ultraembedded / Riscv
Licence: bsd-3-clause
RISC-V CPU Core (RV32IM)
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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
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