PicoRV32 is a CPU core that implements the RISC-V RV32IMC Instruction Set. It can be configured as RV32E, RV32I, RV32IC, RV32IM, or RV32IMC core, and optionally contains a built-in interrupt controller.
Tools (gcc, binutils, etc..) can be obtained via the RISC-V Website.
The examples bundled with PicoRV32 expect various RV32 toolchains to be installed in /opt/riscv32i[m][c]. See
the build instructions below for details.
Many Linux distributions now include the tools for RISC-V (for example
Ubuntu 20.04 has gcc-riscv64-unknown-elf). To compile using those set
TOOLCHAIN_PREFIX accordingly (eg. make TOOLCHAIN_PREFIX=riscv64-unknown-elf-).
PicoRV32 is free and open hardware licensed under the ISC license (a license that is similar in terms to the MIT license or the 2-clause BSD license).
- Features and Typical Applications
- Files in this Repository
- Verilog Module Parameters
- Cycles per Instruction Performance
- PicoRV32 Native Memory Interface
- Pico Co-Processor Interface (PCPI)
- Custom Instructions for IRQ Handling
- Building a pure RV32I Toolchain
- Linking binaries with newlib for PicoRV32
- Evaluation: Timing and Utilization on Xilinx 7-Series FPGAs
- Small (750-2000 LUTs in 7-Series Xilinx Architecture)
- High fmax (250-450 MHz on 7-Series Xilinx FPGAs)
- Selectable native memory interface or AXI4-Lite master
- Optional IRQ support (using a simple custom ISA)
- Optional Co-Processor Interface
This CPU is meant to be used as auxiliary processor in FPGA designs and ASICs. Due to its high fmax it can be integrated in most existing designs without crossing clock domains. When operated on a lower frequency, it will have a lot of timing slack and thus can be added to a design without compromising timing closure.
For even smaller size it is possible disable support for registers x16..x31 as
well as RDCYCLE[H], RDTIME[H], and RDINSTRET[H] instructions, turning the
processor into an RV32E core.
Furthermore it is possible to choose between a dual-port and a single-port register file implementation. The former provides better performance while the latter results in a smaller core.
Note: In architectures that implement the register file in dedicated memory resources, such as many FPGAs, disabling the 16 upper registers and/or disabling the dual-port register file may not further reduce the core size.
The core exists in three variations: picorv32, picorv32_axi and picorv32_wb.
The first provides a simple native memory interface, that is easy to use in simple
environments. picorv32_axi provides an AXI-4 Lite Master interface that can
easily be integrated with existing systems that are already using the AXI
standard. picorv32_wb provides a Wishbone master interface.
A separate core picorv32_axi_adapter is provided to bridge between the native
memory interface and AXI4. This core can be used to create custom cores that
include one or more PicoRV32 cores together with local RAM, ROM, and
memory-mapped peripherals, communicating with each other using the native
interface, and communicating with the outside world via AXI4.
The optional IRQ feature can be used to react to events from the outside, implement fault handlers, or catch instructions from a larger ISA and emulate them in software.
The optional Pico Co-Processor Interface (PCPI) can be used to implement
non-branching instructions in an external coprocessor. Implementations
of PCPI cores that implement the M Standard Extension instructions
MUL[H[SU|U]] and DIV[U]/REM[U] are included in this package.
You are reading it right now.
This Verilog file contains the following Verilog modules:
| Module | Description |
|---|---|
picorv32 |
The PicoRV32 CPU |
picorv32_axi |
The version of the CPU with AXI4-Lite interface |
picorv32_axi_adapter |
Adapter from PicoRV32 Memory Interface to AXI4-Lite |
picorv32_wb |
The version of the CPU with Wishbone Master interface |
picorv32_pcpi_mul |
A PCPI core that implements the MUL[H[SU|U]] instructions |
picorv32_pcpi_fast_mul |
A version of picorv32_pcpi_fast_mul using a single cycle multiplier |
picorv32_pcpi_div |
A PCPI core that implements the DIV[U]/REM[U] instructions |
Simply copy this file into your project.
A basic test environment. Run make test to run the standard test bench (testbench.v)
in the standard configurations. There are other test benches and configurations. See
the test_* make target in the Makefile for details.
