adumont / cerberus2080

FORTH and BIOS for the Cerberus2080

Cerberus2080

• Cerberus2080
• Introduction
• BIOS
  • Build and flash the BIOS
  • Serial console
  • Other available BIOS
• Emulator
  • Build the kernel for the emulator
  • Emulator execution modes
  • Disassembler
• Kernel
  • Build the kernel for Cerberus
  • Disassemble the kernel binary
  • cl65 linker config files
• Cerberus2080 Serial Programmer
  • Send a file to Cerberus:
• Credits
• References

Introduction

This repo gathers my work on the Cerberus2080.

BIOS

See files in bios/CERBERUS_2080_BIOS.

For now I've taken Andy Toone's 0xFE BIOS as code base, and I have made some minor changes:

• Reset vector set at $C000
• Boots in 6502 fast mode by default
• Write default values to pins before setting their mode (we don't want to output unknown/unexpected value in the board)
• Libs are included in the sketch folder, under libraries/
• Enhanced memory dump formatting (all caps, always two digit) with ASCII dump
• After a list command (dump), you can use page-up/down keys to browse the memory.
• You can specify the address where to run your code from with run ADDR (default is C000 for 6502, 0202 for z80).
• Use crc A1 A2 to compute the CRC32 of memory range from address A1 to A2.
• Added a couple of build options (see in Makefile):
  • JINGLE_OFF: disable the boot Jingle
  • SOUND_FEEDBACK_OFF: disable the keys feedback sound

I have also added a couple of Makefiles for make the BIOS building from the repo.

You still need to have the Arduino IDE installed, but you won't need to open it.

TODO: detect the Arduino IDE path. For now, you can change the ARDUINO_DIR variable in your Makefile.

Build and flash the BIOS

By using the Makefile, building and flashing the BIOS is a matter of running a couple of commands:

cd bios/CERBERUS_2080_BIOS
make build
make flash

Notes:

• Be sure to insert the USB FTDI dongle in the right position
• You need to power the Cerberus board before flashing the BIOS (you can see the USB dongle red led is on)

Serial console

0xFE BIOS added a serial access to the BIOS. at 9600 8N1. You can use minicom to access it:

minicom -b 9600

Other available BIOS

• Andy Toone's 0xFE Bios
• Gordon Henderson's BIOS
• Dean Belfield BreakIntoProgram's BIOS
• The Byte Attic's original BIOS

Emulator

I've built an emulator to develop a minimal Kernel. It's based on Py65 for the 6502 emulation, and Curses for the screen rendering.

On one hand, the emulator writes any key pressed to RAM using the MAILBOX/MAILFLAG mechanism, just like CAT in the Cerberus2080.

On the other hand, the emulator intercepts writes to the Display RAM ($F800-$FCAF) and renders the written char on the emulated screen (left pane). Cerberus Character memory isn't emulated.

The right pane shows PC, Clock cycles, the 6502 Registers, some variables watches. In step by step mode, the next disassembled instruction will be shown as well.

Build the kernel for the emulator

In order to build the FORTH kernel for the emulator we have to specify EMULATOR=1:

make clean
EMULATOR=1 make forth.bin
emulator/cerbemu.py -r forth.bin

In the early stage it looked like this:

Now it has evolved like this, showing more debug information:

• 6502 registers and stack
• FORTH registers and stack
• last lines of dissassembled 6502 code
• symbol where we are in the code (using forth.lbl file)

Emulator execution modes

You can alternate between Continuous and Step by step modes:

• Page Down will enter Step by step modes. Press Page Down key to run the next instruction.
• End key will enter Continuous modes

Disassembler

The Emulator will show the next instruction to be run (available in Step by step mode only)

Kernel

In the early stage of this project, I worked on getting the input of the user (key pressed) rendered on the screen in a terminalish user-friendly way:

• handling line overflows
• scrolling up when reaching last line
• handling backspace and return
• display a cursor (static '_', not flashing at the moment)

That's mainly why I made the Emulator. This is what is in the file minikernel.s.

I then worked on merging my FORTH within this kernel. The result is forth.s (the FORTH kernel).

Build the kernel for Cerberus

make clean
EMULATOR=1 make forth.bin
emulator/cerbemu.py -r forth.bin

Disassemble the kernel binary

Useful to check that everything is where/how it should be:

da65 --cpu 65c02 --comments 3 --start-addr $(( 0xC000)) forth.bin | less

cl65 linker config files

There are two cl65 linker config files (layout of the binary by the lcl65 linker):

• lib/emulator.cfg: for the emulator, which include the whole $C000-$FFFF (so it does includes the interrupt vector table)
• lib/cerberus.cfg: for Cerberus2080, cl65 will not include the interrupt vector table.

Cerberus2080 Serial Programmer

The programmer.py script allows to send commands and load files into Cerberus RAM.

$ ./programmer.py -h
usage: programmer.py [-h] [-p PORT] {send,run} ...

Cerberus2080 Serial Programmer

positional arguments:
  {send,run}
    send                Flash a binary file to Cerberus
    run                 Send the run command

optional arguments:
  -h, --help            show this help message and exit
  -p PORT, --port PORT  USB port to use

Written by @adumont

Send a file to Cerberus:

usage: programmer.py send [-h] [-a ADDR] file

positional arguments:
  file                  File to send

optional arguments:
  -h, --help            show this help message and exit
  -a ADDR, --addr ADDR  Address (hexadecimal), default: C000

Example 1: Load file at 0xC000 (default)

$ ./programmer.py send scr1.bin

File is sent and loaded into RAM at 0xC000 unless otherwise specified with -a/--addr.

Example 2: Load file at a specific address, for example 0xB000

$ ./programmer.py send -a B000 scr1.bin

Credits

• TheByteAttic/CERBERUS2080: CERBERUS 2080™, the amazing multi-processor 8-bit microcomputer

References

• CERBERUS 2080™ | The Byte Attic™