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Emulator for the Commander X16 8-bit computer

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Build Status Release License: BSD-Clause Contributors

This is an emulator for the Commander X16 computer system. It only depends on SDL2 and should compile on all modern operating systems.


  • CPU: Full 65C02 instruction set
  • VERA
    • Mostly cycle exact emulation
    • Supports almost all features:
      • composer
      • two layers
      • sprites
      • VSYNC, raster, sprite IRQ
  • Sound
    • PCM
    • PSG
    • YM2151
  • Real-Time-Clock
  • System Management Controller
  • SD card: reading and writing (image file)
  • VIA
    • ROM/RAM banking
    • keyboard
    • mouse
    • gamepads

Missing Features

  • VERA
    • Does not support the "CURRENT_FIELD" bit
    • Interlaced modes (NTSC/RGB) don't render at the full horizontal fidelity
  • VIA
    • Does not support counters/timers/IRQs

Binaries & Compiling

Binary releases for macOS, Windows and x86_64 Linux are available on the releases page.

The emulator itself is dependent only on SDL2. However, to run the emulated system you will also need a compatible

ROM image. This will be loaded from the directory containing the emulator binary, or you can use the
-rom .../path/to/rom.bin

WARNING: Older versions of the ROM might not work in newer versions of the emulator, and vice versa.

You can build a ROM image yourself using the build instructions in the [x16-rom] repo. The

included in the latest release of the emulator may also work with the HEAD of this repo, but this is not guaranteed.

macOS Build

Install SDL2 using

brew install sdl2

Linux Build

The SDL2 development package is available as a distribution package with most major versions of Linux: - Red Hat:

yum install SDL2-devel
- Debian:
apt-get install libsdl2-dev


to build the source. The output will be
in the current directory. Remember you will also need a
as described above.

WebAssembly Build

Steps for compiling WebAssembly/HTML5 can be found here.


You can start

either by double-clicking it, or from the command line. The latter allows you to specify additional arguments.
  • When starting
    without arguments, it will pick up the system ROM (
    ) from the executable's directory.
  • The system ROM filename/path can be overridden with the
    command line argument.
  • -nvram
    lets you specify a 64 byte file for the system's non-volatile RAM. If it does not exist, it will be created once the NVRAM is modified.
  • -keymap
    tells the KERNAL to switch to a specific keyboard layout. Use it without an argument to view the supported layouts.
  • -sdcard
    lets you specify an SD card image (partition table + FAT32).
  • -prg
    lets you specify a
    file that gets injected into RAM after start.
  • -bas
    lets you specify a BASIC program in ASCII format that automatically typed in (and tokenized).
  • -run
    executes the application specified through
    , depending on the load address.
  • -geos
    launches GEOS at startup.
  • -scale
    scales video output to an integer multiple of 640x480
  • -echo
    causes all KERNAL/BASIC output to be printed to the host's terminal. Enable this and use the BASIC command "LIST" to convert a BASIC program to ASCII (detokenize).
  • -warp
    causes the emulator to run as fast as possible, possibly faster than a real X16.
  • -gif [,wait]
    to record the screen into a GIF. See below for more info.
  • -quality
    change image scaling algorithm quality
    • nearest
      : nearest pixel sampling
    • linear
      : linear filtering
    • best
      : (default) anisotropic filtering
  • -log
    enables one or more types of logging (e.g.
    -log KS
    • K
      : keyboard (key-up and key-down events)
    • S
      : speed (CPU load, frame misses)
    • V
      : video I/O reads and writes
  • -debug
    enables the debugger.
  • -dump
    configure system dump (e.g.
    -dump CB
    • C
      : CPU registers (7 B: A,X,Y,SP,STATUS,PC)
    • R
      : RAM (40 KiB)
    • B
      : Banked RAM (2 MiB)
    • V
      : Video RAM and registers (128 KiB VRAM, 32 B composer registers, 512 B pallete, 16 B layer0 registers, 16 B layer1 registers, 16 B sprite registers, 2 KiB sprite attributes)
  • -sound
    can be used to specify the output sound device.
  • -abufs
    can be used to specify the number of audio buffers (defaults to 8). If you're experiencing stuttering in the audio try to increase this number. This will result in additional audio latency though.
  • -rtc
    causes the real-time-clock set to the system's time and date.
  • When compiled with
    #define TRACE
    will enable an instruction trace on stdout.


x16emu -h
to see all command line options.