Run make test_ez to run testbench_ez.v, a very simple test bench that does
not require an external firmware .hex file. This can be useful in environments
where the RISC-V compiler toolchain is not available.
Note: The test bench is using Icarus Verilog. However, Icarus Verilog 0.9.7 (the latest release at the time of writing) has a few bugs that prevent the test bench from running. Upgrade to the latest github master of Icarus Verilog to run the test bench.
A simple test firmware. This runs the basic tests from tests/, some C code, tests IRQ
handling and the multiply PCPI core.
All the code in firmware/ is in the public domain. Simply copy whatever you can use.
Simple instruction-level tests from riscv-tests.
Another simple test firmware that runs the Dhrystone benchmark.
A simple example SoC using PicoRV32 that can execute code directly from a memory mapped SPI flash.
Various scripts and examples for different (synthesis) tools and hardware architectures.
The following Verilog module parameters can be used to configure the PicoRV32 core.
This parameter enables support for the RDCYCLE[H], RDTIME[H], and
RDINSTRET[H] instructions. This instructions will cause a hardware
trap (like any other unsupported instruction) if ENABLE_COUNTERS is set to zero.
Note: Strictly speaking the RDCYCLE[H], RDTIME[H], and RDINSTRET[H]
instructions are not optional for an RV32I core. But chances are they are not
going to be missed after the application code has been debugged and profiled.
This instructions are optional for an RV32E core.
This parameter enables support for the RDCYCLEH, RDTIMEH, and RDINSTRETH
instructions. If this parameter is set to 0, and ENABLE_COUNTERS is set to 1,
then only the RDCYCLE, RDTIME, and RDINSTRET instructions are available.
This parameter enables support for registers the x16..x31. The RV32E ISA
excludes this registers. However, the RV32E ISA spec requires a hardware trap
for when code tries to access this registers. This is not implemented in PicoRV32.
The register file can be implemented with two or one read ports. A dual ported register file improves performance a bit, but can also increase the size of the core.
Set this to 1 if the mem_rdata is kept stable by the external circuit after a
transaction. In the default configuration the PicoRV32 core only expects the
mem_rdata input to be valid in the cycle with mem_valid && mem_ready and
latches the value internally.
This parameter is only available for the picorv32 core. In the
picorv32_axi and picorv32_wb core this is implicitly set to 0.
By default shift operations are performed in two stages: first shifts in units of 4 bits and then shifts in units of 1 bit. This speeds up shift operations, but adds additional hardware. Set this parameter to 0 to disable the two-stage shift to further reduce the size of the core.
By default shift operations are performed by successively shifting by a
small amount (see TWO_STAGE_SHIFT above). With this option set, a barrel
shifter is used instead.
This relaxes the longest data path a bit by adding an additional FF stage at the cost of adding an additional clock cycle delay to the conditional branch instructions.
Note: Enabling this parameter will be most effective when retiming (aka "register balancing") is enabled in the synthesis flow.
This adds an additional FF stage in the ALU data path, improving timing at the cost of an additional clock cycle for all instructions that use the ALU.
Note: Enabling this parameter will be most effective when retiming (aka "register balancing") is enabled in the synthesis flow.
This enables support for the RISC-V Compressed Instruction Set.
Set this to 0 to disable the circuitry for catching misaligned memory accesses.
Set this to 0 to disable the circuitry for catching illegal instructions.
The core will still trap on EBREAK instructions with this option
set to 0. With IRQs enabled, an EBREAK normally triggers an IRQ 1. With
this option set to 0, an EBREAK will trap the processor without
triggering an interrupt.
Set this to 1 to enable the Pico Co-Processor Interface (PCPI).
This parameter internally enables PCPI and instantiates the picorv32_pcpi_mul
core that implements the MUL[H[SU|U]] instructions. The external PCPI
interface only becomes functional when ENABLE_PCPI is set as well.
This parameter internally enables PCPI and instantiates the picorv32_pcpi_fast_mul
core that implements the MUL[H[SU|U]] instructions. The external PCPI
interface only becomes functional when ENABLE_PCPI is set as well.
