main-website/docs/rtl-synthesis.md

RTL Synthesis Guide

This guide covers synthesizing the ReviveSparc Verilog RTL for FPGA deployment.

Overview

The ReviveSparc RTL implements a 6-stage, in-order, dual-issue SPARC V9 pipeline with chip multi-threading. It is written in synthesizable Verilog and targets Xilinx and Intel FPGAs.

Pipeline Stages

Fetch   --> Decode --> Execute --> Memory --> Writeback --> Commit
  |          |           |           |           |           |
  v          v           v           v           v           v
 ICache   Decode     ALU/MUL    D-Cache    RegFile    TLB/MMU

Prerequisites

• Verilator 5.x (for simulation)
• Vivado 2024.x or Quartus 23.x (for synthesis)
• Python 3.10+ (for build scripts)
• FuseSoC (optional, for IP management)

Building the RTL Simulation

make rtl

This compiles the RTL with Verilator and produces a simulation binary.

Running RTL Tests

make rtl-test

This runs the complete RTL test suite, including unit tests for each pipeline stage and integration tests for the full core.

FPGA Synthesis

Xilinx Vivado

cd rtl/syn/vivado
vivado -mode batch -source synth.tcl

This generates a bitstream for the default target board.

Target Boards

Board	Speed	Resources Used	Status
Xilinx VC707 (Virtex-7)	50 MHz, 4 cores	65% LUT, 48% BRAM	Verified
Xilinx KC705 (Kintex-7)	75 MHz, 2 cores	55% LUT, 35% BRAM	Verified
Xilinx Arty A7 (Artix-7)	40 MHz, 1 core	70% LUT, 60% BRAM	Verified
Intel Arria 10 GX	100 MHz, 4 cores	60% ALM, 40% M20K	Verified
Intel Cyclone V	50 MHz, 2 cores	72% ALM, 55% M20K	Beta

Configuration

RTL parameters are set in rtl/config.vh:

`define NUM_CORES        4
`define THREADS_PER_CORE 4
`define L1I_SIZE         16384
`define L1D_SIZE         16384
`define L2_SIZE          1048576
`define PIPELINE_STAGES  6

Running on FPGA

1. Program the FPGA with the generated bitstream
2. Connect a USB-UART cable (default: 115200 baud, 8N1)
3. Load a program via the boot ROM interface:

./tools/fpga-load.py /dev/ttyUSB0 kernel.bin

4. The core resets and begins execution. Output appears on the serial console.

Boot ROM

The boot ROM loads a program from the UART interface into memory and starts execution at the entry point. The boot ROM firmware is in soft/bootrom/ and is pre-synthesized into the bitstream.

Debug Interface

The RTL includes a JTAG-like debug interface accessible via the emulator:

./sparc-emu --connect-fpga /dev/ttyUSB1

This connects the emulator's debugger to the FPGA-resident cores, allowing register inspection and breakpoint management.

Performance Results

Configuration	FPGA	Frequency	DMIPS
1 core, 4 threads	Artix-7	40 MHz	45
2 cores, 4 threads	Kintex-7	75 MHz	168
4 cores, 4 threads	Virtex-7	50 MHz	320
4 cores, 4 threads	Arria 10	100 MHz	720

Contributing RTL Changes

1. Write a Verilog testbench in rtl/tb/
2. Verify with Verilator: make rtl-test
3. Ensure lint passes: make rtl-lint
4. Submit a pull request with simulation results