What is ARA?

In previous blog posts I’ve mentioned some terms like “65xx ISA” and “Addressable Register Architecture (ARA).”  I suspect most software engineers have conceptual knowledge of instruction sets and their classification. As an example, the x86 instruction set is much more complex than the reduced instruction set of the ARM. This is where the terms CISC and RISC get thrown around and most conversations end. I felt compelled to do more research to deepen my conceptual knowledge and find out where ARA fits in the landscape of ISAs. That’s my goal of this post. If you’re looking for something more in-depth then you’ll likely have to keep looking but check out the Fun Facts before you go. A special thanks to my uncle Bill Mensch for providing information on ARA and tolerating my basic knowledge questions.

In order for me to understand ARA, I needed to get a better understanding of ISA. So, like most, I looked to the internet. I was not setting out to do a “deep dive” but just to gather enough information to get some context. The “aha” moment for me came when I discovered the separation between the ISA and the underlying microarchitecture of a microprocessor. A microarchitecture is a given way to implement an Instruction Set Architecture. I immediately saw similarities in the relationship between the Java Virtual Machine (JVM) specification and runtime being analogous to an ISA and microarchitecture. Oracle maintains JVM specifications (e.g. Java SE 7) that can be thought of like an Instruction Set Architecture. As most software engineers know, Oracle also offers a implementation (or runtime) of the JVM specification similar to how a microarchitecture implements an ISA. There are 40+ implementations of the JVM specification including the Dalvik and ART JVMs that make up the Android runtime. So, now it was clear to me the role of an ISA and I can start pestering uncle Billy about ARA. 🙂

Bill told me that he coined the term Addressable Register Architecture (ARA) about 5 years ago to describe the 65xx ISA. According to him, “…there had to be an essential architectural advantage to explain the extremely wide and successful application of the 6502 and it’s decedents (W65C02, W65C02S & W65C816)“. Bill explains his realization to me in an email:

…I realized that the wider data widths such as 64-bit floating point operations might be the key. So it was always related to by Motorola and others as “memory mapped IO” but no one actually looked at the fact that the floating point registers were in “page Zero” and could have been anywhere in memory. Page Zero was selected because it uses less machine cycles. Using a C compiler, other registers are stored in memory. Actually, all IO registers are “addressed” and that is how I arrived at defining the 65xx as an ARA. – Bill Mensch, Oct 2015

ARA gives 65xx ISA a unique identity which is neither CISC nor RISC. So, in my “layman’s world” I think of ARA as an ISA that is the “best of both worlds” sitting nicely in between CISC & RISC. This is where I leave you to continue exploring and digging deeper if you desire. This is a fascinating world that is extremely complex.

Before you leave, take a look at the Fun Facts, I think you’ll be surprised!

Fun Facts

Fun Fact #1: Did you know that a microarchitecture has more affect on power, energy and/or performance than instruction set architectures, e.g. CISC or RISC? Here’s a great EETimes blog post (RISC vs CISC: What’s the Difference?) that reveals this finding of a 4 year study from the Vertical Research Group.

Fun Fact #2: “The ARM engineers Steve Furber and Sophie (then Roger) Wilson visited me in my WDC Mesa offices the November-December time frame in 1983 asking me to design a 32-bit microprocessor which could have been best described as a RISC processor. They were partially funded by Apple for around $3M and Apple used the Acorn RISC Machine (ARM) in the failed Newton PDA. Acorn RISC Machine became Advanced RISC Machine and in an attempt to get away from the RISC identity became just ARM.” – Bill Mensch, June 2015

…Bill respectfully declined to design the 32-bit microprocessor for the ARM engineers.

Fun Fact #3: The 6502 made an appearance in the first Terminator movie! The T-800 HUD (Head Up Display) features 6502 assembly code. It was determined to be Apple-II code taken from Nibble Magazine. (http://www.pagetable.com/?p=64) The feature image is from the T-800 HUD.


The W65C265SXB 65xxcelr8r Board

I’m a software engineer working as a consultant for Chariot Solutions developing software solutions for clients to solve business problems. My path to becoming a software engineer started in hardware specifically integrated circuits while working at the Western Design Center (WDC) in the late 1980’s. It was at WDC where I learned how to layout circuits in a work-study program while studying Electrical Engineering at Arizona State University. Later, I changed majors and finished school with a B.S. in Computer and Information Sciences.

The last few years I’ve been focused on mobile applications primarily developing native Android applications. It’s very satisfying building apps that run on devices that you can hold in your hands. A refreshing change from building enterprise server-side software that runs on virtual machines in a cloud located somewhere on this planet. I still enjoy developing apps for mobile devices however, I find myself wanting to get back to my roots and get “closer to the metal”…to quote my Uncle Bill Mensch who taught me about integrated circuits years ago. Uncle Bill and I continue to stay in contact and have had many discussions on technology, philosophy, Embedded Intelligence, business, and family.

So, I’m one of 11 million professional software developers in the world today. My goal is to explore 65xx technology and share my experiences through the eyes of a software engineer. The first step starts with the W65C265SXB.

W65C265SXB Board

What is a W65C265SXB? The W65C265SXB (Standard chip Xxcelr8r Board) is a board designed for developing Hi-Rel (High Reliability) low-power applications for the W65C265S microcontroller. You can read about all the features of this board in the datasheet. WDC explained to me that their microcontrollers are general purpose making them ideal for use in industrial control systems, e.g. weigh belt conveyor systems, industrial refrigeration, access control, general communication, etc. The W65C265S is built on the 65xx Instruction Set Architecture (ISA), which is a very efficient ISA (benchmark comparison). Most microcontrollers today are RISC-based like the Atmel ATmega328 used in the popular Arduino Uno board. The difference in instruction set architectures is an important distinction. There are many RISC-based microcontrollers in the market however, there are only two (W65C134S & W65C265S) that are based on 65xx ISA.
The W65C265SXB is based around the W65C265S which is a feature rich 8/16–bit microcomputer based on the W65C816 microprocessor.
The first thing I did after unboxing my board was to plug in a USB cable into the J5 connector then into my MacBook Pro. Next, I downloaded the Python Terminal interface created by WDC. I had to make a few changes to the python script to run on my Mac since it was built to run on Windows. Finally, I ran the script and pressed the S1 reset button and …
Terminal Interface
…my board comes alive. 🙂 What you’re seeing is the initial output of the W65C265S internal Monitor ROM.
The monitor acts as a simple operating system, allowing the user to examine registers and memory, load and save programs, and debug applications in RAM.
The Monitor provides you with an interface into the mask ROM of the W65C265S. The Monitor was written in Assembly language (code listing) in the early 1990’s. As a software engineer, I find this pretty amazing that a simple operating system written in Assembly over 20 years ago can still be a valuable asset!

So my journey begins once again. I feel like it’s “back to the future” for me. It’s been 20 years since I last worked with 65xx technology. Now as I look at the W65C265SXB I wonder, can this technology still be relevant after all these years of technological advancement? I plan on finding out however, I bet I already know the answer. 😉