MCL65+ Instructions

The following are some notes on how to install and configure a system to load code into the MCL65+.

ONE: The first step is to load a special version of the Arduino GUI and the Teensyduino tool on top of it. Personally, I am running Arduino GUI version 1.8.13 on my computer.

Here is a link to the Teensyduino installation instructions:

TWO: The MCL65+ is implemented using a Teensy 4.1, so the Arduino tools need to be configured for this board. Select the Teensy 4.12 board, the CPU Speed set to 816Mhz, and the Optimize to “Fastest”.

THREE: I have uploaded a few versions of the MCL65+ to GitHub which include a generic 6502 as well as a version for the Apple II+. Each of them have the ability to run in accelerated modes.

Link to GitHub MCL65+ source:

FOUR: Remember to cut the solder-pad jumper indicated below so that the MCL65+ and Teensy 4.1 board can be powered using the computer’s power supply alone. If this jumper is not cut, then there will be a connection between the USB and computer power supplies!

FIVE: The MCL65+ supports four acceleration modes. Modes 0,1,2 and 3.

0x0 – All 6502 bus cycles executed using external memory accesses to the motherboard – No acceleration
0x1 – Reads use cycle accurate internal memory and writes pass through to motherboard – No acceleration
0x2 – Reads accelerated using internal memory and writes pass through to motherboard – Accelerated mode
0x3 – All reads and writes use non-cycle accurate accelerated internal memory – Greatest acceleration mode

Please note that the computer’s video address range cannot be accelerated to mode 0x3 because in this mode writes do not go to physical memory.

The acceleration mode can be changed in three ways: 1) statically in the C code. 2) Using the JTAG UART. 3) Using a series of computer keyboard keystrokes which is currently only supported on the Apple II.

Method 1 – Change acceleration mode statically in the C code. The code snippet for the Apple II+ version is below. You can set the mode to a specific number, or you can set it to ‘mode’ which allows it to be changed with either the UART or the keystrokes.

Method 2 – Change acceleration mode using the JTAG UART. Using a serial terminal such as the Arduino’s Serial Monitor or TeraTerm, simply send type the digit 0,1,2 or 3 to change the acceleration mode.

Method 3 -Using a series of computer keyboard keystrokes which is currently only supported on the Apple II. Press a keystroke sequence of the left-arrow(L), right-arrow(R), left-arrow(L), then the number to change the acceleration mode. Note that this method only works if the Apple II program or game is scanning for keystrokes. If not, then the UART method of acceleration must be used. For example: press LRL3 to enter acceleration mode 3

SIX: Partitioning address ranges. As seen above, you can allocate different acceleration modes to different address ranges. In the example above you can see the range 0x0400 to 0x0C00 was set to acceleration mode 0x1 which means all CPU writes will go out to the physical video RAM on the motherboard. Other ranges specific to the Apple II were partitioned and assigned their own acceleration mode. You can partition these ranges to any target computer. The default is acceleration mode 0x0 in which all 6502 accesses go out to the computer’s physical motherboard which is identical to the original 6502.


  • When accelerating the Apple II, the disk drive will not be able to work. This is because the firmware driver uses software loops which are dependent on the speed of the CPU. When accelerated, these loops run too fast for the disk drive to respond!
  • Be careful not to accelerate accesses to video memory ranges or else the display will no longer work.
  • Some computers use software to draw to the display which may be dependent on the speed of the CPU. No acceleration could be achievable in this case.
  • To achieve the maximum acceleration, all RAM and BIOS ROMs should be copied into the internal memory and use acceleration mode 0x3. Only the computer’s video RAM should use mode 0x2.

MCL65+ Instructions

MCL64 – World’s Fastest Commodore 64

Another day another stunt! But here were are: The world’s fastest Commodore 64? 🙂

When I locate most of the C64’s memory ranges inside of the micro controller and disable cycle accurate mode (1Mhz native mode) the 6510 is emulated at over 600Mhz on a dual-issue superscalar processor, so it’s no surprise that we can achieve such a speed improvement!

The project is now on GitHub:

I am basing the speed increase on a small BASIC test which counts the “jiffies” during a loop of code. This is the method used by the YouTube host “8-Bit Show and Tell” during his demonstration of the Super CPU Commodore 64 accelerator board:

In cycle-accurate mode my MCL64 measures the same results as the host, but when comparing the Super CPU accelerated to 20Mhz and the MCL64 in its accelerated mode, the MCL64 is roughly 2X faster than the Super CPU (in raw accelerated mode). The Super CPU has an additional “optimized” BASIC acceleration mode which further speeds up the hardware; however if the MCL64 implemented this optimization it would most likely again be much faster than the Super CPU.

