Amiga 500 for MEGA65

Experience the Commodore Amiga 500 on your MEGA65!

This core turns the MEGA65 into an Amiga 500 with the original OCS chipset (PAL), a cycle accurate 68000 CPU, 512 KB of Chip RAM and a 512 KB memory expansion in the trapdoor slot (known as Slow RAM, this is what the classic Commodore A501 expansion did). The Amiga therefore has 1 MB of RAM in total.

The core is work in progress and not officially released yet. There are rough edges and missing features, but the basics work: Workbench 1.3 boots from a mounted ADF disk image and classic demos and games load and run.

Credits

• This core is based on the Minimig-AGA core of the MiSTer project. Minimig was originally created by Dennis van Weeren and has been improved by many others over the years.
• The CPU is fx68k by Jorge Cwik, a cycle accurate implementation of the 68000.
• sy2002 ported the core to the MEGA65 in 2026.
• The core uses the MiSTer2MEGA65 framework and QNICE-FPGA for FAT32 support (loading the Kickstart ROM, mounting disks) and for the on-screen-menu.

Features

• Amiga 500, OCS chipset, PAL
• Cycle accurate 68000 CPU
• 512 KB Chip RAM plus 512 KB Slow RAM (trapdoor expansion), 1 MB in total
• One floppy drive (df0:): mount standard 880 KB *.adf disk images via the on-screen-menu, read and write
• Kickstart 1.3
• Real Amiga mouse in port 1, joystick in port 2, exactly like on a real Amiga
• MEGA65 keyboard mapped to the Amiga keyboard
• Interlace ("laced") modes with a built-in flicker fixer on HDMI
• Analog output in parallel to HDMI: scandoubled 31 kHz VGA or raw 15 kHz RGB for CRTs (SCART), selectable in the menu
• Battery-backed real-time clock

Kickstart ROM

The core needs the Kickstart 1.3 ROM (revision 34.5, the 256 KB version that shipped with the Amiga 500). Put it on your SD card as

/amiga/kick.rom

as a raw dump of exactly 256 KB (262,144 bytes), no byte swapping. Without this file the core stops with an error message. Kickstart is copyrighted software, so it is not part of this repository or of any release; you need to obtain a legal copy yourself, for example from Cloanto's Amiga Forever.

Floppy disks

Press Help to open the menu and mount a *.adf image via the ADF: item. The disk boots after mounting. Disks are read/write: when the Amiga writes to a disk — saving a file, formatting, storing a high score — the change is written back to the *.adf file on your SD card.

To eject the disk, open the menu with Help, move the highlight to the ADF: item and press Space — exactly like the C64 core. On an empty drive that same Space opens the file browser to mount a disk instead, so one key both mounts and ejects.

Saving happens in the background, so the Amiga never stalls. After a write, the MEGA65's drive LED turns yellow while the change is being saved and green again once it is safely on the SD card. Please wait until the LED has stayed green for a few seconds before you unmount a disk, swap disks, reset, or switch the machine off — the yellow light can briefly come back on as more data is flushed. Switching off while it is yellow loses the not-yet-saved changes, exactly like ejecting a real floppy while its drive light is still on.

Mouse and joystick

Plug the mouse into port 1 and the joystick into port 2, exactly like on a real Amiga. The only officially supported mouse is the original Amiga "tank mouse" (or an adapter that behaves like one). Commodore C64 mice do not work: neither the 1350 ("joystick mouse") nor the 1351 (proportional mouse) speaks the Amiga's protocol. We may add support for them in a future version.

The Amiga tank mouse works out of the box: movement and the left button behave just like on the original machine.

The right mouse button (in Workbench it pulls down the menu bar) is the tricky one. On a real Amiga the mouse signals it on a special line that the Amiga's Paula chip actively drives high. The MEGA65 can only read that line, not drive it, so it cannot sense the right button of an original tank mouse. This is a hardware property, identical on every MEGA65 model from R3 to R6. The built-in answer is always available: hold the Run/Stop key as a right mouse button (hold it while moving the mouse to open the Workbench menus).