If both ENABLE_MUL and ENABLE_FAST_MUL are set then the ENABLE_MUL setting will be ignored and the fast multiplier core will be instantiated.
This parameter internally enables PCPI and instantiates the picorv32_pcpi_div
core that implements the DIV[U]/REM[U] instructions. The external PCPI
interface only becomes functional when ENABLE_PCPI is set as well.
Set this to 1 to enable IRQs. (see "Custom Instructions for IRQ Handling" below for a discussion of IRQs)
Set this to 0 to disable support for the getq and setq instructions. Without
the q-registers, the irq return address will be stored in x3 (gp) and the IRQ
bitmask in x4 (tp), the global pointer and thread pointer registers according
to the RISC-V ABI. Code generated from ordinary C code will not interact with
those registers.
Support for q-registers is always disabled when ENABLE_IRQ is set to 0.
Set this to 0 to disable support for the timer instruction.
Support for the timer is always disabled when ENABLE_IRQ is set to 0.
Produce an execution trace using the trace_valid and trace_data output ports.
For a demontration of this feature run make test_vcd to create a trace file
and then run python3 showtrace.py testbench.trace firmware/firmware.elf to decode
it.
Set this to 1 to initialize all registers to zero (using a Verilog initial block).
This can be useful for simulation or formal verification.
A 1 bit in this bitmask corresponds to a permanently disabled IRQ.
A 1 bit in this bitmask indicates that the corresponding IRQ is "latched", i.e. when the IRQ line is high for only one cycle, the interrupt will be marked as pending and stay pending until the interrupt handler is called (aka "pulse interrupts" or "edge-triggered interrupts").
Set a bit in this bitmask to 0 to convert an interrupt line to operate as "level sensitive" interrupt.
The start address of the program.
The start address of the interrupt handler.
When this parameter has a value different from 0xffffffff, then register x2 (the
stack pointer) is initialized to this value on reset. (All other registers remain
uninitialized.) Note that the RISC-V calling convention requires the stack pointer
to be aligned on 16 bytes boundaries (4 bytes for the RV32I soft float calling
convention).
A short reminder: This core is optimized for size and fmax, not performance.
Unless stated otherwise, the following numbers apply to a PicoRV32 with ENABLE_REGS_DUALPORT active and connected to a memory that can accommodate requests within one clock cycle.
The average Cycles per Instruction (CPI) is approximately 4, depending on the mix of instructions in the code. The CPI numbers for the individual instructions can be found in the table below. The column "CPI (SP)" contains the CPI numbers for a core built without ENABLE_REGS_DUALPORT.
| Instruction | CPI | CPI (SP) |
|---|---|---|
| direct jump (jal) | 3 | 3 |
| ALU reg + immediate | 3 | 3 |
| ALU reg + reg | 3 | 4 |
| branch (not taken) | 3 | 4 |
| memory load | 5 | 5 |
| memory store | 5 | 6 |
| branch (taken) | 5 | 6 |
| indirect jump (jalr) | 6 | 6 |
| shift operations | 4-14 | 4-15 |
When ENABLE_MUL is activated, then a MUL instruction will execute
in 40 cycles and a MULH[SU|U] instruction will execute in 72 cycles.
When ENABLE_DIV is activated, then a DIV[U]/REM[U] instruction will
execute in 40 cycles.
When BARREL_SHIFTER is activated, a shift operation takes as long as
any other ALU operation.
The following dhrystone benchmark results are for a core with enabled
ENABLE_FAST_MUL, ENABLE_DIV, and BARREL_SHIFTER options.
Dhrystone benchmark results: 0.516 DMIPS/MHz (908 Dhrystones/Second/MHz)
For the Dhrystone benchmark the average CPI is 4.100.
Without using the look-ahead memory interface (usually required for max clock speed), this results drop to 0.305 DMIPS/MHz and 5.232 CPI.
The native memory interface of PicoRV32 is a simple valid-ready interface that can run one memory transfer at a time:
output mem_valid
output mem_instr
input mem_ready
output [31:0] mem_addr
output [31:0] mem_wdata
output [ 3:0] mem_wstrb
input [31:0] mem_rdata
The core initiates a memory transfer by asserting mem_valid. The valid
signal stays high until the peer asserts mem_ready. All core outputs
are stable over the mem_valid period. If the memory transfer is an
instruction fetch, the core asserts mem_instr.
