This Z80 Computer Bootstraps Itself


[Plasmode] has created several Z80-compatible board designs, at least four of them using the oddball Z280. The Z280 was a special variant of a Z80 that could bootstrap itself with no external PROM, making it ideal for anyone trying to build a system on a breadboard. According to his post, the cost to build the board is about $35.

Although the 8080 CPU got a lot of glory, it was much harder to use than the Zilog Z80. The Z80 only required a single clock and power supply, so it was much easier to build a system, even on a breadboard. On top of that, the bus wasn’t multiplexed and it could refresh DRAM memory by itself. Maybe that’s why you can still get Z80-derived chips readily. There was one thing, though, you needed an EPROM or some other way to run some initial code to bootstrap your system. Zilog knew this a problem. In those days, you had to use a special tool to burn a PROM and unless it was erasable (and you had the special UV light to erase it), any mistakes cost you a chip.

With the Z280, it was possible to load files via the bootloader to make the device program its own EPROM, as this board does. The bootloader is simple. It loads 256 bytes of memory from the serial port and runs it. The chip has two modes with a 16-bit data bus and 24 address bits. However, it can also operate in a Z80-compatible mode. The chip had many innovative features like a memory management unit and cache, but failed to become a success.

As a CP/M board, though, this should be an easy build. The CPU runs with a 12 MHz bus and has a cool megabyte of memory split between RAM and EPROM. There’s a 44-pin IDE interface and two RC2014 expansion connectors.

While $35 doesn’t seem like much, you can get by with a lot less using a classic Z80. If you don’t mind using an Arduino for support, you can spend as little as $4.

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