This is where I worked (link below), they gave me my first job coding when I was 17. On the first page of the brochure, the card in the lower left was what we c

This is where I worked (link below), they gave me my first job coding when I was 17. On the first page of the brochure, the card in the lower left was what we called the dpc-816. It had a Z80 and an 80186 chip on it. The silver heat sink covers the 80186 and the Z80 is just to the left, vertically oriented. I worked on parts of the BIOS for it. Both the 8 and 16 bit BIOSes scanned for a sequence, ESC Ctrl-\, which would switch the terminal from one CPU to the other. Both CPUs could execute simultaneously on the same bus, with shared RAM. They solved the arbitration problem very simply; they inverted the clock from the 80186 and fed the inverted clock into the Z80. This way whichever CPU was going for the bus first would pull the HLT pin on the other via some TTL logic, so on the next rising edge of the clock, the other chip would stop. When the bus cycle was complete, some TTL would drop the HLT pin. http://www.vintagecomputer.net/ace/ACEDiscovery500Ads.pdf http://www.vintagecomputer.net/ace/ACEDiscovery500Ads.pdf