VLSI design of a twin register file for reducing the effects of conditional branches in a pipelined architecture

Abstract

Pipelining is a major organizational technique which has been used by computer engineers to enhance the performance of computers. Pipelining improves the performance of computer systems by exploiting the instruction level parallelism of a program. In a pipelined processor the execution of instructions is overlapped, and each instruction is executed in a different stage of the pipeline. Most pipelined architectures are based on a sequential model of program execution in which a program counter sequences through instructions one by one.A fundamental disadvantage of pipelined processing is the loss incurred due to conditional branches. When a conditional branch instruction is encountered, more than one possible paths are following the instruction. The correct path can be known only upon the completion of the conditional branch instruction. The execution of the next instruction following a conditional branch cannot be started until the conditional branch instruction is resolved, resulting in stalling of the pipeline. One approach to avoid stalling is to predict the path to be executed and continue the execution of instructions along the predicted path. But in this case an incorrect prediction results in the execution of incorrect instructions. Hence . the results of these incorrect instructions have to be purged. Also, the instructions in the various stages of the pipeline must be removed and the pipeline has to start fetching instructions from the correct path. Thus incorrect prediction involves a flushing of the pipeline. This thesis proposes a twin processor architecture for reducing the effects of conditional branches. In such an architecture, both the paths following a conditional branch are executed simultaneously on two processors. When the conditional branch is resolved, the results of the incorrect path are discarded. Such an architecture requires a special purpose twin register file. It is the purpose of this thesis to design a twin register file consisting of two register files which can be independently accessed by the two processors. Each of the register files also has the capability of being copied into the other, making the design of the twin register file a complicated issue. The special pwpose twin register file is designed using layout tools Lager and Magic. The twin register file consists of two three-port register files which are capable of executing the 'read', 'write' and 'transfer' operations. The transfer of data from one register f.tle to another is accomplished in a single phase of the cl<X!k. The functionality of a 32-word-by-16-bit twin register file is verified by simulating it on IRSIM. The timing requirements for the read, write and transfer operations are detennined by simulating the twin register file on SPICE.