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Z80 Mark II    By now I had spent considerable time and energy on the microcomputer and had started to formulate some future plans. The present design’s biggest problem was that it was designed around the Signetics 2650 microprocessor. While it was a fine microprocessor, it had not caught on and had been rather bypassed. Also, it had one big fault as a general purpose microprocessor and that is that it had a rigid stack design that was only five levels deep. In my software, I had got around this by programming my own stacks but this slowed things down and was ungainly. Also, it complicated subroutine calling and return. So I called the current design “Mark I” and started to work on Mark II.

I decided that Mark II would use the Zilog Z80A microprocessor. This was an improvement on the Intel 8080A microprocessor and was a very popular and powerful processor with a huge instruction set. I also decided to do away with the directly accessed video memory with its composite video generating circuitry and to incorporate a dedicated video chip, the Motorola MC6847. The power supply, the front panel, the memory and the I/O were all still usable. I planned to add an RS-232 serial interface, interrupt handling, timer chip and other enhancements.

Construction began in late 1979. All the software had to be rewritten for the new processor and I took the opportunity to revamp many of the routines. The results were a vast improvement over Mark I.

At about that time I became interested in the Forth computer language. I was intrigued by its small size and yet powerful capabilities. So I read everything I could find on the subject and then wrote a Forth system to run on my Z80A microcomputer. Now I had a high-level language available.