Keyboard Layout

The X16 uses a PS/2 keyboard, and the ROM currently supports several different layouts. The following table shows their names, and what keys produce different characters than expected:

|Name |Description |Differences| |------|------------------------|-------| |en-us |US |[

] ⇒ [←], [~] ⇒ [π], [\] ⇒ [£]|
|en-gb |United Kingdom         |[
] ⇒ [←], [~] ⇒ [π]| |de |German |[§] ⇒ [£], [´] ⇒ [^], [^] ⇒ [←], [°] ⇒ [π]| |nordic|Nordic |key left of [1] ⇒ [←],[π]| |it |Italian |[\] ⇒ [←], [&vert;] ⇒ [π]| |pl |Polish (Programmers) |[`] ⇒ [←], [~] ⇒ [π], [\] ⇒ [£]| |hu |Hungarian |[\] ⇒ [←], [&vert;] ⇒ [π], [§] ⇒ [£]| |es |Spanish |[&vert;] ⇒ π, \ ⇒ [←], Alt + [<] ⇒ [£]| |fr |French |[²] ⇒ [←], [§] ⇒ [£]| |de-ch |Swiss German |[^] ⇒ [←], [°] ⇒ [π]| |fr-be |Belgian French |[²] ⇒ [←], [³] ⇒ [π]| |fi |Finnish |[§] ⇒ [←], [½] ⇒ [π]| |pt-br |Portuguese (Brazil ABNT)|[\] ⇒ [←], [&vert;] ⇒ [π]|

Keys that produce international characters (like [ä] or [ç]) will not produce any character.

Since the emulator tells the computer the position of keys that are pressed, you need to configure the layout for the computer independently of the keyboard layout you have configured on the host.

Use the F9 key to cycle through the layouts, or set the keyboard layout at startup using the

command line argument.

The following keys can be used for controlling games:

|Keyboard Key | SNES Equivalent | |--------------|----------------| |Ctrl | B | |Alt | Y | |Space | SELECT | |Enter | START | |Cursor Up | UP | |Cursor Down | DOWN | |Cursor Left | LEFT | |Cursor Right | RIGHT |

Functions while running

  • Ctrl
    will reset the computer.
  • Ctrl
    will paste the clipboard by injecting key presses.
  • Ctrl
    will save a system dump (configurable with
    ) to disk.
  • Ctrl
    will toggle full screen mode.
  • Ctrl
    will toggle warp mode.

On the Mac, use the

key instead.

GIF Recording

With the argument

, followed by a filename, a screen recording will be saved into the given GIF file. Please exit the emulator before reading the GIF file.

If the option

is specified after the filename, it will start recording on
POKE $9FB5,2
. It will capture a single frame on
POKE $9FB5,1
and pause recording on
POKE $9FB5,0
returns a 128 if recording is enabled but not active.

BASIC and the Screen Editor

On startup, the X16 presents direct mode of BASIC V2. You can enter BASIC statements, or line numbers with BASIC statements and

the program, just like on Commodore computers.
  • To stop execution of a BASIC program, hit the
    key (
    in the emulator), or
    Ctrl + C
  • To insert characters, first insert spaces by pressing
    Shift + Backspaces
    , then type over those spaces.
  • To clear the screen, press
    Shift + Home
  • The X16 does not have a

SD Card Images

The command line argument

lets you attach an image file for the emulated SD card. Using an emulated SD card makes filesystem operations go through the X16's DOS implementation, so it supports all filesystem operations (including directory listing though
command channel commands using the
statement) and guarantees full compatibility with the real device.