So what works depends on what you plug in:

What you use	Move + left button	Right (and middle) button
Original Amiga "tank" mouse	Yes	Run/Stop key
Original Amiga "tank" mouse with DIY adapter (see below)	Yes	Yes
Adapter that actively drives the line (e.g. Micro Tom, USBAMI)	Yes	Yes
Faithful Tank Mouse replica that actively drives the line	Yes	Yes
mouSTer in Amiga-mouse mode	Yes	Run/Stop key (see note)
Commodore 1350 (C64 "joystick mouse")	No	No (maybe supported later)
Commodore 1351 (C64 "proportional mouse")	No	No (maybe supported later)

Note on the mouSTer: it emulates a real tank mouse so faithfully that it inherits the exact same limitation. A future mouSTer firmware option, or a tiny adapter with a built-in pull-up resistor, could make its right button work natively. I opened an issue in the mouSTer GitHub repo, you might want to "like" it to motivate the maintainer to implement the feature: willyvmm/mouSTer#53

A simple DIY adapter makes the right button work, even with an original tank mouse or a mouSTer. The only missing piece is the pull-up that a real Amiga's Paula chip provides, and you can add it externally: build a straight-through DB9 male-to-female passthrough (all nine pins wired 1:1) and solder two resistors inside the shell, roughly 2 kΩ from pin 7 (+5V) to pin 9 (right button) and another 2 kΩ from pin 7 to pin 5 (middle button). With that adapter in line, the right (and middle) button of any faithful passive mouse works natively. A more detailed build tutorial is coming soon to the doc folder.

One heads-up for actively-driving adapters: if you unplug one while the Amiga is running, the right button can stay "stuck" for up to half a minute (Workbench shows its menu bar and stops redrawing) before it clears on its own. Just give it a moment after swapping devices.

No mouse? Drive the pointer from the keyboard

Have no Amiga mouse or adapter at hand? You can still operate Workbench. The Amiga's operating system can move the mouse pointer from the keyboard, and that feature works on this core too. It is provided by Intuition (the Amiga's windowing system), so it is available in Workbench and other OS-friendly programs — but not in games or demos that take over the machine.

The MEGA65 keys map onto the Amiga's built-in combinations like this:

Action	Keys
Move the pointer	MEGA + ↑ ↓ ← →
Move the pointer faster	MEGA + Shift + ↑ ↓ ← →
Left mouse button	MEGA + Alt
Right mouse button	Run/Stop

MEGA is the Amiga's left Amiga key and Alt is its left Alt, so MEGA + Alt is exactly the Amiga's built-in "left click". The pointer keeps accelerating the longer you hold an arrow, so tap the keys for fine positioning and hold them to cross the screen.

On a real Amiga the right button is Right Amiga + Right Alt, but the MEGA65 keyboard has no Right Alt key. That is no problem here: as described above, AExp maps Run/Stop straight to the right mouse button, so hold Run/Stop whenever you would right-click.

Keyboard

The most important mappings:

MEGA65	Amiga
MEGA	Left Amiga
RESTORE	Right Amiga
CTRL + MEGA + RESTORE	Ctrl + Left Amiga + Right Amiga (reset)
Run/Stop	Right mouse button (hold)
F1 F3 F5 F7 F9	F1, F3, F5, F7, F9
Shift + F-key	F2, F4, F6, F8, F10 (as printed on the MEGA65 keycaps)
Help	Opens and closes the core's menu

Esc, Tab and Caps Lock work as expected. Amiga keys that have no MEGA65 counterpart (for example the right Alt key and most of the numeric keypad) cannot be typed at the moment.

Video: HDMI

HDMI outputs 720p at 50 Hz (16:9) by default. The first HDMI: menu entry offers the other 50 Hz modes: 576p at 50 Hz in 4:3 or 5:4.

An OCS PAL Amiga is a 50 Hz machine, so only faithful 50 Hz modes are offered.

The second HDMI: menu entry, directly below the display mode, selects the scaling filter:

Filter	Look
No Filter	nearest neighbor: maximum sharpness, visible pixel stairs
Sharp Bilinear	pixel sharp, but with softened stair edges
Bicubic	smooth all-round interpolation
Smooth	soft polyphase scaling
Lanczos	crisp polyphase scaling; the default
Scanlines	Lanczos plus visible scanlines
CRT (S-Video)	scanlines plus a slightly softened picture, like S-Video
CRT (Composite)	scanlines plus heavy horizontal blur, like an antenna or composite cable

These two features both fight "flicker", but they cure two entirely different things — one the shimmer of interlaced screens, the other a periodic hitch in smooth motion:

Interlace flicker fixer (automatic)

Laced screens such as the 640x512 Workbench or the interlaced pictures that demos love are woven into a stable, full-resolution HDMI picture — the same job the A3000's "Amber" chip or an Indivision does on real hardware. This runs automatically; there is no menu entry for it. Demos that flicker on purpose (alternating two images at 50 Hz to fake extra colors, transparency or glowing lights) keep flickering: that is the intended look, and only a CRT softens it. If you want the full story about Amiga video modes and flicker, read doc/video_modes.md.

