cradek / zarc

ZARC

Overview

The Z80 Anachronistic Retro Computer (ZARC) is a Zilog Z80-based 8-bit computer. Some features were inspired by a number of other machines, including the DEC PDP-8, BBC Microcomputer and Sinclair Spectrum.

Front Panel

The most significant features are:

• Zilog Z80 CPU running at 8 MHz.
• Physical front panel with LEDs (“blinkenlights”) and switches.
• 1 MB of battery-backed static RAM with write protection. This simulates core memory, allowing use of techniques previously used on such machines.
• A flexible memory mapping system is used to map the RAM into the 16-bit space directly addressable by the CPU.
• The core computer is a mixture of CMOS and TTL with no programmable logic.
• I/O is implemented in an Altera Cyclone II FPGA.
• NMI-based single-step for software debugging, including the capability of debugging interrupt routines.

I/O includes:

• Two RS-232 ports.
• Real time clock (RTC).
• MMC memory card interface.

Software features include:

• Monitor with disassembler and memory page management.
• CP/M 2.2 using MMC card for file storage.
• File system checker (fsck)
• File transfer over serial link (stran)
• ZX81 emulator (zx81)
• Bitcoin miner (bitcoin)

The CP/M system is capable of running most widely available CP/M software.

Documentation

• Hardware
• Monitor
• CP/M, including BIOS and installation
• File system checker
• File transfer over serial link

History

When I was at school many years ago in the era of Sinclair ZX81s and BBC Microcomputers, I built two Z80 machines. The first was called "Phoenix I", and was a simple single-board machine with very limited capabilities. The I/O was six 7-segment filament displays, a re-purposed calculator keypad and (later) a three-channel sound chip. This was later dismantled and the parts re-used for the "Phoenix 2". This was a much more sophisticated machine, with 192 KB of RAM, a "high resolution" (256 x 192) two bits per pixel video display, a full alphanumeric keyboard and data storage on cassette tape. It ran a modified Sinclair Spectrum basic, and a modified version of a game called Manic Miner. Unfortunately, I have nothing left of either of these machines.

ZARC could reasonably have been called "Phoenix 3", though I think ZARC is slightly more imaginative. In part, this is the machine I would like to have had in the 1980s. Both hardware and software are much more sophisticated, and the visual appeal of a "proper" front panel is nice. Lockdowns associated with the COVID-19 plague gave me the opportunity to make this machine a reality.