  Energy efficiency has become an increasingly important concern in computer
architecture due to the end of Dennard scaling. Heterogeneity has been explored
as a way to achieve better energy efficiency and heterogeneous
microarchitecture chips have become common in the mobile setting.
  Recent research has explored using heterogeneous-ISA, heterogeneous
microarchitecture, general-purpose cores to achieve further energy efficiency
gains. However, there is no open-source hardware implementation of a
heterogeneous-ISA processor available for research, and effective research on
heterogeneous-ISA processors necessitates the emulation speed provided by FPGA
prototyping. This work describes our experiences creating JuxtaPiton by
integrating a small RISC-V core into the OpenPiton framework, which uses a
modified OpenSPARC T1 core. This is the first time a new core has been
integrated with the OpenPiton framework, and JuxtaPiton is the first
open-source, general-purpose, heterogeneous-ISA processor. JuxtaPiton inherits
all the capabilities of OpenPiton, including vital FPGA emulation
infrastructure which can boot full-stack Debian Linux. Using this
infrastructure, we investigate area and timing effects of using the new RISC-V
core on FPGA and the performance of the new core running microbenchmarks.