This is the YouTube demonstration of the Super CPU running in its 20Mhz acceleration mode:

This is the MCL64 running in its accelerated mode which is more than 2X faster than the Super CPU:

Here are a few C64 cartridge images which I loaded into the MCL64:

MCL64 – World’s Fastest Commodore 64

MCL64 – MOS 6510 Emulator works in Commodore 64

I received my MC64 PCB in the mail today which I plugged it into my Commodore 64 and was happy to get some decent results!

The MCL64 is a port of my MCL65+ project to the MOS 6510 pinout so it can be used as a drop-in replacement for the Commodore 64’s CPU. It uses my 6502 emulator which runs on the Teensy 4.1 which is an Arduino-like board which runs at 600Mhz+ and has 1MB of memory. The 6510 emulation can either be cycle accurate or it can run significantly faster than the original processor!

I am able to boot to BASIC and run a print “Hello World” program, so one of the next steps will be to try running some of the C64 cartridge images directly from the processor’s on-board RAM. I will probably need to add some code to support the C64’s bank switching internally to do this.

I will post the source files on GitHub shortly.

So far so good!

MCL64 – MOS 6510 Emulator works in Commodore 64

EPROM Emulator

I thought an EPROM emulator would be a cool application of a Teensy 4.0 and a small PCB.

The EPROM is emulated using a Teensy 4.0 which has more than enough speed to sample the 16 address lines, retrieve the data from a 64KB array, and then drive the data lines.

It supports EPROMs sizes up to the 64KB 27C512. Uploading a new ROM image takes seconds and is as simple as updating the data array then reprogramming the it using the Arduino’s IDE.

The small PCB contains three SMT buffers to convert voltages since the Teensy is not 5V tolerant. This board plugs into the EPROM socket and the Teensy plugs into this board.

The setup you see below is somewhat tall because, rather than soldering the Teensy directly to the board, I used sockets between the Teensy and the PCBA which added a lot of height. Without these sockets it would be much shorter.

The PCB Files and source code are on Github:

Below are a few pictures of it emulating the 32KB ROM in an IBM PCjr.

EPROM Emulator

MCL65+ World’s Fastest Apple II+

Another flashy title, but again probably true! The MCL65+, when running in accelerated mode is, I estimate, more than ten times faster than a stock 1Mhz Apple II+! This was accomplished by emulating all of the computer’s ROM and RAM in the 600Mhz microcontroller’s memory. Just the I/O and video memory ranges were left as regular 6502 bus access to the motherboard which run at 1Mhz.

The MCL65+ is a 6502 accelerator card which uses a 600Mhz Arduino Teensy4.1 microcontroller to emulate a 6502 microprocessor as well as its bus interface signals. It was designed to be a drop-in replacement for the original 6502 processor found in computers like the VIC-20, the early Apple computers, and others.

I took some videos of two BASIC programs I made to measure the system’s performance before and after the acceleration. One is the classic x=x+1, print x, goto 10 program and the other prints an array of characters. Both very simple however the accelerated speed increase is dramatic.. the text just flies by!

I was surprised that the video and keyboard worked so well under acceleration! The next thing I need to try is booting the computer from either the 5.25″ diskette drive, or a compact flash drive emulator…

Here are some videos of it running with acceleration enabled and disabled. Please note that these programs print a lot to the screen which is accessed via the 1Mhz 6502 bus to the video memory and slows the test down. If less is printed to the screen the acceleration is even faster…

Cycle accurate mode:
Accelerated mode:

MCL65+ World’s Fastest Apple II+

World’s Fastest VIC-20

I thought I would give this post a catchy title, and I believe it is probably true! The MCL65+ runs an emulated 6502 on a 600Mhz microcontroller, so when it is not running cycle-accurate it is quite a bit faster than the original 1Mhz 6502 in a VIC-20.

Here are some of the details:

The MCL65+ can emulate the complete 64KB of the 6502’s address range at 600Mhz, so I was able to add certain components to see what worked and what didn’t. It turns out that the VIC-20 BASIC was not tolerant of much acceleration… When I ran an accelerated ZeroPage and Stack range the performance boost was only about 15%. This is because when I tried to accelerate the BIOS and video regions, the VIC-20 video would no longer work. I guess there are timing dependancies with the BIOS that must not be exceeded.

I had better luck with some of the cartridge games. Some of them actually ran better when they were accelerated because, at the normal clock speed, they were slow and less responsive! When accelerating the game and VIC-20 memory ranges they ran much faster which was more enjoyable. Donkey Kong, Pac-Man, and Jungle Hunt all ran well at the accelerated speed. Defender was a little too fast to control!

I will post some videos of the accelerated games so you can behold the World’s Fastest VIC-20! 🙂

World’s Fastest VIC-20