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RISC-V Core

Github: http://github.com/ultraembedded/riscv

A 32-bit RISC-V core written in Verilog and an instruction set simulator supporting RV32IM.
This core has been tested against a co-simulation model and exercised on FPGA.

For a higher performance dual issue CPU with branch prediction, see my latest RISC-V core here; http://github.com/ultraembedded/biriscv

Overview

Features

  • 32-bit RISC-V ISA CPU core.
  • Support RISC-V integer (I), multiplication and division (M), and CSR instructions (Z) extensions (RV32IMZicsr).
  • Supports user, supervisor and machine mode privilege levels.
  • Basic MMU support - capable of booting Linux with atomics (RV-A) SW emulation.
  • Implements base ISA spec v2.1 and privileged ISA spec v1.11.
  • Verified using Google's RISCV-DV random instruction sequences using cosimulation against C++ ISA model.
  • Support for instruction / data cache, AXI bus interfaces or tightly coupled memories.
  • Configurable number of pipeline stages and result forwarding options.
  • Synthesizable Verilog 2001, Verilator and FPGA friendly.
  • Coremark: 2.94 CoreMark/MHz
  • Dhrystone: 1.25 DMIPS/MHz ('legal compile options' / 337 instructions per iteration)
  • Want higher performance (4.1CM/MHz / 1.9DMIPS/MHz) - see my improved core.

Configuration

Param Name Valid Range Description
SUPPORT_SUPER 1/0 Enable supervisor / user privilege levels.
SUPPORT_MMU 1/0 Enable basic memory management unit.
SUPPORT_MULDIV 1/0 Enable HW multiply / divide (RV-M).
SUPPORT_LOAD_BYPASS 1/0 Support load result bypass paths.
SUPPORT_MUL_BYPASS 1/0 Support multiply result bypass paths.
SUPPORT_REGFILE_XILINX 1/0 Support Xilinx optimised register file.
EXTRA_DECODE_STAGE 1/0 Extra decode pipe stage for improved timing.
MEM_CACHE_ADDR_MIN 32'h0 - 32'hffffffff Lowest cacheable memory address.
MEM_CACHE_ADDR_MAX 32'h0 - 32'hffffffff Highest cacheable memory address.

Directories

Name Contents
core/riscv RISC-V pipelined RV32IM CPU core (Verilog)
isa_sim Instruction set simulator (C)
top_tcm_axi/src_v Example instance with 64KB DP-RAM & AXI Interfaces
top_tcm_axi/tb System-C testbench for the core
top_cache_axi/src_v Example instance with instruction and data caches.
top_cache_axi/tb System-C testbench for the core

Example Core Instance (with TCM memory)

The top (top_tcm_axi/src_v/riscv_tcm_top.v) contains;

  • Instances one of the above cores, adding RAM and standard bus interfaces.
  • 64KB dual ported RAM for (I/D code and data).
  • AXI4 slave port for loading the RAM, DMA access, etc (including support for burst access).
  • AXI4-Lite master port for CPU access to peripherals.
  • Separate reset for CPU core to dual ported RAM / AXI interface (to allow program code to be loaded prior to CPU reset de-assertion).

Memory Map

Range Description
0x0000_0000 - 0x0000_ffff 64KB TCM Memory
0x0000_2000 Boot address (configurable, see RISCV_BOOT_ADDRESS)
0x8000_0000 - 0xffff_ffff Peripheral address space (from AXI4-L port)

Interfaces

Name Description
clk_i Clock input
rst_i Async reset, active-high. Reset memory / AXI interface.
rst_cpu_i Async reset, active-high. Reset CPU core (excluding AXI / memory).
axi_t_* AXI4 slave interface for access to 64KB TCM memory.
axi_i_* AXI4-Lite master interface for CPU access to peripherals.
intr_i Active high interrupt input (for connection external int controller).

Testbench

A basic System-C / Verilator based testbench for the core is provided.

Dependencies;

  • gcc
  • make
  • libelf
  • System-C (specify path using SYSTEMC_HOME)
  • Verilator (specify path using VERILATOR_SRC)

To build the testbench;

cd top_tcm_axi/tb
make

To run the provided test executable;

cd top_tcm_axi/tb
make run

Example Core Instance (with caches)

The top (top_cache_axi/src_v/riscv_top.v) contains;

  • Instances one of the above cores, adding RAM and standard bus interfaces.
  • 16KB 2-way set associative instruction cache
  • 16KB 2-way set associative data cache with write-back and allocate on write.
  • 2 x AXI4 master port for CPU access to instruction / data / peripherals.

Interfaces

Name Description
clk_i Clock input
rst_i Async reset, active-high. Reset memory / AXI interface.
axi_i_* AXI4 master interface for CPU access to instruction memory.
axi_d_* AXI4 master interface for CPU access to data / peripheral memories.
intr_i Active high interrupt input (for connection external int controller).
reset_vector_i Boot vector.

Execution Example