C64 Emulator in a FPGA

This project implement a C64 computer in FPGA logic. I made this to learn how to write RTL code. I tried to make this implenentaion as close to the real C64 as possible, usign scematics and datasheets. Unlike most other C64 fpga implementations this emulator outputs the composite video signal, ie there is no VGA or HDMI output.

I tried to make the code a potable as I could but I tested this on a Digilent Nexys 4 board, with an Artix 7 fpga. Porting this to other boards shoudl be faily easy. But note that the clock generator must be replaced and maybe something need to be done about the RAM and ROMS. In the Artix 7 theese are translated in to Block rams, which is think is a Xilinx specific thing.

Directory structure

assets    : Rom images for the C64
modules   : Functional implentation
sim       : Simulation projects
syn       : Synthesis project
tools     : Tools for controlling the C64
testbench : Toplevel testbenches
top       : Toplevel modules

How to build

This porject uses the hdlmake tool to generate make files. Its is also assumed that Xilinx Vivado is installed and is in the system PATH. To install hdlmake run

python -m pip install hdlmake

To run the toplevel C64 simulation

cd sim/c64
python -m hdlmake
make clean && make && ./c64_tb.vpp

To run synthesis and build the FPGA bitstream

cd syn/nexys4
python -m hdlmake
make clean && make

This will after some time create the file c64.runs/impl_1/nexys4_c64.bit This which can be programmed to the FPGA.

Nexyx 4 implementation

When runnign the Nexyxs 4 board will show the memory bus address on the right 4 digits of the 7 segment display. (it show snapshots of the addresses because it changes really fast). The left most 4 digits indicates the sound waveform.

• The composite video(PAL) is output on the RED pin of the VGA connector, so simple cable can be made to connect the ground and red pin of the VGA connecotr the tv composite input.

  As an expeirment the video output is also tranmitted as a RF signal on the blue pin of the VGA connector. The signal is tranmitted at 55Mhz corresponding to VHF channel C3. On my setup I just plug in a jump wire into VGA connector then my TV is able to pickup the signal. Audio is transmitted on the green pin of the VGA connector.

WARNING: Tranmitting TV signal on VHF C3 is most likely a violation of RF regulatory requirements in your country. (How ever the power will not be very high). Also I did not implement propper SSB modulation so the signal has a mirror down in channel C3 so it actually occupies a bandwith of 10Mhz.

• Audio is also output on the Nexsys minijack connector.

• A keyboard can be connected to the Nexys4 usb port.

• They 5 navigation buttons controls the joystick A or B port, swith 15 switches between port A and B.

• All switches should be on up position(towards the 7 segment digits). SW0 and SW1 controls the EXTROM and GAME c64 pins of the C64. Switching SW1 down will boot the cartrige image, which is a C64 diagnostic card.

• The USB UART is attracted to the special C64 debug interface(see below)

Debug Interface

In order to debug the C64 core a debug interface has been made. The debug interface is controlled with a normal UART, and is able to do DMA transfers using the expansion port of the C64.

I've made a python script which is able to read and write the bus using debug interface.

Uploading and running a PRG file:

python3 tools/c64debug.py -prg ~/prg/BoulderDash.prg -c RUN

Usage:

usage: c64debug.py [-h] [-read READ] [-prg PRG] [-snd] [-write WRITE] [-offset OFFSET] [-serial SERIAL] [-cmd CMD]

Read or wirte C64 bus.

optional arguments:
  -h, --help      show this help message and exit
  -read READ      read a number of bytes(in hex)
  -prg PRG        upload prg file
  -snd            play sound
  -write WRITE    Write a hex sequence
  -offset OFFSET  offset in hex
  -serial SERIAL  Serial device to use
  -cmd CMD        Inject a line into the BASIC keyboard buffer followed by RETURN.

Simulation

Using verialtor it is possible to run a behavioural simulation of the emualtor almost realtime. The simulator has sound support but it sounds very bad. The simulation project is in the folder sim/c64

./c64hdl  -p ~/prg/ik_plus.prg

Scripts for testing the code with the vice emualtor test suite are also provided. to run the vice test suite colone the test suite

svn co  https://svn.code.sf.net/p/vice-emu/code/testprogs
cd testprogs/testbench
cp ../../vice-tests/* .
cp ../../assets/*.mif .

edit test testbench.sh and add

        c64hdl)
                target="$1"
            ;;

Run a test

./testbench.sh  c64hdl 

Or run a subset of the tests

./testbench.sh  c64hdl cia

Showcase

Known issues

• RF modulation does not use SSB modulation, and occupies ca 12mhz bandwith as opposed to a 6 mhz BW.
• Sprites can have artifacts when they reach the right side of the border.
• Several of the undocumented MOS6510 opcodes does not work
• A lot of games crashes, there are propably lots of reasons for this.

License

This project contains code from:

• Arlet Ottens, for the CPU implementation
• Rayne, for the MOS6526 CIA imeplentation

This code is distributed under 3 clause BSD license.