In a read transfer mem_wstrb has the value 0 and mem_wdata is unused.
The memory reads the address mem_addr and makes the read value available on
mem_rdata in the cycle mem_ready is high.
There is no need for an external wait cycle. The memory read can be implemented
asynchronously with mem_ready going high in the same cycle as mem_valid, or
mem_ready being tied to constant 1.
In a write transfer mem_wstrb is not 0 and mem_rdata is unused. The memory
write the data at mem_wdata to the address mem_addr and acknowledges the
transfer by asserting mem_ready.
The 4 bits of mem_wstrb are write enables for the four bytes in the addressed
word. Only the 8 values 0000, 1111, 1100, 0011, 1000, 0100, 0010,
and 0001 are possible, i.e. no write, write 32 bits, write upper 16 bits,
write lower 16, or write a single byte respectively.
There is no need for an external wait cycle. The memory can acknowledge the
write immediately with mem_ready going high in the same cycle as
mem_valid, or mem_ready being tied to constant 1.
The PicoRV32 core also provides a "Look-Ahead Memory Interface" that provides all information about the next memory transfer one clock cycle earlier than the normal interface.
output mem_la_read
output mem_la_write
output [31:0] mem_la_addr
output [31:0] mem_la_wdata
output [ 3:0] mem_la_wstrb
In the clock cycle before mem_valid goes high, this interface will output a
pulse on mem_la_read or mem_la_write to indicate the start of a read or
write transaction in the next clock cycle.
Note: The signals mem_la_read, mem_la_write, and mem_la_addr are driven
by combinatorial circuits within the PicoRV32 core. It might be harder to
achieve timing closure with the look-ahead interface than with the normal
memory interface described above.
The Pico Co-Processor Interface (PCPI) can be used to implement non-branching instructions in external cores:
output pcpi_valid
output [31:0] pcpi_insn
output [31:0] pcpi_rs1
output [31:0] pcpi_rs2
input pcpi_wr
input [31:0] pcpi_rd
input pcpi_wait
input pcpi_ready
When an unsupported instruction is encountered and the PCPI feature is
activated (see ENABLE_PCPI above), then pcpi_valid is asserted, the
instruction word itself is output on pcpi_insn, the rs1 and rs2
fields are decoded and the values in those registers are output
on pcpi_rs1 and pcpi_rs2.
An external PCPI core can then decode the instruction, execute it, and assert
pcpi_ready when execution of the instruction is finished. Optionally a
result value can be written to pcpi_rd and pcpi_wr asserted. The
PicoRV32 core will then decode the rd field of the instruction and
write the value from pcpi_rd to the respective register.
When no external PCPI core acknowledges the instruction within 16 clock
cycles, then an illegal instruction exception is raised and the respective
interrupt handler is called. A PCPI core that needs more than a couple of
cycles to execute an instruction, should assert pcpi_wait as soon as
the instruction has been decoded successfully and keep it asserted until
it asserts pcpi_ready. This will prevent the PicoRV32 core from raising
an illegal instruction exception.
Note: The IRQ handling features in PicoRV32 do not follow the RISC-V Privileged ISA specification. Instead a small set of very simple custom instructions is used to implement IRQ handling with minimal hardware overhead.
The following custom instructions are only supported when IRQs are enabled
via the ENABLE_IRQ parameter (see above).
The PicoRV32 core has a built-in interrupt controller with 32 interrupt inputs. An
interrupt can be triggered by asserting the corresponding bit in the irq
input of the core.
When the interrupt handler is started, the eoi End Of Interrupt (EOI) signals
for the handled interrupts go high. The eoi signals go low again when the
interrupt handler returns.
The IRQs 0-2 can be triggered internally by the following built-in interrupt sources:
| IRQ | Interrupt Source |
|---|---|
| 0 | Timer Interrupt |
| 1 | EBREAK/ECALL or Illegal Instruction |
| 2 | BUS Error (Unalign Memory Access) |
This interrupts can also be triggered by external sources, such as co-processors connected via PCPI.
