RIBBBIT65: What Is It?

RIBBBIT65: What Is It?

There are a lot of ideas out there in the world about what a computer should be, and what a computer should be able to do.

Certainly a computer should be able to execute specified commands in a specified order, retrieve and store data, and be able to perform arithmetic and logical operations on that data.  It should be able to accept some input, and provide some output.  Its execution of instructions should be able to be altered either by data values or input received.

Beyond that, the intended application goes a long way toward dictating what the specifications and requirements are which will drive the design.

So, what is the RIBBBIT65, beyond being a computer?  What kind of computer is it?

The RIBBBIT65 is an 8-bit microcomputer, based around the Western Design Center's W65C02 microprocessor family.

It is intended primarily as general purpose computer with an eye on learning.

The computing ecosystem is radically different today than it was when the MOS 6502 was first introduced in 1975.  Today, our computers have so much spare horsepower, we can emulate whole computer systems in our web browsers.  Almost everybody who could read this post has a computer of some kind and access to the Internet.  Today, you really don't need "the actual hardware" to enjoy a completely reasonable emulation or simulation of using old hardware, if that's your interest.

I'm not going to lie, though.  There are some of us, for whom the simulation is insufficient.  Some of us want to poke pins with probes.  Some of us want to talk to the CPU on its own terms.  There are some times when the simulation and the emulation are not what is needed.  Sometimes you actually need the hardware.

There are people out there who have done amazing work, re-implementing Commodore, Sinclair, and Atari systems with blends of new and old hardware, and I applaud their efforts.  Reverse-engineering a VIC-II or an ANTIC can't be an easy task.  For me, though, with each ASIC and ULA there is a definition of borders and an imposition of constraints.  With every piece of "other people's software" between me and the CPU, the machine itself becomes more diffuse and meaningless.  

I wanted a chance to work down on the bare iron, and stand up a computer.  A system.  I wanted the feeling of discovery and triumph that came from wrangling a box of rattly parts and wires into a working machine with peripherals and a workable but simple interface.  Something that would allow me to demonstrate, tame, and use most of the core functionality from the dawn of the microcomputer age.  

I have mixed feelings about things like floppy disks and CRT monitors.  Aesthetically, I find them appealing.  I like the idea of them -- the close relationship between the abstract and the concrete... between ethereal electrical fluctuations and real, physical phenomena.  Functionally, I feel they were good for their time, but I have no practical use for them today.

I had struggled for a long time with the conceptual leap required for me to grok machine language.  Ben Eater's "Hello World From Scratch" video straightened most of that out for me.  I finally saw a path to the kind of machine that interested me.

What kind of machine is that?  It's the RIBBBIT65, of course.

The RIBBBIT65 is a general purpose computer.  It's not a games machine or an industrial controller -- not by design, anyway.  Initially, I just wanted to make a board that would give me a good base-line 6502-based system that I could experiment with, that only required that I knew the 6502 itself, the basic layout of the memory map (e.g. where RAM, ROM, and IO are).

RIBBBIT65_COREplus

The RIBBBIT65_COREplus board satisfies that need for me.  It's got all the basic functionality, and conforms to a fairly straightforward model - 32K ROM, 32K RAM.  You want to do more with it, you can hang stuff off the bus.

The RIBBBIT65 CoreROM is currently in development, but at this stage, it has a working monitor program, and a framework for an abstracted IO handler and interrupt handler.  The CoreROM is divided into 4 8K segments, with one (the lowest 8K segment) being set aside as ROM that can be paged-in and -out as needed.

The RIBBBIT65 ROMxpander is still in early stages of development, but the intent is to provide 3 32K user-installable ROMs (intended to changed infrequently) which are each broken up into 4 8K segments, which can be paged in for the lower 8K of the CoreROM.  One possible application would be one ROM chip would contain up to 4 programming languages or tools, while another might contain up to 4 productivity tools.  The ROMxpander is also planned to accept an 8K "ROM cartridge" with the ability to quickly and easily switch ROM-based programs.

RIBBBIT65 SystemIO

The RIBBBIT65 SystemIO board provides a set of peripherals and related functionality to let the RIBBBIT65 interact with the world.  It has a serial console, which nearly all modern "retro-computer" kits have, but it also has 4 additional general purpose RS232 ports.  It has 2 general purpose W65C22 Versatile Interface Adapters, and a third for driving an LCD character display.  There are plans for storage devices, SPI & I2C adapters, and an interrupt controller to allow multiple devices to send interrupts to the CPU (provided the software is setupt to handle them properly).  Space has also been set aside for a "character addressable video display adapter" on the SystemIO bus, which could allow something on the order of Sinclair ZX Spectrum graphics, full-screen editing, and so on.

The RIBBBIT65 UserIO board is, at this stage, mostly just conceptual, but it is intended to leverage the SystemIO board, and provide an additional 16 ports for user-defined peripherals, with interrupt handling for upto 7 of them.

Finally, the RIBBBIT65-ITX (in development) is intended to provide a subset of RIBBBIT65 functionality in a Mini-ITX form factor to permit use in a variety of applications and settings, in an enclosure of the user's choosing, and connecting to industry standard ATX power and front-panel connections.