We are going to blink the LED (P72) on W65C265SXB, which is located in the middle of the board just above the W65C265S 16-bit Microcontroller chip. Blinking the LED in the embedded world is synonymous with writing a “Hello World!” program in software. I’ve written many programs over the years in numerous programming languages, however, those pale in comparison to controlling something tangible like an LED at the lowest level.

Ok, so there are few ways in which you can control the LED (P72) on the W65C265SXB:

I might be able to guess your reaction to this list… “c program, not a big deal…I just need a little time to get back up-to-speed”, “assembly language, ah, hmm…I’m sure I can find some examples on the internet”, “S28 record, what is that?” and finally “opcodes, that sounds hard!” I’m going start from the ground up by using opcodes. Why? Well, I believe the best way to an understanding of how to program 65xx technology is to start with a simple exercise at the lowest level by talking its language, i.e. machine code.

Software engineers gain in-depth knowledge of their tools and high-level programming languages over years of experience. However, few engineers seek to decode the output of a compiler (e.g. Assembly language or machine code) or hexdump of an executable mainly because it’s unnecessary, which I agree. I’m going to provide you with a glimpse “under the compiler” in order to gain a deeper level of understanding. If you haven’t been here before, enjoy this new world that’s been here all along. So, let’s get started with a quick review of the Monitor ROM.

Step 1: Monitor ROM


As I mentioned previously, the Monitor ROM (aka Monitor or Mensch Monitor) provides you with an interface into the mask ROM of the W65C265S. The Monitor was designed specifically for the W65C265S and it resides in the 8K byte mask ROM.

The W65C265S has a 8K byte mask ROM from $00:E000 to $00:FFFF. This may be enabled or disabled by hardware or software. The mask ROM is added to the chip as part of the manufacturing process. – W65C265S Monitor ROM REFERENCE MANUAL

The Monitor is the main reason why you may have decided to first buy the W65C265SXB over the other boards. I think of the Monitor like a Bash shell for the W65C265S. It behaves more like a REPL with its rich interactive environment to load, run and debug programs. Moreover, I like the idea of having a board with a built-in tool to assist with learning the technology. I simply need a terminal application, usb cable and board.

Step 2: Serial Terminal


CoolTerm is the terminal application I’m using to connect to the 265SXB via the J5 USB connector. CoolTerm is great cross-platform terminal built for communication with hardware connected to serial ports. The default serial port options in CoolTerm work the 265SXB. You will have to specify the port where the board is connected. If you do not see the connected port in the dropdown list simply click on the Re-Scan Serial Ports button to re-populate the Ports list.

Some other great terminal interfaces include:

Once you have CoolTerm configured, simply click the Connect button and you’ll see one ‘.’ in the upper left corner of the screen. Also, the bottom footer will indicate ‘Connected’. Then press the Reset button (RESB) on the board and you will see the initial screen of the Monitor.

(See attached images as reference)

Ok, our tools are setup and we’re ready to go! I really like the simplicity.

Step 3: How to blink the LED Using Opcodes


Yes, that’s right opcodes (aka machine code)! We don’t need no assembler! Well, not for this simple task. Opcodes (operation codes) are individual instructions from a machine’s language. A machine’s language is defined by an Instruction Set Architecture (ISA). So, since we’re working with 65xx technology we’ll be using instructions from the 65xx ISA called ARA (Addressable Register Architecture).

The 65xx ISA was used to define RISC within the Microprocessor Report many years ago.

According to the datasheet, we’ll be writing our opcodes in cache memory locations within $00:0200-$00:7FFF. Great, but where is that exactly? You can find the chip set on the board labeled “32KByte SRAM”.

Alternative 265SXB Guide

Let’s blink the LED and then explain what is happening. In the Monitor User Console:

  1. Press ‘M’ key
  2. Enter 002000. This is the starting memory address.
  3. Enter the following 12-byte sequences of hex characters and then press
  4. Enter. A9 00 8D 23 DF 00 A9 04 8D 23 DF 00
  5. Press the ‘J’ key
  6. Enter 002000 to turn ON the LED
  7. Press the ‘J’ key
  8. Enter 002006 to turn OFF the LED


Congratulations! You just turned an LED on/off with machine code, woot!

You can find this project on Instructables at :