Images must be greater than 32 MB in size and contain an MBR partition table and a FAT32 filesystem. The file
in this repository is an empty 100 MB image in this format.

On macOS, you can just double-click an image to mount it, or use the command line:

# hdiutil attach sdcard.img
/dev/disk2              FDisk_partition_scheme
/dev/disk2s1            Windows_FAT_32                  /Volumes/X16 DISK
# [do something with the filesystem]
# hdiutil detach /dev/disk[n] # [n] = number of device as printed above

On Linux, you can use the command line:

# sudo losetup -P /dev/loop21 disk.img
# sudo mount /dev/loop21p1 /mnt # pick a location to mount it to, like /mnt
# [do something with the filesystem]
# sudo umount /mnt
# sudo losetup -d /dev/loop21

On Windows, you can use the OSFMount tool.

Host Filesystem Interface

If the system ROM contains any version of the KERNAL, and there is no SD card image attached, the LOAD (

) and SAVE (
) KERNAL calls (and BASIC statements) are intercepted by the emulator for device 8 (the default). So the BASIC statements will target the host computer's local filesystem:

Note that this feature is very limited! Manually reading and writing files (e.g.

in BASIC) is not supported by the host filesystem interface. Use SD card images for this.

The emulator will interpret filenames relative to the directory it was started in. On macOS, when double-clicking the executable, this is the home directory.

To avoid incompatibility problems between the PETSCII and ASCII encodings, you can

  • use lower case filenames on the host side, and unshifted filenames on the X16 side.
  • use
    to switch to the X16 to ISO mode for ASCII compatibility.
  • use
    to switch to the upper/lower character set for a workaround.

Dealing with BASIC Programs

BASIC programs are encoded in a tokenized form, they are not simply ASCII files. If you want to edit BASIC programs on the host's text editor, you need to convert it between tokenized BASIC form and ASCII.

  • To convert ASCII to BASIC, reboot the machine and paste the ASCII text using
    Ctrl + V
    Cmd + V
    ). You can now run the program, or use the
    BASIC command to write the tokenized version to disk.
  • To convert BASIC to ASCII, start x16emu with the
    the BASIC file, and type
    . Now copy the ASCII version from the terminal.

Using the KERNAL/BASIC environment

Please see the KERNAL/BASIC documentation.


The debugger requires

to start. Without it it is effectively disabled.

There are 2 panels you can control. The code panel, the top left half, and the data panel, the bottom half of the screen. You can also edit the contents of the registers PC, A, X, Y, and SP.

The debugger uses its own command line with the following syntax:

|Statement|Description| |---------|----------------------------------------------------------------------------------------------------| |d %x|Change the code panel to view disassembly starting from the address %x.| |m %x|Change the data panel to view memory starting from the address %x.| |b %s %d|Changes the current memory bank for disassembly and data. The %s param can be either 'ram' or 'rom', the %d is the memory bank to display.| |r %s %x|Changes the value in the specified register. Valid registers in the %s param are 'pc', 'a', 'x', 'y', and 'sp'. %x is the value to store in that register.|

The debugger keys are similar to the Microsoft Debugger shortcut keys, and work as follows

|Key|Description | |---|---------------------------------------------------------------------------------------| |F1 |resets the shown code position to the current PC | |F2 |resets the 65C02 CPU but not any of the hardware. | |F5 |is used to return to Run mode, the emulator should run as normal. | |F9 |sets the breakpoint to the currently code position. | |F10|steps 'over' routines - if the next instruction is JSR it will break on return. | |F11|steps 'into' routines. | |F12|is used to break back into the debugger. This does not happen if you do not have -debug| |TAB|when stopped, or single stepping, hides the debug information when pressed |


is selected the STP instruction (opcode $DB) will break into the debugger automatically.

