PicoRV32 - A Size-Optimized RISC-V CPU

PicoRV32 is a CPU core that implements the RISC-V RV32I Instruction Set.

Tools (gcc, binutils, etc..) can be obtained via the RISC-V Website.

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:

• Small (~1000 LUTs in a 7-Series Xilinx FGPA)
• High fMAX (~250 MHz on 7-Series Xilinx FGPAs)
• Selectable native memory interface or AXI4-Lite master

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 single-port and a dual-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 two variations: picorv32 and picorv32_axi. The former provides a simple native memory interface, that is easy to use in simple environments, and the latter provides an AXI-4 Lite Master interface that can easily be integrated with existing systems that are already using the AXI standard.

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.

Parameters:

The following Verilog module parameters can be used to configure the PicoRV32 core.

ENABLE_COUNTERS (default = 1)

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.

ENABLE_REGS_16_31 (default = 1)

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.

ENABLE_REGS_DUALPORT (default = 1)

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.

LATCHED_MEM_RDATA (default = 0)

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.

ENABLE_PCPI (default = 0)

Set this to 1 to enable the Pico Co-Processor Interface (PCPI).

ENABLE_IRQ (default = 0)

Set this to 1 to enable IRQs.

MASKED_IRQ (default = 32'h 0000_0000)

A 1 bit in this bitmask corresponds to a permanently disabled IRQ.

PROGADDR_RESET (default = 32'h 0000_0000)

The start address of the program.

PROGADDR_IRQ (default = 32'h 0000_0010)

The start address of the interrupt handler.

Performance:

A short reminder: This core is optimized for size, not performance.

Unless stated otherwise, the following numbers apply to a PicoRV32 with ENABLE_REGS_DUALPORT active and connected to a memory that can accomodate requests within one clock cycle.

The average Cycles per Instruction (CPI) is 4 to 5, 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

Dhrystone benchmark results: 0.309 DMIPS/MHz (544 Dhrystones/Second/MHz)

For the Dhrystone benchmark the average CPI is 4.167.

PicoRV32 Native Memory Interface

This section is under construction.

Pico Co-Processor Interface (PCPI)

This section is under construction.

Custom Instructions for IRQ Handling

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 interrupts. 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	SBREAK or Illegal Instruction
2	BUS Error (Unalign Memory Access)

The core has 4 additional 32-bit registers q0 .. q3 that are used for IRQ handling. When an IRQ triggers, the register q0 contains the return address and q1 contains a bitmask of all active IRQs. This means one call to the interrupt handler might need to service more than one IRQ when more than one bit is set in q1.

Registers q2 and q3 are uninitialized and can be used as temporary storage when saving/restoring register values in the IRQ handler.

getq rd, qs

This instruction copies the value from a q-register to a general-purpose register. This instruction is encoded under the custom0 opcode:

0000000 00000 000XX 000 XXXXX 0001011
f7      rs2   qs    f3  rd    opcode

Example assembler code using the custom0 mnemonic:

Instruction	Assember Code
getq x5, q2	custom0 5, 2, 0, 0
getq x3, q0	custom0 3, 0, 0, 0
getq x1, q3	custom0 1, 3, 0, 0

setq qd, rs

This instruction copies the value from a general-purpose register to a q-register. This instruction is encoded under the custom0 opcode:

0000001 00000 XXXXX 000 000XX 0001011
f7      rs2   rs    f3  qd    opcode

Example assembler code using the custom0 mnemonic:

Instruction	Assember Code
setq q2, x5	custom0 2, 5, 0, 1
setq q0, x3	custom0 0, 3, 0, 1
setq q3, x1	custom0 3, 1, 0, 1

retirq

Return from interrupt. This instruction copies the value from q0 to the program counter and re-enables interrupts. This instruction is encoded under the custom0 opcode:

0000010 00000 00000 000 00000 0001011
f7      rs2   rs    f3  rd    opcode

Example assembler code using the custom0 mnemonic:

Instruction	Assember Code
retirq	custom0 0, 0, 0, 2

maskirq

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. This instruction is encoded under the custom0 opcode:

0000011 00000 XXXXX 000 XXXXX 0001011
f7      rs2   rs    f3  rd    opcode

Example assembler code using the custom0 mnemonic:

Instruction	Assember Code
maskirq x1, x2	custom0 1, 2, 0, 3

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.

waitirq

Pause execution until an interrupt triggers. This instruction is encoded under the custom0 opcode. The bitmask of pending IRQs is written to rd.

0000100 00000 00000 000 XXXXX 0001011
f7      rs2   rs    f3  rd    opcode

Example assembler code using the custom0 mnemonic:

Instruction	Assember Code
waitirq x1	custom0 1, 0, 0, 4

timer

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 00000 XXXXX 000 XXXXX 0001011
f7      rs2   rs    f3  rd    opcode

Example assembler code using the custom0 mnemonic:

Instruction	Assember Code
timer x1, x2	custom0 1, 2, 0, 5

Building a pure RV32I Toolchain:

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:

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

sed -i 's|--enable-languages|--with-arch=RV32I &|' Makefile.in
sed -i 's|asm volatile|value = 0; // &|' newlib/newlib/libc/machine/riscv/ieeefp.c

mkdir build; cd build
../configure --with-xlen=32 --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, which are using the name prefix riscv64-unknown-elf- by default.

Evaluation: Timing on Xilinx 7-Series FPGAs

The following table lists the maximum clock speeds that PicoRV32 can run at on Xilinx 7-Series FPGAs. This are the values reported by Vivado 2015.1 post place&route static timing analysis (report_timing).

Device	Speedgrade	Clock Period (Freq.)
Xilinx Artix-7T	-1	5.1 ns (196 MHz)
Xilinx Artix-7T	-2	4.1 ns (243 MHz)
Xilinx Artix-7T	-3	3.6 ns (277 MHz)
Xilinx Kintex-7T	-1	3.3 ns (303 MHz)
Xilinx Kintex-7T	-2	2.6 ns (384 MHz)
Xilinx Kintex-7T	-3	2.5 ns (400 MHz)
Xilinx Virtex-7T	-1	3.1 ns (322 MHz)
Xilinx Virtex-7T	-2	2.6 ns (384 MHz)
Xilinx Virtex-7T	-3	2.4 ns (416 MHz)

Todos:

• Optional FENCE support
• Optional write-through cache
• Optional support for compressed ISA
• Improved documentation and examples
• Code cleanups and refactoring of main FSM