Resurrecting the SuperH Architecture (2015)

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Resurrecting the SuperH architecture

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By Nathan Willis June 10, 2015

LinuxCon Japan

Processor architectures are far from trivial; untold millions of dollars and many thousands of hours have likely gone into the creation and refinement of the x86 and ARM architectures that dominate the CPUs in Linux boxes today. But that does not mean that x86 and ARM are the only architectures of value, as Jeff Dionne, Rob Landley, and Shumpei Kawasaki illustrated in their LinuxCon Japan session "Turtles all the way down: running Linux on open hardware." The team has been working on breathing new life into a somewhat older architecture that offers comparable performance to many common system-on-chip (SoC) designs—and which can be produced as open hardware.

The architecture in question is Hitachi's SuperH, whose instruction set was a precursor to one used in many ARM Thumb CPUs. But the patents on the most important SuperH designs have all expired—and more will be expiring in the months and years to come—which makes SuperH a candidate for revival. Dionne, Landley, and Kawasaki's session [PDF] outlined the status of their SuperH-based "J2" core design, which can be synthesized in low-cost FGPAs or manufactured in bulk.

Dionne started off the talk by making a case for the value of running open-source software on open hardware. That is a familiar enough position, of course, but he went on to point out that a modern laptop contains many more ARM and MIPS processors than it does x86 processors. These small processors serve as USB and hard-drive controllers, run ACPI and low-level system management services, and much more. Thus, the notion of "taking control of your hardware" has to include these chips as well.

He then asked what constitutes the minimal system that can run Linux. All that is really needed, he said, is a flat, 32-bit memory address space, a CPU with registers to hold instructions, some I/O and storage (from which the kernel and initramfs can be loaded), and a timer for interrupts. That plus GCC is sufficient to get Linux running—although it may not be fast, depending on the specifics. One does not even need a cache, floating-point unit, SMP, or a memory-management unit (MMU).

At this point, Landley chimed in to point out that Dionne had been the maintainer of uClinux, which was an active project maintaining Linux on non-MMU systems up through 2003, when Dionne handed off maintainership to others where, unfortunately, development slowed down considerably. The requirements for running Linux are quite low, though; many of the open-hardware boards popular today (such as the Raspberry Pi) throw in all sorts of unnecessary extras.

That brings us to SuperH, which Dionne said was developed with a "massive research and development outlay." The SuperH SH2 was a highly optimized design, employing a five-stage Harvard RISC architecture with an instruction-set density considerably ahead of its contemporaries. That density is a common way to measure CPU efficiency, he explained; a dense architecture requires fewer instructions and thus fewer clock cycles to perform a given task. Most of a CPU's clock cycles are spent waiting for something, he said; waiting for instructions is such a bottleneck that if you can get them fast enough, "it almost doesn't matter what your clock speed is."

The SuperH architecture is so dense that a 2009 research paper [PDF] plotted it ahead of every architecture other than x86, x86_64, and CRIS v32. ARM even licensed the SuperH patent portfolio to create its Thumb instruction set in the mid-1990s.

Fortunately, the patents are now expiring. The last of the SH2 patents expired in 2014, with more to come. The SH2 processor was, he said, used in the Sega Saturn game console; the SH4 (found in the Sony Sega Dreamcast) will have the last of its patents expire in 2016. Though they are older chips, they were used in relatively powerful devices.

In preparation for this milestone, Dionne, Landley, and others have been working on J2, a clean-room re-implementation of the SH2 that is implemented as a "core design kit." The source for the core is written in VHDL, and it can be synthesized on a Xilinx Spartan6 FPGA. The Spartan6 is a low-cost platform (boards can be purchased for around $50), but it also contains enough room to add additional synthesized components—like a serial controller, memory controller, digital signal processor, and...

Resurrecting the SuperH Architecture (2015)

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