The core has 4 additional 32-bit registers q0 .. q3 that are used for IRQ
handling. When the IRQ handler is called, the register q0 contains the return
address and q1 contains a bitmask of all IRQs to be handled. This means one
call to the interrupt handler needs to service more than one IRQ when more than
one bit is set in q1.
When support for compressed instructions is enabled, then the LSB of q0 is set when the interrupted instruction is a compressed instruction. This can be used if the IRQ handler wants to decode the interrupted instruction.
Registers q2 and q3 are uninitialized and can be used as temporary storage
when saving/restoring register values in the IRQ handler.
All of the following instructions are encoded under the custom0 opcode. The f3
and rs2 fields are ignored in all this instructions.
See firmware/custom_ops.S for GNU assembler macros that implement mnemonics for this instructions.
See firmware/start.S for an example implementation of an interrupt handler assembler wrapper, and firmware/irq.c for the actual interrupt handler.
This instruction copies the value from a q-register to a general-purpose register.
0000000 ----- 000XX --- XXXXX 0001011
f7 rs2 qs f3 rd opcode
Example:
getq x5, q2
This instruction copies the value from a general-purpose register to a q-register.
0000001 ----- XXXXX --- 000XX 0001011
f7 rs2 rs f3 qd opcode
Example:
setq q2, x5
Return from interrupt. This instruction copies the value from q0
to the program counter and re-enables interrupts.
0000010 ----- 00000 --- 00000 0001011
f7 rs2 rs f3 rd opcode
Example:
retirq
The "IRQ Mask" register contains a bitmask of masked (disabled) interrupts. This instruction writes a new value to the irq mask register and reads the old value.
0000011 ----- XXXXX --- XXXXX 0001011
f7 rs2 rs f3 rd opcode
Example:
maskirq x1, x2
The processor starts with all interrupts disabled.
An illegal instruction or bus error while the illegal instruction or bus error interrupt is disabled will cause the processor to halt.
Pause execution until an interrupt becomes pending. The bitmask of pending IRQs
is written to rd.
0000100 ----- 00000 --- XXXXX 0001011
f7 rs2 rs f3 rd opcode
Example:
waitirq x1
Reset the timer counter to a new value. The counter counts down clock cycles and
triggers the timer interrupt when transitioning from 1 to 0. Setting the
counter to zero disables the timer. The old value of the counter is written to
rd.
0000101 ----- XXXXX --- XXXXX 0001011
f7 rs2 rs f3 rd opcode
Example:
timer x1, x2
TL;DR: Run the following commands to build the complete toolchain:
make download-tools
make -j$(nproc) build-tools
The default settings in the riscv-tools build scripts will build a compiler, assembler and linker that can target any RISC-V ISA, but the libraries are built for RV32G and RV64G targets. Follow the instructions below to build a complete toolchain (including libraries) that target a pure RV32I CPU.
The following commands will build the RISC-V GNU toolchain and libraries for a
pure RV32I target, and install it in /opt/riscv32i:
# Ubuntu packages needed:
sudo apt-get install autoconf automake autotools-dev curl libmpc-dev \
libmpfr-dev libgmp-dev gawk build-essential bison flex texinfo \
gperf libtool patchutils bc zlib1g-dev git libexpat1-dev
sudo mkdir /opt/riscv32i
sudo chown $USER /opt/riscv32i
git clone https://github.com/riscv/riscv-gnu-toolchain riscv-gnu-toolchain-rv32i
cd riscv-gnu-toolchain-rv32i
git checkout 411d134
git submodule update --init --recursive
mkdir build; cd build
../configure --with-arch=rv32i --prefix=/opt/riscv32i
make -j$(nproc)
The commands will all be named using the prefix riscv32-unknown-elf-, which
makes it easy to install them side-by-side with the regular riscv-tools (those
are using the name prefix riscv64-unknown-elf- by default).