Flicker-free: smooth motion (menu entry)

The third HDMI: menu entry, Flicker-free (on by default), keeps the HDMI picture perfectly smooth. An Amiga runs a hair below 50 Hz while HDMI is locked to exactly 50 Hz, so without correction the picture drops or repeats one frame roughly every twelve seconds — a small judder or tear, most visible on horizontal scrollers. Flicker-free nudges the Amiga clock by a fraction of a percent so its frame rate averages exactly 50 Hz and the seam disappears. Turn it off for the analog VGA / 15 kHz outputs: there it would make the sync frequency step, which analog monitors dislike. (With it on, the machine also runs about 0.16 % fast, so software clocks gain a few seconds per hour — turn it off if you need authentic timing.)

Video: VGA port (analog RGB)

The VGA connector always carries the picture in parallel to HDMI. The VGA: menu selects one of three modes:

• Standard (default): the Amiga's 15.6 kHz picture is line-doubled to 31 kHz so that VGA monitors accept it. Note that it is still a 50 Hz signal, which not every flat panel likes.
• 15 kHz with HS/VS: the raw 15.6 kHz RGB signal with separate horizontal and vertical sync, for retro monitors with a VGA-style input.
• 15 kHz with CSYNC: the raw 15.6 kHz RGB signal with composite sync, which is what RGB SCART cables and most CRT setups expect.

On a 15 kHz CRT you get the most authentic Amiga picture possible: interlace is displayed natively by the tube (no flicker fixer needed) and the intentional flicker effects of demos melt on the phosphor exactly as their authors intended.

Careful: a regular VGA monitor shows no picture at all in the 15 kHz modes — including the on-screen-menu. If you locked yourself out, connect an HDMI display and switch back there; both outputs share the same menu.

Screen centering

The Amiga's picture may not sit perfectly centered on your screen — an old quirk that every faithful Amiga recreation shares. AExp fixes it without re-flashing: drop a small aexp_screen.cfg file into /amiga and the core re-centers the picture, adjusting automatically for each Amiga screen mode. Two ready-made files ship with the core (one for 16:9 displays, one tuned like a 4:3 monitor); pick the one that looks best, or fine-tune your own with the included aexp_screen_cfg.py tool. HDMI and VGA are centered independently. The full guide is in doc/screen_adjust.md.

Audio

Audio is available on HDMI and on the 3.5 mm jack, carrying Paula's output as-is.

Real-time clock

AExp can feed the Amiga the MEGA65's own battery-backed clock, so Workbench shows the real date and time and your files get proper timestamps. It takes a minute to set up, and Kickstart 1.3 has two quirks worth knowing about (the year can come out as 1978, and the time can be an hour off — both with simple fixes, neither a fault of AExp). The full walkthrough is in doc/RTC.md.

Constraints and roadmap

At this moment the core is an alpha version. The largest known gaps:

• Only one floppy drive (df0:)
• No hard disk support
• OCS and PAL only: no ECS, no AGA, no NTSC, no Fast RAM

The list of work-in-progress builds lives in doc/inofficial.md.

Installation

There is no official release on the MEGA65 Filehost yet. If you have a *.cor or *.bit file of one of the work-in-progress builds:

1. Use a FAT32 formatted SD card with a maximum capacity of 32 GB. The card in the back slot has precedence over the card in the bottom slot.
2. Copy the Kickstart ROM to /amiga/kick.rom as described above.
3. Optional: copy the aexp-<version>.cfg file that comes with the build into /amiga so that the core remembers your menu settings. Without the file nothing breaks, your settings are just not saved. The file name contains the core version, so after an upgrade you need the matching file and need to re-select your settings once.
4. Put your *.adf disk images into /amiga, the file browser starts there.
5. Flash the *.cor file using the MEGA65's bitstream utility, or, if you have a JTAG adaptor, load the *.bit file directly with the M65 tool: m65 -q yourbitstream.bit.
6. Press Help as soon as the core is running to mount a disk and to configure the core.

