Glacial - microcoded RISC-V core designed for low FPGA resource utilization

Copyright 2018 Eric Smith [email protected]

Hosted at the glacial Github repository.

Introduction

Glacial is an RV32I core designed for the 2018 RISC-V SoftCPU Contest, for the "smallest implementation" categories. Glacial is implemented as a microcoded processor core with a very simple 8-bit data path. The microcode, scratchpad, and RISC-V memory are all stored in the same 8-bit wide RAM.

Glacial is compliant with the RISC-V Instruction Set Manual, Volume I: User Level ISA, Document Version 2.2, dated 2017-05-07.

Glacial implements only the minimal subset of the Volume II: Privileged Architecture specification needed to pass the RV32I compliance tests and to run the Zephyr kernel. Glacial implements M-mode only. Numerous deviations from the Privileged Architecture specification exist; for example, all of the implmented CSRs are fully writeable, though writing them may not have the intended effects.

Microarchitecture inspiration

In order to achieve minimal FPGA resource utilization, a microcoded architecture is used, trading off FPGA logic cells for additional static RAM and slower execution. Microcoding was invented by M.V. Wilkes, and described in "The Best Way to Design an Automated Calculating Machine", Manchester University Computer Inagural Conf., 1951, pp. 16-18.

The initial IBM System/360 machines announced in 1964 may have been the first mass-produced microcoded computers. The System/360 Model 30, in particular, is notable as it used microcoding with 8-bit data paths to implement a 32-bit architecture. For further information, see S.G. Tucker, "Microprogram control for System/360", IBM Systems Journal, 6(4), 1967, pp. 222-241.

Some specific features of the Glacial microarchitecture were drawn from well-known minicomputers and microprocessors of the 1960s and 1970s:

• DEC PDP-1 (1960): bit-mapped operate instruction (popularized by the later PDP-8, 1964)
• DEC PDP-5 (1963): short-form page zero addressing (popularized by the PDP-8)
• IBM System/360 Model 30 (1964): 32-bit processor architecture microcoded using 8-bit data paths
• IBM System/360 Model 25 (1968): microcode and scratchpad stored in same physical memory array as user memory
• DEC PDP-11 (1970): register indirect with postincrement memory addressing mode
• General Instruments PIC1650 (1976): skip on memory bit set or clear (popularized by Microchip PIC16C family)

The doc directory contains a Glacial Microarchitecture document in LaTeX, available in PDF form in the Glacial wiki.

Acknowledgements

Antti Lukats provided a huge amount of assistance, including a bootloader for the Microsemi SmartFusion2, and huge amounts of general advice on the mailing list and on a wiki he set up.

Charles Papon provided an example of assembly language code to output the compliance test result signatures to a UART.

Nelson Ribeiro provided advice on porting the Zephyr RTOS to a new platform.

Thanks also to the RISC-V Foundation and its sponsors for organizing the contest, and to Microsemi and Lattice for providing FPGA development boards.

Hardware Requirements

The Glacial core is written in non-vendor-specific Verilog, and should synthesize for any FPGA. Glacial was specifically intended to work with two of the FPGA boards specified for the contest:

• Lattice iCE40 UltraPlus iCE40UP5K, using the iCE40 UltraPlus MDP board
• Microsemi SmartFusion2 M2S025, using the Future Electronics Creative Development Board

Software Requirements

All development was done on Linux, and instructions are only provided for Linux. The author specifically used Fedora 28 on an x86_64 platform.

• Building the microcode and memory images requires GNU Make and Python 3.
• Verilog simulation requires Verilator.
• Compiling the RISC-V compliance tests requires the toolchain provided by the RISC-V GNU Compiler Toolchain: https://github.com/riscv/riscv-gnu-toolchain (The compliance tests will not easily build with the Zephyr SDK toolchain due to problems with compiler options)
• Compiling Zephyr requires the Zephyr SDK.
• Building the FPGA image for the Lattice iCE40 UltraPlus requires Lattice iCEcube2 software.
• Building the FPGA image for the Microsemi SmartFusion2 requires Microsemi Libero SoC Design Software.

Subdirectories

• ucode: microcode source code
• ucode/tools: microcode assembler, simulator, memory utility
• verilog: Verilog core and testbench
• riscv-compliance: compliance tests (use "glacial" branch)
• zephyr: Zephyr RTOS and examples (use "glacial" branch)

Instructions

install/build RISC-V GNU Compiler Toolchain

This must be done to produce a 32-bit toolchain, which seems quite tricky, and the details are beyond the scope of this README.

Set the path to the tools (assuming installed in /opt/riscv32):

export PATH=/opt/riscv32/bin:$PATH

install Zephyr SDK

This must be done to get the toolchain used for the Zephyr demos. The installation details are beyond the scope of this README.

Set the path to the tools (assuming installed in /opt/zephyr-sdk):

export PATH=/opt/zephyr-sdk/sysroots/x86_64-pokysdk-linux/usr/bin/riscv32-zephyr-elf/:$PATH
export ZEPHYR_TOOLCHAIN_VARIANT=zephyr
export ZEPHYR_SDK_INSTALL_DIR=/opt/zephyr-sdk

clone repositories and set GLACIAL environment variable

git clone --recurse-submodules https://github.com/brouhaha/glacial.git
cd glacial
export GLACIAL=`pwd`

build microcode

make -C ucode

build Verilator simulator

make -C verilog

run RV32I compliance tests on Verilator simulator

make -C riscv-compliance RISCV_PREFIX=riscv32-unknown-elf- RISCV_TARGET=glacial RISCV_DEVICE=rv32i

This will report "OK: 55/55" to indicate that all 55 rv32i tests passed, then will attempt to run rv32im tests, which is expected to fail, as Glacial only supports rv32i. This appears to be due to a design defect in the riscv-compliance top-level Makefile.

build Zephyr (NOT WORKING)

cd zephyr
source zephyr-env.sh
cd samples/philosophers
mkdir build-glacial
cd build-glacial
cmake -DBOARD=glacial ..
make
cd ../synchronization
mkdir build-glacial
cd build-glacial
cmake -DBOARD=glacial ..
make
cd ../..