computer (EISC) is introduced that addresses the issues of memory size and performance in embedded microprocessor systems. The architecture exhibits an efficient fixed length 16-bit instruction set with short length offset and immediate operands. The offset and immediate operands can be extended to 32 bits via the operation of an extension flag. The code density of the EISC instruction set and its memory transfer erformance is shown to be significantly higher than current architectures making it a suitable candidate for the next generation of embedded computer systems. The compact EISC instruction set introduces data dependencies that seemingly limit deep pipeline and superscalar implementations. This paper suggests a mechanism by which these dependencies might be removed in hardware

Randomized cache placement for eliminating conflicts

Applications with regular patterns of memory access can experience high levels of cache conflict misses. In shared-memory multiprocessors conflict misses can be increased significantly by the data transpositions required for parallelization. Techniques such as blocking which are introduced within a single thread to improve locality, can result in yet more conflict misses. The tension between minimizing cache conflicts and the other transformations needed for efficient parallelization leads to complex optimization problems for parallelizing compilers. This paper shows how the introduction of a pseudorandom element into the cache index function can effectively eliminate repetitive conflict misses and produce a cache where miss ratio depends solely on working set behavior. We examine the impact of pseudorandom cache indexing on processor cycle times and present practical solutions to some of the major implementation issues for this type of cache. Our conclusions are supported by simulations of a superscalar out-of-order processor executing the SPEC95 benchmarks, as well as from cache simulations of individual loop kernels to illustrate specific effects. We present measurements of instructions committed per cycle (IPC) when comparing the performance of different cache architectures on whole-program benchmarks such as the SPEC95 suite.Peer ReviewedPostprint (published version