Scor­pio News


October–December 1987 – Volume 1. Issue 4.

Page 7 of 55

Preview of the GM890 Z280 Processor Board

by Gerry Dickson

This is a preview of the GM890 processor board, from Gemini. It uses the state of the art Zilog Z280 microprocessor, a super-updated version of the Zilog Z80.

I have used the board for 3 weeks. I persuaded Gemini to let me have an advance version of the board. The board is an 8″ x 8″ 80-BUS board. The board I have does not have the usual solder resist and contains some wire link modifications that Gemini say is for multi-processing.

I am told that the final version, due in August, may be slightly different from this one, but I must say that I was really impressed. The speed of which the board operates is so much faster that the Z80 and there are numerous extra facilities. One thing I must point out straight away is that this board is fully compatible with the existing Z80 board, so it can be plugged straight into your 80-BUS system, in place of a GM811 or GM813, and be in use immediately.


  • The board with 256K of RAM and a Boot EPROM. There aroe four 32 pin sockets on the board designed to accept EPROMs or static RAM IC. Link options allow the eventual use of the complete range of memory ICs up to 1 Megabye (278152 ?). Each socket can optionally be battery-backed from the on-board battery.
  • The board can be supplied fitted with 1MByte DRAMs, if required. Being 18-Pin DRAM sockets, 4MByte DRAMs may be fitted when available.
  • Battery backed real-time clock, using the 146818 clock IC.
  • Gemini compatible PIO, using a fast Z80 PIO. Pin outs are identical with the GM811/​GM813 and it is both hardware & software compatible.
  • Gemini UART using the WD8250 UART. Pin outs are identical with the GM811/​GM813 and it is both hardware & software compatible.
  • A second UART, within the Z280.
  • Four, DMA Channels, Three 16-bit counters timers, 256 Byte cache, Dual (System/​User) addressing modes, separate 64K code/​data/​Op system.


The GM890 board contains a “state of the art” Z280 processor. This CPU, from Zilog is 100% upwards code compatible with the Zilog Z80 and the Hitachi HD64180. (also called Z180). The Z280 supplied on the board was a 10MHz version. Versions up to 25MHz are also supported on the board. The Arithmetic Logic Unnit (ALU)Logic is now completely 16-Bit.

I found that “pure processing”, Z80 code ran approximately 3 times faster that the GM813. A speed improvement of 300% !!!!


The board was plugged into a standard Gemini Galaxy in place of the GM813. I found that all my existing applications software worked at vastly increased speed. Pascal compilation speed was amazingly fast. The screen displays come & go immediately. Compilation to the “M” drive was around 3 times fatser than the same operation on the GM813.

Both printer & serial ports were checked OK to both types of printers.


I tried out a few of the enhancements, using both Turbo Pascal and the GemZap assembler. The additional instructions were easy to try out, using “DB” in Gemzap and “In-line” code for Turbo Pascal.

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