Effectively keyboard routines only work when the debugger is running normally. Single stepping through keyboard code will not work at present.




Copyright (c) 2019-2020 Michael Steil <[email protected]>,, et al. All rights reserved. License: 2-clause BSD

Release Notes

Release 38 ("Kyoto")

  • CPU
    • added WAI, BBS, BBR, SMB, and RMB instructions [Stephen Horn]
  • VERA
    • VERA speed optimizations [Stephen Horn]
    • fixed raster line interrupt [Stephen Horn]
    • added sprite collision interrupt [Stephen Horn]
    • fixed sprite wrapping [Stephen Horn]
    • added VERA dump, fill commands to debugger [Stephen Horn]
    • fixed VRAM memory dump [Stephen Horn]
  • SD card
    • SD card write support
    • Ctrl+D/Cmd+D detaches/attaches SD card (for debugging)
    • improved/cleaned up SD card emulation [Frank van den Hoef]
    • SD card activity/error LED support
    • VERA-SPI: support Auto-TX mode
  • misc
    • added warp mode (Ctrl+'+'/Cmd+'+' to toggle, or
    • added '-version' shell option [Alice Trillian Osako]
    • new app icon [Stephen Horn]
    • expose 32 bit cycle counter (up to 500 sec) in emulator I/O area
    • zero page register display in debugger [Mike Allison]
    • Various WebAssembly improvements and fixes [Sebastian Voges]

Release 37 ("Geneva")

  • VERA 0.9 register layout [Frank van den Hoef]
  • audio [Frank van den Hoef]
    • VERA PCM and PSG audio support
    • YM2151 support is now enabled by default
    • added
      to specify number of audio buffers
  • removed UART [Frank van den Hoef]
  • added window icon [Nigel Stewart]
  • fixed access to paths with non-ASCII characters on Windows [Serentty]
  • SDL HiDPI hint to fix mouse scaling [Edward Kmett]

Release 36 ("Berlin")

  • added VERA UART emulation (
  • correctly emulate missing SD card
  • moved host filesystem interface from device 1 to device 8, only available if no SD card is attached
  • require numeric argument for
    to auto-run test
  • fixed JMP (a,x) for 65c02
  • Fixed ESC as RUN/STOP [Ingo Hinterding]

Release 35

  • video optimization [Neil Forbes-Richardson]
  • added
    to launch GEOS on startup
  • added
    to launch (graphics) unit test on startup
  • debugger
    • switch viewed RAM/ROM bank with
      numpad +
      numpad -
    • optimized character printing [Kobrasadetin]
  • trace mode:
    • prepend ROM bank to address in trace
    • also prints 16 bit virtual regs (graph/GEOS)
  • fixes
    • initialize memory to 0 [Kobrasadetin]
    • fixed SYS hex argument
    • disabled "buffer full, skipping" and SD card debug text, it was too noisy

Release 34

  • PS/2 mouse
  • support for text mode with tiles other than 8x8 [Serentty]
  • fix: programmatic echo mode control [Mikael O. Bonnier]

Release 33

  • significant performance optimizations
  • VERA
    • enabled all 128 sprites
    • correct sprite zdepth
    • support for raster IRQs
  • SDL controller support using
    [John J Bliss]
  • 65C02 BCD fixes [Norman B. Lancaster]
  • feature parity with new LOAD/VLOAD features [John-Paul Gignac]
  • default RAM and ROM banks are now 0, matching the hardware
  • GIF recording can now be controlled from inside the machine [Randall Bohn]
  • Debugging
    • Major enhancements to the debugger [kktos]
    • -echo
      will now encode non-printable characters like this: \X93 for CHR$(93),
      as well as pasting accepts this convention again
    • -echo raw
      for the original behavior
    • -echo iso
      for correct character encoding in ISO mode
    • -ram
      to specify RAM size; now defaults to 512