Developers

Want to build the core from source? Here is the whole path from a fresh clone to a *.cor file. This core is built on the MiSTer2MEGA65 (M2M) framework, whose Wiki is the authoritative reference for the build environment and its operating-system specific details.

What you need

• Xilinx Vivado 2022.2 to synthesize the FPGA bitstream. The free ML Standard Edition covers the MEGA65's Artix-7 (XC7A200T). Vivado runs on Linux and Windows only — there is no macOS build.
• A bash shell with GCC, make, awk and git, to build the QNICE helper CPU's tool chain and the on-screen-menu firmware.
• A MEGA65 (R3/R3A, R4, R5 or R6) and a legal Kickstart 1.3 ROM (see the Kickstart ROM section above) to actually run the result.

Operating-system hints for the bash tool chain:

• Linux: install build-essential (or your distribution's GCC and make packages), gawk and git. Everything, including Vivado, runs natively.
• macOS: xcode-select --install provides the compiler and make; git and awk are already there. You can build the tool chain and the firmware natively, but since Vivado has no macOS build you need to run the synthesis on Linux or Windows — for example in a Linux VM (Parallels, UTM, VirtualBox) that mounts this working folder.
• Windows: Vivado runs natively. For the bash tool chain use WSL2 (Ubuntu) or MSYS2 / Git Bash.

Build the core

1. Clone with all submodules (the Minimig core, the M2M framework and QNICE-FPGA):

   git clone --recursive https://github.com/sy2002/AExp.git
   cd AExp

   Already cloned without --recursive? Pull the submodules in afterwards:

   git submodule update --init --recursive

2. Build the QNICE tool chain. This compiles the assembler, the QNICE C compiler, etc. natively for your operating system:

   cd M2M/QNICE/tools
   ./make-toolchain.sh

   Answer every prompt by pressing Enter. When it finishes, return to the repository root (cd ../../..).

3. Open the Vivado project for your board and generate the bitstream. There is one project per MEGA65 revision:

   Board	Vivado project
   R3 / R3A	CORE/CORE-R3.xpr
   R4	CORE/CORE-R4.xpr
   R5	CORE/CORE-R5.xpr
   R6	CORE/CORE-R6.xpr

   Run Generate Bitstream. Vivado rebuilds the QNICE on-screen-menu firmware automatically in a pre-synthesis step, so there is nothing else to prepare. The bitstream ends up in CORE/CORE-R3.runs/impl_1/mega65_r3.bit (substitute your board).

4. Turn the *.bit into a MEGA65 *.cor file with coretool, part of the MEGA65 tools:

   cd CORE/CORE-R3.runs/impl_1
   coretool -B AExp-WIP-V1-A3-R3.cor --bit mega65_r3.bit --target mega65r3 --bit-name "Amiga 500 for MEGA65" --bit-version "WIP-V1-A3"

   Use the target string that matches your board — mega65r3, mega65r4, mega65r5 or mega65r6 — and the version string from the CORE_VERSION constant in CORE/vhdl/config.vhd. Unlike the C64 core, the Amiga core registers no MEGA65 file type (ADFs are mounted from inside its own menu), so no --flags or --caps arguments are needed. The make_release.py packaging script runs this step for you and prefers coretool when both it and bit2core are installed.

5. Deploy and run. Copy the *.cor to the MEGA65 (or, with a JTAG adaptor, flash the *.bit directly with m65 -q mega65_r3.bit) and follow the Installation steps above. Remember that the Kickstart ROM at /amiga/kick.rom is mandatory — without it the core stops at an error screen.

Settings file

For the core to remember your menu settings, the SD card needs an aexp-<version>.cfg file in /amiga (see Installation). Release packages made with make_release.py already contain the matching file. If you build from source yourself, create one with default settings using the M2M helper; the auto argument reads the required size straight from config.vhd:

cd M2M/tools
./make_config.sh aexp-WIP-V1-A3 auto

Run it from inside M2M/tools — the auto argument reads the required size from config.vhd via a relative path. Use the same <version> as the CORE_VERSION constant in CORE/vhdl/config.vhd.

Going deeper

• doc/how_to_port.md is the engineering reference for this port: the M2M architecture, the MiSTer-to-MEGA65 porting walkthrough and a Quartus-to-Vivado pattern catalog.
• The M2M Wiki documents the build environment in depth and explains the QNICE debug console — a real-time serial log and interactive monitor, available if you have a JTAG adaptor.