SimCoupe Circle — SAM Coupé Emulator for Raspberry Pi

A bare-metal port of SimCoupe (SAM Coupé emulator) running on Raspberry Pi 2B/3B/4B/400 using the Circle C++ framework.

Introduction

SimCoupe emulates the SAM Coupé — a British Z80-based home computer released in 1989 by Miles Gordon Technology. This version brings SimCoupe to Raspberry Pi as a bare-metal application, eliminating the need for a full operating system.

This port is based on SimCoupe by Simon Owen, adapted to run on Circle, a C++ bare-metal programming environment for Raspberry Pi.

Features

• Bare-metal execution — No Linux required; runs directly on Raspberry Pi hardware
• Multicore architecture:
  • Core 0: USB host, scheduler, input handling, VCHIQ audio transport
  • Core 1: Z80 CPU emulation and video rendering
  • Core 2: Audio synthesis (SAA1099, SID, DAC)
• USB input — Keyboard, mouse and gamepads (up to 2)
• VCHIQ audio — HDMI or headphone jack output via GPU audio service
• FAT filesystem — Load ROMs and disk images from SD card
• Framebuffer video — 800x600, 8-bit palette (Pi 2/3) or 32-bit ARGB (Pi 4/400)

Supported Hardware

Raspberry Pi	Kernel File	CPU	Audio Default	Video
Pi 2B	kernel7.img	Cortex-A7	Headphones	8-bit palette
Pi 3B	kernel8-32.img	Cortex-A53	Headphones	8-bit palette
Pi 4B	kernel7l.img	Cortex-A72	HDMI	32-bit ARGB
Pi 400	kernel7l.img	Cortex-A72	HDMI	32-bit ARGB

Pi 400 notes: Must use HDMI0 port (closest to USB-C power). No headphone jack.

Audio System

All audio goes through VCHIQ (VideoCore Host Interface Queue), routed to HDMI or headphone jack:

• Sample rate: 44100 Hz stereo (matching upstream SimCoupe)
• Auto-detection: Pi 4/400 defaults to HDMI, Pi 2/3 to headphones
• Override: Set sounddev=hdmi or sounddev=headphones in cmdline.txt
• Ring buffer: Core 2 generates audio, Core 0 transports to GPU via VCHIQ
• Flow control: GPU callback reports buffered bytes, poll rate-limited

Audio Pipeline

Core 2 (emulator)         Ring Buffer          Core 0 (poll)           GPU
  Sound::FrameUpdate() --> s_ring[] ---------> circle_audio_poll() --> VCHIQ --> speaker
  SAA1099 + DAC + SID      ~1s stereo          1 frame per poll        HDMI/jack
  44100 Hz stereo           176K s16            via WriteSamples()

Requirements

• Toolchain: arm-none-eabi-gcc (ARM cross-compiler)
• Build tools: CMake 3.16+, GNU Make
• SD Card: FAT32 formatted, 512MB+ recommended
• SAM Coupé ROM: samcoupe.rom (included in simcoupe/Resource/)

Installing the toolchain

macOS:

brew install --cask gcc-arm-embedded
brew install cmake

Linux (Debian/Ubuntu):

sudo apt install gcc-arm-none-eabi cmake make

Building

# Build for a specific platform
./build.sh pi2      # Raspberry Pi 2B
./build.sh pi3      # Raspberry Pi 3B (default)
./build.sh pi4      # Raspberry Pi 4B / 400

# Build all platforms
./build.sh all

# Create release ZIP
./build.sh release

# Clean everything
./build.sh clean

SD Card Setup

Format your SD card as FAT32 and copy all of the following files to the root. Without the firmware files the Pi will not boot.

File	Source	Required
bootcode.bin	circle/boot/	Pi 2B/3B
start.elf	circle/boot/	Pi 2B/3B
start4.elf	circle/boot/	Pi 4B/400
fixup.dat	circle/boot/	Pi 2B/3B
fixup4.dat	circle/boot/	Pi 4B/400
armstub7-rpi4.bin	circle/boot/	Pi 4B/400
bcm2711-rpi-4-b.dtb	circle/boot/	Pi 4B
bcm2711-rpi-400.dtb	circle/boot/	Pi 400
config.txt	simcoupe/sdcard/	All
cmdline.txt	simcoupe/sdcard/	Optional
kernel7.img	simcoupe/	Pi 2B
kernel8-32.img	simcoupe/	Pi 3B
kernel7l.img	simcoupe/	Pi 4B/400
simcoupe/samcoupe.rom	simcoupe/Resource/	All
simcoupe/atom.rom	simcoupe/Resource/	Optional
simcoupe/atomlite.rom	simcoupe/Resource/	Optional
simcoupe/sp0256-al2.bin	simcoupe/Resource/	Optional
simcoupe/*.dsk	your collection	Optional

The Pi firmware auto-selects the correct kernel based on the board. You can place all three kernel files on the same SD card.

