Commander X16

Commander X16
Type:	Home Computer
Producer:	http://www.CommanderX16.com
Price:	Varies
Released:	2023
Discontinued:
Processor:	WDC 65C02 at 8-MHz
Memory:	512K ROM + 40K program RAM + 512K banked RAM*
OS:	Enhanced KERNAL and enhanced BASIC 2.0.
Info:

The Commander X16 (colloquially known as the X16) is a modern (retro) hardware platform. It is intended to be a hobby computer built from modern, mostly off-the-shelf parts, with KERNAL compatibility with the Commodore family of computers. It is the brainchild of David "the 8 Bit Guy" Murray, who wanted an accessible and affordable platform with late-80s sound and graphics. In his own words:

I wanted a computer that was similar to the Commodore 64, but made from all off-the-shelf components.

Its hardware includes a real 65C02 CPU. It is simple enough that someone can understand the entire system. It is capable enough to create a wide retro ecosystem. Its largely off-the-shelf hardware solves retro-computing problems of today: namely, ever-increasing costs, auction site price gouging/sniping, and the unreliability of 40-year old hardware.

Origin

In 2019, the popular YouTube program The 8-Bit Guy posted two videos titled "Building My Dream Computer", where he discussed hardware considerations in building a modern 8-bit computer. By the end of the second video, he had settled on a 6502-derived CPU, the Commodore 64 KERNAL, and an architecture based loosely on the VIC-20. He discussed requirements for sound and proposed potential sound chips.

KERNAL Compatibility

The project's goal is not to create a C64 clone. However, a primary goal is to use a 6502 processor, the Commodore KERNAL and an extended BASIC 2.0. Thus the machine is generally KERNAL-compatible with Commodore machines. Serial IEC allows file transfer by floppy diskette, and an SD card slot permits very fast file loading.

VERA's Proof of Concept

A follow-on possibility is in repackaging the X16's graphics FPGA "VERA" to extend the graphics and sound capabilities of Commodore and non-Commodore machines, including the C64 and the VIC-20.

Hardware specifications

Physically the system is a Micro ATX motherboard with all-DIP socketed chips.

CPU & Clock

• Western Design Center 65C02S, a static descendant of the 6502, running at 8 MHz.
• Battery-backed Microchip MCP7940N real-time-clock chip. It provides a real-time clock/calendar, two alarms and 64 bytes of RAM. It is accessible through the I2C bus.

I/O

The X16 has four expansion slots with access to the CPU data bus. Like the C64 and the VIC-20, the system contains an IEC compatible disk drive port, as well as an I2C bus. It also has two exposed SNES game controller connectors, with two internal pin headers that can support two more SNES connectors. It accepts a PS/2 keyboard and mouse.

• The IEC port is Device 8.
• The SD Card slot is also Device 8, but can be set to Device 9.

Memory and Operating System

The memory model is inspired by the VIC-20 with 40k of "low RAM". 1 KB of this used by the system for I/O. A 512K ROM is split into 32 banks of 16K each; Bank 0 holds the Commodore KERNAL, with DOS functions and a 16 bit ABI, and an upgraded BASIC 2.0. The X16 also ships with 512K "high" static RAM, which is split into 64 banks of 8K. High RAM is expandable up to 2MiB. RAM bank 0 is a scratch space for the operating system.

The memory map is inspired by the VIC-20, which had a no-fuss division of RAM, I/O addresses, and ROM. The Commander X16 builds on this concept by using an 8K window for banked RAM, and using the top 16K of memory for banked ROM:

Banking logic allows up to 2MB of high RAM for data, machine language, etc, and 128K for ROM. Commander X16 Memory Map

Memory map

Address Range	Size	Description
0x0000-0x0001	2 bytes	RAM, ROM bank control bytes
0x0002-0x0021	32 bytes	16 bit ABI registers
0x0022-0x007F	94 bytes	User zero page
0x0080-0x00FF	128 bytes	KERNAL and BASIC zero page variables
0x0100-0x01FF	256 bytes	Accumulator stack
0x0200-0x03FF	512 bytes	KERNAL and BASIC variables, vectors
0x0400-0x07FF	1024 bytes	User space
0x0800-0x9EFF	38,656 bytes	BASIC program/variable RAM
0x9F00-0x9FFF	256 bytes	I/O registers
0xA000-0xBFFF	8192 bytes*	RAM Bank window (512K total, expandable to 2MB)
0xC000-0xFFFF	16,384 bytes*	ROM Bank window (512K total)

ABI

The "ABI Registers" are virtual 16 bit registers r0 through r15, which are located in zero page locations $02 through $21: r0 = r0L = $02, r0H = $03, r1 = r1L = $04 etc. (The registers start at $02 instead of $00 to allow compatibility with 65xx systems that have a processor port at $00/$01.)

Banks

To switch the current RAM bank:

POKE 0, B

...where B is the bank number, from 0 to 255. Note that Bank 0 is used by the KERNAL for variables and buffers, so user RAM begins at Bank 1.

To switch the current ROM bank:

POKE 1, B

...where B is the bank number, from 0 to 31. Bank 0 is the KERNAL + BASIC.

ROM banks include things such as:

   - KERNAL
   - Monitor
   - Character sets
   - Keyboard layout
   - CBDOS for SD cards
   - GEOS KERNAL
   - BASIC
   - Assembler/Monitor
   - And room for more.

