Retro Computing seems to be becoming more and more popular, and I'm all for it. My first computer came as a blank printed circuit board (PCB) and bags full of resistors, capacitors, integrated circuits (ICs) and IC sockets that I had to solder to the board myself. There was no concept of buying a motherboard and a video card, loading Windows, plugging in USB mouse and keyboard, and hooking up an HDMI monitor. None of that stuff existed way back in 1979.

My computer used an 8-bit CPU - the 6502 - which was commonly used by Commodore and Acorn Computers in the BBC Micro. It was a simple CPU and very easy to design a computer around. Its clock speed was 750 kHz (0.75 MHz). Note that is Megahertz - not the Gigahertz of today's CPUs.
It had 256 bytes of user RAM, which I eventually extended up to 8 kilobytes. That might seem ridiculously small by today's standards, but we didn't have high-resolution graphics to play with, and we could cram a lot of functionality into small amounts of memory.

The computer kit was made by Tangerine Computer Systems, and was called the Microtan 65. In its most basic form, it was programmed by punching in hexadecimal machine code. You certainly learned how computers worked by doing that. Then I upgraded to a full ASCII keyboard and was able to type in 6502 assembly mnemonics, such as "LDA 0" instead of "A9 00" for Load Accumulator with zero.

I was amazed to find that the 6502 CPU is still being made by Western Design Center (WDC). However, they have updated it somewhat. It is now totally static so that the clock speed can be slowed down to a standstill, which is great for debugging. At the other end of the scale, the maximum clock speed has been increased from the original 1MHz to 14MHz.

Retro, but Modern

I began to wonder what a 14MHz Microtan 65 would be like. So, I have decided to try and build one. It's going to be a functional replica, but made with modern components, which means I'll be able to use the full 64kb address range for RAM, ROM and I/O. My original Microtan had memory chips that were 4 bits wide so I needed 2 chips to make up 1kb memory, so my 8kb needed 16 chips. With today's memory, I can add 32kb in just one chip.

One thing about the Microtan 65 was, like many other computers of its day, it have UHF output, which plugged into the aerial socket of a regular television. I plan to use VGA instead, so that I can use a more modern monitor. As the video RAM was taken from the system memory, high resolution and colour were not available. With modern memory banking, a faster CPU, faster memory, and even dual-port memory, I plan to overcome that limitation eventually. But first, I want to get a basic clone of the original Microtan 65 up and running.