The easiest way to set up the SD card is to use ./build.sh release which creates a ready-to-use ZIP file.

config.txt

The included config.txt supports all Pi models with conditional sections:

arm_64bit=0
initial_turbo=0
dtparam=audio=on

[pi4]
kernel=kernel7l.img
armstub=armstub7-rpi4.bin
max_framebuffers=2

cmdline.txt

Optional kernel parameters:

sounddev=hdmi          # Force HDMI audio (auto-detected on Pi 4/400)
sounddev=headphones    # Force headphone jack (auto-detected on Pi 2/3)

If cmdline.txt is empty or absent, audio destination is auto-detected.

Controls

Input	Function
USB Keyboard	SAM Coupé keyboard
USB Mouse	SAM Coupé mouse
USB Gamepad	Joystick 1/2

Function Keys

Key	Function
F1	Insert disk (drive 1)
Shift+F1	Eject disk (drive 1)
Alt+F1	New disk (drive 1)
F2	Insert disk (drive 2)
Shift+F2	Eject disk (drive 2)
Alt+F2	New disk (drive 2)
F3	Tape browser
Shift+F3	Eject tape
F4	Import data
Shift+F4	Export data
Alt+F4	Exit (halt)
F5	Toggle TV aspect ratio
F6	Toggle smoothing
Shift+F6	Toggle motion blur
F8	Toggle fullscreen
F9	Debugger
Shift+F9	Save screenshot (PNG)
F10	Options menu
F11	NMI
F12	Reset
Ctrl+F12	Exit (halt)
Numpad 9	Boot from drive 1

Architecture

+-----------------------------------------------------------+
|                      Raspberry Pi                         |
+------------------+------------------+---------------------+
|  Core 0          |  Core 1          |  Core 2             |
|                  |                  |                     |
|  USB HCI         |  Z80 Emulation   |  Audio Synthesis    |
|  Scheduler       |  Video Rendering |  SAA1099 + DAC +SID |
|  KB/Mouse/Gamepad|  Frame Update    |  Sound::FrameUpdate |
|  VCHIQ Audio Poll|                  |  Ring Buffer Write  |
+------------------+------------------+---------------------+
|  Circle Framework + Multicore-safe Linux Compat Layer     |
|                                                           |
|  VCHIQSound   CScreenDevice   CEMMCDevice   CUSBHCIDevice |
|  (Audio)      (Pi4 Video)     (SD Card)     (USB)         |
+-----------------------------------------------------------+
|  Hardware                                                 |
|  HDMI/Jack Audio  |  HDMI Video  |  SD Card  |  USB       |
+-----------------------------------------------------------+

Key Implementation Details

• Video: Pi 2/3 use CBcmFrameBuffer (8-bit palette, GPU converts). Pi 4/400 use CScreenDevice (32-bit ARGB, software palette LUT) because VCHIQ init disrupts raw framebuffers on BCM2711
• Audio: Push-based VCHIQ (adapted from BMC64). Ring buffer decouples emulator (Core 2) from VCHIQ transport (Core 0). Re-START mechanism recovers from GPU pipeline drain
• Multicore locks: linux_multicore_fix.cpp provides atomic rwlock, mutex, semaphore and completion overrides for Circle's Linux compat layer (originals assert Core 0 only)

Credits

SimCoupe Original

• Simon Owen — SimCoupe emulator (https://github.com/simonowen/simcoupe)
• Allan Skillman — Original SimCoupe 0.72
• Dave Laundon — CPU contention and sound enhancements
• Dave Hooper — Phillips SAA 1099 chip emulator

Circle Framework

• Rene Stange — Circle bare-metal C++ environment (https://github.com/rsta2/circle)

Circle Bare-Metal Port

• RQ Radionics & Rodolfo Guerra

License

This project is licensed under the GNU General Public License v2.0. See simcoupe/License.txt for details.