Abstract A Comparison of Scalable Superscalar Processors

The poor scalability of existing superscalar processors has been of great concern to the computer engineering community. In particular, the critical-path lengths of many components in existing implementations grow as O(n’) where n is the fetch width, the issue width, or the window size. This paper describes two scalable processor architectures, the Ultrascalar I and the Ultrascalar II, and compares their VLSI complexities (gate delays, wire-length delays, and area.) Both processors are implemented by a large collection of ALUs with controllers (together called execution stations) connected together by a network of parallel-prefix tree circuits. A fattree network connects an interleaved cache to the execution stations. These networks provide the full functionality of superscalar processors including renaming, out-of-order execution, and speculative execution. The difference between the processors is in the mechanism used to transmit register values from one execution station to another. Both architectures use a parallel-prefix tree to communicate the register values between the execution stations. The Ultrascalar I transmits an entire copy of the register file to each station, and the station chooses which register values it needs based on the instruction. The Ultrascalar I uses an H-tree layout. The Ultrascalar II uses a mesh-of-trees and carefully sends only the register values that will actually be needed by each subtree to reduce the number of wires required on the chip. The complexity results are as follows: The complexity is described for a processor which has an instruction-set architecture containing L logical registers and can execute n instructions in parallel. The chip provides enough memory bandwidth to execute up to M(n) memory operations per cycle. (M is assumed to have a certain regularity property.) In all the processors, the VLSI area is the square of the wire delay. The Ultrascalar I has gate delay O(log n) and wire-dela