Release 32

  • correct ROM banking
  • VERA emulation optimizations [Stephen Horn]
  • added
    option to allow writing RAM, CPU state or VERA state to disk [Nils Hasenbanck]
  • added
    option to change scaling algorithm; now defaults to "best" [Maurizio Porrato]
  • output of
    can now be fed into UNIX pipes [Anonymous Maarten]
  • relative speed of emulator is shown in the title if host can't keep up [Rien]
  • fix: 6502 BCD arithmetic [Rien]
  • fix: colors (white is now white) [Rien]
  • fix: sprite flipping [jjbliss]

Release 31

  • VERA 0.8 register layout
  • removed
    (character ROM is now part of
  • GIF recording using
    [Neil Forbes-Richardson]
  • numpad support [Maurizio Porrato]
  • fake support of VIA timers to work around BASIC RND(0)
  • default ROM is taken from executable's directory [Michael Watters]
  • emulator window has a title [Michael Watters]
  • -debug
    allows specifying a breakpoint [Frank Buss]
  • package contains the ROM symbols in
  • support for VERA SPI

Release 30

Emulator: * VERA can now generate VSYNC interrupts * added

for setting the keyboard layout * added
for integer scaling of the window [Stephen Horn] * added
to enable various logging features (can also be enabled at runtime (POKE $9FB0+) [Randall Bohn]) * changed
to be an option to
* emulator detection: read $9FBE/$9FBF, must read 0x31 and 0x36 * fix:
no longer corrupt BASIC programs. * fix:
into RAM bank [Stephen Horn] * fix: 2bpp and 4bpp drawing [Stephen Horn] * fix: 4bpp sprites [MonstersGoBoom] * fix: build on Linux/ARM

Release 29

  • better keyboard support: if you pretend you have a US keyboard layout when typing, all keys should now be reachable [Paul Robson]
  • -debug
    will enable the new debugger [Paul Robson]
  • runs at the correct speed (was way too slow on most machines)
  • keyboard shortcuts work on Windows/Linux:
    Ctrl + F/R/S/V
  • Ctrl + V
    pastes the clipboard as keypresses
  • -bas file.txt
    loads a BASIC program in ASCII encoding
  • -echo
    prints all BASIC/KERNAL output to the terminal, use it with LIST to convert a BASIC program to ASCII
  • -run
    acts like
    , but also autostarts the program
    SYS 65535
    exit the emulator and save memory into the host's storage
  • the packages now contain the current version of the Programmer's Reference Guide (HTML)
  • fix: on Windows, some file load/saves may be been truncated

Release 28

  • support for 65C02 opcodes [Paul Robson]
  • keep aspect ratio when resizing window [Sebastian Voges]
  • updated sprite logic to VERA 0.7 – the layout of the sprite data registers has changed, you need to change your code!

Release 27

  • Command line overhaul. Supports
  • ROM and char filename defaults, so x16emu can be started without arguments.
  • Host Filesystem Interface supports
  • macOS and Windows packaging logic in Makefile

Release 26

  • better sprite support (clipping, palette offset, flipping)
  • better border support
  • KERNAL can set up interlaced NTSC mode with scaling and borders (compile time option)

Release 25

  • sdcard: fixed
    to load to the correct addressg
  • sdcard: all temp data will be on bank #255; current bank will remain unchanged
  • DOS: support for DOS commands ("UI", "I", "V", ...) and more status messages (e.g. 26,WRITE PROTECT ON,00,00)
  • BASIC:
    command. Without argument: print disk status; with "$" argument: show directory; with "8" or "9" argument: switch default drive; otherwise: send DOS command; also accessible through F7/F8
  • Vera: cycle exact rendering, NTSC, interlacing, border

Release 24

  • SD card support
    • pass path to SD card image as third argument
    • access SD card as drive 8
    • the local PC/Mac disk is still drive 1
    • modulo debugging, this would work on a real X16 with the SD card (plus level shifters) hooked up to VIA#2PB as described in sdcard.c in the emulator surce

Release 23

  • Updated emulator and ROM to spec 0.6 – the ROM image should work on a real X16 with VERA 0.6 now.

