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What's Old Is New


The CRISS CP/M retro computer is a highly-polished CP/M 2.2 computer built with an array of modern microcontrollers.

If you want to really understand how a computer works, one of the best ways to do that is to build your own. Modern computers are orders of magnitude more complex than the comparatively simple computer-in-a-keyboard machines of yesteryear, so many people that want to tackle this challenge build retro computers reminiscent of the machines available in the 1970s and 1980s. Retro computers also hold sentimental value for those that grew up during the early days of the personal computing revolution.

Hardware hacker Igor has built a very refined retro computer, called the CRISS CP/M, that also incorporates some modern conveniences. While retro hardware purists may object to Igor’s strategy of using modern components, you would not know there was anything modern in the case by using the computer. It can emulate the hardware of the TRS-80, Microbee, Kaypro, or Robotron 1715 perfectly.

The completely open source design of the computer makes use of modern Microchip microcontrollers -- the ATmega1284P emulates a Z80 CPU, an ATmega328P stands in as a peripheral controller, and the VGA controller makes use of ATmega328P and ATtiny13 chips. One of the modern conveniences is a ENC28J60 chip that serves as the Ethernet controller. 64 kilobytes of RAM is included. All chips are in DIP format, which makes it easier to trace signals and understand the inner workings of the computer, and also adds to the retro feel of the machine.

The monochrome VGA can display up to 80 by 25 characters, with five different character sets to choose from. A 160 by 96 pixel graphical mode is also available. With the CRISS CP/M running CP/M 2.2 right out of the box, a slew of programming languages, text processors, games, and business software can be used. For loading all of this software, there is another modern convenience available -- an SD card reader provides the computer with mass storage.

If you are concerned about the dangers of retro computing, rest assured, Igor makes it clear that CRISS CP/M is "the SAFEST computer in the world." I think something may have been lost in translation there, but in all seriousness, this looks like a very well thought out, and highly polished build. For those that are interested, kits and assembled computers are available.

Nick Bild

Nick Bild

R&D, creativity, and building the next big thing you never knew you wanted are my specialties.

CRISS CP/M Provides Modern Hardware For A Classic OS


August 30, 2021

Today you might choose run Windows, Linux, MacOS or some other OS on your computer. Back in the 1980s however, you generally had little choice: a certain home computer came with a certain OS, and that was it. If yours was based on a Z80 processor, chances are it ran CP/M. While differences in hardware often made direct data exchange difficult, CP/M provided at least a basic level of software compatibility between various Z80-based computers. Although eventually supplanted by MS-DOS (which initially aimed to be compatible with CP/M), enthusiasts kept the classic OS running on old hardware throughout the 90s and even beyond.

Igor decided to make a 21st-century CP/M machine by designing the CRISS, a single-board computer based mainly on AVR microcontrollers. The CPU is a 20 MHz ATMEGA1284P, which imitates a 4 MHz Z80 through machine-code emulation. A pair of ATMEGA328s run the peripheral controller and a VGA output, so the CRISS can be used with modern monitors. True to its heritage however, the image is monochrome green-on-black, looking instantly familiar to users of Kaypros, Osbornes and other contemporary CP/M machines.

Software is loaded through an SD card that holds floppy images. The CRISS can directly run programs written for the Kaypro II and Robotron 1715 computers, although other platforms can be supported as well with a software upgrade. Igor shows it running programs ranging from the Turbo Pascal compiler to games like Xonix and Tetris.

Housed in a neat little case, the CRISS can communicate with standard PS/2 keyboards and serial printers. Even an Ethernet port is provided for those willing to experiment with network connectivity (a rare feature in the 1980s).

We love seeing modern retro builds like this; similar projects we’ve covered before include the compact ZZ80MB and the huge Z20X. Others have used different ways of running CP/M on modern hardware, such as booting it directly on a Raspberry Pi or emulating an Altair on an ESP32.

Robin Kearey

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