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 currently feature complete and in beta testing phase for the first official release version 1. As far as we can assess this at the moment: it is rock solid and runs 99.9% of all games and demos.

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 and raw Amiga keyboard mode
• 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
• Adjustable picture, per Amiga screen mode: HDMI crop plus analog position (pan) and analog overscan, via a config file and helper tool
• 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. 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 original Amiga "tank mouse" is the directly supported passive mouse; compatible active adapters are listed below. 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). This works when your keyboard is in the MEGA65 mode. In the positional Amiga keyboard mode this substitute moves to the ↑ symbol key (left of RESTORE), because there Run/Stop is the Amiga's Esc. Both keyboard modes are summarized in the Keyboard section below.

So what works depends on what you plug in:

What you use	Move + left button	Right / middle buttons
Original Amiga "tank" mouse	Yes	Right via Run/Stop; no middle
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	Yes(*)
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. Therefore you need to download firmware version 3.23.5313 or newer. With new firmware, mouSTer supports a new setting in the [mouse] section: activeplotlines=true. With that, you can use the right button on your mouse, without it, you need to stick to Run/Stop. Here is an example of a known-to-work MOUSTER.INI.

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.

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 (Amiga mode: MEGA + F13)
Right mouse button	Run/Stop (Amiga mode: ↑ key)

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". (In the positional Amiga keyboard mode the left Alt key moves to F13, so use MEGA + F13 there.) The pointer keeps accelerating the longer you hold an arrow, so tap the keys for fine positioning and hold them to cross the screen.

Keyboard

The MEGA65 keyboard drives the Amiga, and you choose how. Two mapping modes are available in the menu's Keyboard section:

• MEGA65 mode (default) — the cap is law: you get exactly the character printed on the MEGA65 keycap, including the front-face symbols typed with MEGA (so { is MEGA+:, ~ is MEGA+,, and so on). Best if the MEGA65 is the keyboard you know.
• Amiga mode — positional: each key sends the Amiga key in the same place on a real Amiga keyboard, so the shifted number row and a few punctuation keys follow the Amiga's own labels. Best for Amiga muscle memory and for games such as Pinball Dreams that require the original Amiga mapping.

The most important keys (the meanings below are for the default MEGA65 mode; where Amiga keyboard mode differs from the MEGA65 keyboard mode is noted in the right column):

MEGA65 keyboard	Amiga
MEGA	Left Amiga; in MEGA65 mode it also selects front-face symbols
CTRL + MEGA + RESTORE	Ctrl + Left Amiga + Right Amiga (reset)
RESTORE	Right Amiga · Amiga mode: Right Alt
Run/Stop	Right mouse button, hold · Amiga mode: Esc
F1 F3 F5 F7 F9	F1, F3, F5, F7, F9 (MEGA65 mode)
Shift + F1/F3/F5/F7/F9	F2, F4, F6, F8, F10 (MEGA65 mode)
Top row from Run/Stop through F11	Esc, F1–F10 (Amiga mode)
Help	Amiga Help; default menu key

In the default MEGA65 mode Esc, Tab and Caps Lock work as expected. In Amiga mode the entire top row is positional — Run/Stop is Esc, Esc is F1, Alt is F2, Caps Lock is F3 … F11 is F10 — the right mouse button moves to the ↑ symbol key (left of RESTORE), and F13 becomes Left Alt. The full per-key breakdown is in the guide below.

By default Help opens the menu, but you can reassign it — to F11, F13 or MEGA+Run/Stop — in the Keyboard menu, which reserves Help solely for the Amiga. In MEGA65 mode, F11 and F13 are clean menu keys that send nothing to the Amiga. In Amiga mode they also send F10 and Left Alt respectively; see the guide for the effect of every menu-key choice.

The full keyboard guide — complete per-mode tables for typing, special keys, menu keys, and steering the mouse from the keyboard — is in doc/keyboard.md.

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.

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 adjustment

The Amiga's picture may not sit perfectly on your screen — an old quirk that every faithful Amiga recreation shares. AExp fixes it: drop a small aexp_screen.cfg file into /amiga and the core adjusts the picture, automatically per Amiga screen mode. Three independent controls are available: HDMI crop re-frames the picture on HDMI; analog position moves the complete analog picture (OSM included) left/right/up/down in all three VGA modes; analog overscan hides or reveals the Amiga border edges, which some demos fill with odd-looking material. 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. 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 in beta testing phase for the first official release version 1. It is feature complete, so - among other things - the following known gaps will remain in version 1:

• Kickstart 1.3 (ROM size limited to 256kB)
• 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. Optional: copy the aexp_screen.cfg file into /amiga and the core adjusts the picture, automatically per Amiga screen mode.
5. Put your *.adf disk images into /amiga, the file browser starts there.
6. 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.
7. 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? In doc/developers.md you will find 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.