Release 22

SYS65375 (SWAPPER) now also clears the screen, avoid ing side effects.

Release 21

  • support for $ and % number prefixes in BASIC
  • support for C128 KERNAL APIs LKUPLA, LKUPSA and CLOSE_ALL

Release 20

  • Toggle fullscreen using
    Cmd + F
    Cmd + return
  • new BASIC instructions and functions:
    • MON
      : enter monitor; no more SYS65280 required
    • VPEEK(bank, address)
    • VPOKE bank, address, value
      VPOKE4,0,VPEEK(4,0) OR 32
      [for 256 color BASIC]

Release 19

  • fixed cursor trail bug
  • fixed f7 key in PS/2 driver
  • f keys are assigned with shortcuts now: F1: LIST F2: <enter monitor> F3: RUN F4: <switch 40/80> F5: LOAD F6: SAVE" F7: DOS"$ <doesn't work yet> F8: DOS <doesn't work yet>

Release 18

  • Fixed scrolling in 40x30 mode when there are double lines on the screen.

Release 17

  • video RAM support in the monitor (SYS65280)
  • 40x30 screen support (SYS65375 to toggle)

Release 16

  • Integrated monitor, start with SYS65280
    is now 3*8 KB:
    • 0: BASIC (bank 0 at $C000)
    • 1: KERNAL ($E000)
    • 2: UTIL (bank 1 at $C000)

Release 15

  • correct text mode video RAM layout both in emulator and KERNAL

Release 14

  • KERNAL: fast scrolling
  • KERNAL: upper/lower switching using CHR$($0E)/CHR$($8E)
  • KERNAL: banking init
  • KERNAL: new PS/2 driver
  • Emulator: VERA updates (more modes, second data port)
  • Emulator: RAM and ROM banks start out as all 1 bits

Release 13

  • Supports mode 7 (8bpp bitmap).

Release 12

  • Supports 8bpp tile mode (mode 4)

Release 11

  • The emulator and the KERNAL now speak the bit-level PS/2 protocol over VIA#2 PA0/PA1. The system behaves the same, but keyboard input in the ROM should work on a real device.

Release 10

updated KERNAL with proper power-on message

Release 9

  • LOAD and SAVE commands are intercepted by the emulator, can be used to access local file system, like this:


  • No device number is necessary. Loading absolute works like this:


Release 8

  • New optional override load address for PRG files:

    ./x64emu rom.bin chargen.bin basic.prg,0401

Release 7

  • Now with banking.
    to switch the RAM bank at $A000.
    to switch the ROM bank at $C000. The ROM file at the command line can be up to 72 KB now (layout: 0: bank 0, 1: KERNAL, 2: bank 1, 3: bank 2 etc.), and the RAM that
    Cmd + S
    saves is 2088KB ($0000-$9F00: regular RAM, $9F00-$9FFF: unused, $A000+: extra banks)

Release 6

  • Vera emulation now matches the complete spec dated 2019-07-06: correct video address space layout, palette format, redefinable character set

Release 5

  • BASIC now starts at $0401 (39679 BASIC BYTES FREE)

Release 4

  • Cmd + S
    now saves all of memory (linear 64 KB for now, including ROM) to
    , etc. You can extract parts of it with Unix "dd", like:
    dd if=memory.bin of=basic.bin bs=1 skip=2049 count=38655

Release 3

  • Supports PRG file as third argument, which is injected after "READY.", so BASIC programs work as well.

Release 2

  • STOP key support

Release 1

  • 6502 core, fake PS/2 keyboard emulation (PS/2 data bytes appear at VIA#1 PB) and text mode Vera emulation
  • KERNAL/BASIC modified for memory layout, missing VIC, Vera text mode and PS/2 keyboard

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