Sound

An on-board Yamaha YM2151 provides MIDI-like sound. Additionally, the VERA module has a simpler PSG with some PCM playback and SID-like capability.

VERA

A single FGPA module, named VERA, supplies graphics, basic PSG and PCM sound, and SD card access for the X16. It was designed by Frank van den Hoef.

Graphics

The graphics capabilities of the X16 are provided by the VERA module. It has 128 KB RAM, and has multiple output formats (including VGA) at a fixed resolution of 640x480@60HZ. It supports two layers with various tile and bitmap modes, and up to 128 sprites. Its color palette is 256 colors, using a total range of 4096 colors.

VERA supports per-tile h-flip, v-flip, 4-bit palette offset, and a 10 bit tile index. All these modes support smooth scrolling.

Sound

VERA has a programmable 16-channel stereo sound generator (PSG) based loosely on the SID; it is also capable of PCM playback.

VERA's PSG has 64 registers devoted to 16 voices, from $1F9C0 - $1F9FF:

VERA's Programmable Sound Generator registers

Voice	Registers	Voice	Registers	Voice	Registers	Voice	Registers
0	$1F9C0 - $1F9C3	4	$1F9D0 - $1F9D3	8	$1F9E0 - $1F9E3	12	$1F9F0 - $1F9F3
1	$1F9C4 - $1F9C7	5	$1F9D4 - $1F9D7	9	$1F9E4 - $1F9E7	13	$1F9F4 - $1F9F7
2	$1F9C8 - $1F9CB	6	$1F9D8 - $1F9DB	10	$1F9E8 - $1F9EB	14	$1F9F8 - $1F9FB
3	$1F9CC - $1F9CF	7	$1F9DC - $1F9DF	11	$1F9EC - $1F9EF	15	$1F9FC - $1F9FF

Each block of four registers are as follows:

• Bytes 0 and 1: frequency LSB, MSB
• Byte 2: channel (right = bit 7, left = bit 6), volume (bits 0-5)
• Byte 3: waveform (bits 6 and 7), pulse width (bits 0-5)

The waveform bits are defined as:

• 00 pulse
• 01 sawtooth
• 10 triangle
• 11 noise

There are no ADSR envelope controls in VERA itself, nor filters. ADSR must be managed by software.

Emulator

An official emulator is being co-developed right along side the hardware. The emulator is supported for Windows, Mac, and Linux.

Unofficial emulators are being developed for iPad and Android, derived from the source of the official emulator.

Software Compatibility

Applications

The X16 has an augmented Commodore KERNAL, improved BASIC 2.0 (with a DOS wedge and additional commands imported from the C128 and other places), a machine language monitor, an assembly language editor, and more.

BASIC

Commander X16 BASIC 2.0 is largely compatible with VIC-20 and C64 BASIC 2.0, with several new commands. Many routines and hardware addresses are different, breaking compatibility with programs using PEEK, POKE, WAIT, and SYS commands.

BASIC has new support for graphics, I/O, memory, hexadecimal and binary literals, and more. Refer to the system documentation for details. The X16 is "as compatible as possible" with BASIC tokenization through V10. Keywords added to X16 BASIC that also exist in other versions of BASIC match the token, and new keywords are encoded in the ranges $CE-$80+ and $FE-$80+.

KERNAL

The X16 contains an augmented C64 KERNAL. It supports the X16's video modes, peripherals, timekeeping and simple memory management.

For applications to remain compatible between different versions of the ROM, they can rely upon the KERNAL API. The KERNAL fully supports the C64 KERNAL API. In addition, the X16 supports a subset of the C128 API.

Binary

Most binary programs will rely upon specific addressing, and will thus need to be reassembled or recompiled with corrected addresses. Further, the differences in VERA from the Commodore graphics will require different handling as well, generally rendering binary files incompatible.

Assembler

There is a machine language monitor in ROM, based upon that of the Final Cartridge III, and using the same commands.

Cross-Compiler Support

The Commander X16 is a supported target in the cc65 cross-compiler.

Legality of the modified KERNAL and BASIC

The software of the computer is based on leaked Commodore 64 KERNAL and BASIC source code. David requested permission from the current owners of the software (Cloanto) but didn't get a reply. However, "Peri Fractic" was able to get permission from Cloanto to use the BASIC and KERNAL ROMs.Template:Citation needed

Michael Steil has also added new KERNAL routines, based on routines present in the C128 and other systems, for dealing with sprites, date/time, joysticks, mouse, graphics mode, and memory handling.

Should it ever need them, the Commander X16 emulator project has access to the MEGA 65's open ROMs which are open sourced under the LGPL v3 license..

See Also

Official Documentation

• Official Commander X16 Website
• Official X16 and VERA Programmers Reference Guides (Github)
• Official X16 Emulator (Github)
• Official X16 Demos

Tools and Tutorials

• Aloevera - a build tool that converts PNG to VERA text, tiles, sprites, and bitmaps.
• Dansbo's "Hello, World!" in assembly for the x16
• Blog tutorial on passing assembly arguments inline to a subroutine.

8 Bit Guy X16-Related Videos

• Video: Building my Dream Computer — Part 1
• Video: Building my Dream Computer — Part 2
• Video: Building a VIC-20 Kit Computer and X16 Update.