Alternatively you can simply use one of the following make targets from PicoRV32's
Makefile to build a RV32I[M][C] toolchain. You still need to install all
prerequisites, as described above. Then run any of the following commands in the
PicoRV32 source directory:
| Command | Install Directory | ISA |
|---|---|---|
make -j$(nproc) build-riscv32i-tools |
/opt/riscv32i/ |
RV32I |
make -j$(nproc) build-riscv32ic-tools |
/opt/riscv32ic/ |
RV32IC |
make -j$(nproc) build-riscv32im-tools |
/opt/riscv32im/ |
RV32IM |
make -j$(nproc) build-riscv32imc-tools |
/opt/riscv32imc/ |
RV32IMC |
Or simply run make -j$(nproc) build-tools to build and install all four tool chains.
By default calling any of those make targets will (re-)download the toolchain
sources. Run make download-tools to download the sources to /var/cache/distfiles/
once in advance.
Note: These instructions are for git rev 411d134 (2018-02-14) of riscv-gnu-toolchain.
The tool chains (see last section for install instructions) come with a version of the newlib C standard library.
Use the linker script firmware/riscv.ld for linking binaries against the newlib library. Using this linker script will create a binary that has its entry point at 0x10000. (The default linker script does not have a static entry point, thus a proper ELF loader would be needed that can determine the entry point at runtime while loading the program.)
Newlib comes with a few syscall stubs. You need to provide your own implementation
of those syscalls and link your program with this implementation, overwriting the
default stubs from newlib. See syscalls.c in scripts/cxxdemo/
for an example of how to do that.
The following evaluations have been performed with Vivado 2017.3.
The picorv32_axi module with enabled TWO_CYCLE_ALU has been placed and
routed for Xilinx Artix-7T, Kintex-7T, Virtex-7T, Kintex UltraScale, and Virtex
UltraScale devices in all speed grades. A binary search is used to find the
shortest clock period for which the design meets timing.
See make table.txt in scripts/vivado/.
| Device | Device | Speedgrade | Clock Period (Freq.) |
|---|---|---|---|
| Xilinx Kintex-7T | xc7k70t-fbg676-2 | -2 | 2.4 ns (416 MHz) |
| Xilinx Kintex-7T | xc7k70t-fbg676-3 | -3 | 2.2 ns (454 MHz) |
| Xilinx Virtex-7T | xc7v585t-ffg1761-2 | -2 | 2.3 ns (434 MHz) |
| Xilinx Virtex-7T | xc7v585t-ffg1761-3 | -3 | 2.2 ns (454 MHz) |
| Xilinx Kintex UltraScale | xcku035-fbva676-2-e | -2 | 2.0 ns (500 MHz) |
| Xilinx Kintex UltraScale | xcku035-fbva676-3-e | -3 | 1.8 ns (555 MHz) |
| Xilinx Virtex UltraScale | xcvu065-ffvc1517-2-e | -2 | 2.1 ns (476 MHz) |
| Xilinx Virtex UltraScale | xcvu065-ffvc1517-3-e | -3 | 2.0 ns (500 MHz) |
| Xilinx Kintex UltraScale+ | xcku3p-ffva676-2-e | -2 | 1.4 ns (714 MHz) |
| Xilinx Kintex UltraScale+ | xcku3p-ffva676-3-e | -3 | 1.3 ns (769 MHz) |
| Xilinx Virtex UltraScale+ | xcvu3p-ffvc1517-2-e | -2 | 1.5 ns (666 MHz) |
| Xilinx Virtex UltraScale+ | xcvu3p-ffvc1517-3-e | -3 | 1.4 ns (714 MHz) |
The following table lists the resource utilization in area-optimized synthesis for the following three cores:
-
PicoRV32 (small): The
picorv32module without counter instructions, without two-stage shifts, with externally latchedmem_rdata, and without catching of misaligned memory accesses and illegal instructions. -
PicoRV32 (regular): The
picorv32module in its default configuration. -
PicoRV32 (large): The
picorv32module with enabled PCPI, IRQ, MUL, DIV, BARREL_SHIFTER, and COMPRESSED_ISA features.
See make area in scripts/vivado/.
| Core Variant | Slice LUTs | LUTs as Memory | Slice Registers |
|---|---|---|---|
| PicoRV32 (small) | 761 | 48 | 442 |
| PicoRV32 (regular) | 917 | 48 | 583 |
| PicoRV32 (large) | 2019 | 88 | 1085 |