picorv32/README.md

PicoRV32 - A Size-Optimized RISC-V CPU
======================================

PicoRV32 is a CPU core that implements the [RISC-V RV32I Instruction Set](http://riscv.org/).

Tools (gcc, binutils, etc..) can be obtained via the [RISC-V Website](http://riscv.org/download.html#tab_tools).

PicoRV32 is free and open hardware licensed under the [ISC license](http://en.wikipedia.org/wiki/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
- 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 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.

The optional IRQ feature can be used to react to events from the outside, implemnt
fault handlers, or catch instructions from a larger ISA and emulate them in
software.

The optional Pico Co-Prosessor Interface (PCPI) can be used to implement
non-branching instructions in an external coprocessor. An implementation
of a core that implements the `MUL[H[SU|U]]` instructions is provided.


Files in this Repository
------------------------

#### README.md

You are reading it right now.

#### picorv32.v

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_pcpi_mul`     | A PCPI core that implements the `MUL[H[SU|U]]` instructions   |

Simply copy this file into your project.

#### Makefile and testbench.v

A basic test environment run `make test`, `make test_sp` and/or `make test_axi` to run
the test firmware in different environments.

#### firmware/

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.

#### tests/

Simple instruction-level tests from [riscv-tests](https://github.com/riscv/riscv-tests).

#### dhrystone/

Another simple test firmware that runs the Dhrystome benchmark.

#### scripts/

Various scripts and examples for different (synthesis) tools and hardware architectures.


Verilog Module 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.

This parameter is only available for the `picorv32` core. In the
`picorv32_axi` core this is implicitly set to 0.

#### ENABLE_PCPI (default = 0)

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

#### ENABLE_MUL (default = 0)

This parameter internally enables PCPI and instantiates the `picorv32_pcpi_mul`
core that implements the `MUL[H[SU|U]]` instructions. The external CPCI
interface only becomes functional when ENABLE_PCPI is set as well.

#### 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.


Cycles per Instruction 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 |

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.

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 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 | SBREAK 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`.

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](firmware/custom_ops.S) for GNU assembler macros that
implement mnemonics for this instructions.

See [firmware/start.S](firmware/start.S) for an example implementaion of an
interrupt handler assember wrapper, and [firmware/irq.c](firmware/irq.c) for
the actual interrupt handler.

#### getq rd, qs

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

#### setq qd, rs

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

#### retirq

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

#### 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.

    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.

#### waitirq

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

#### 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 ----- XXXXX --- XXXXX 0001011
    f7      rs2   rs    f3  rd    opcode

Example:

    timer x1, x2


Building a pure RV32I Toolchain
-------------------------------

The default settings in the [riscv-tools](https://github.com/riscv/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 and Utilization 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 with `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) |

The following table lists the resource utilization in area-optimized synthesis,
as reported by Vivado 2015.1 post optimization with `report_utilization`. The
"small" core is PicoRV32 configured down to a RV32E cpu, the "regular" core is
PicoRV32 with its default settings and the "large" core is PicoRV32 with
enabled PCPI, IRQ and MUL features.

| Core Variant       | Slice LUTs | LUTs as Memory |
|:------------------ | ----------:| --------------:|
| PicoRV32 "small"   |        855 |             48 |
| PicoRV32 "regular" |        996 |             48 |
| PicoRV32 "large"   |       1814 |             88 |

*Note: Most of the size reduction in the "small" core comes from eliminating
the counter instructions, not from reducing the size of the register file.*


Todos:
------

- Optional support for compressed ISA
- Improved documentation and examples