Answer 6—The Z80 improved on the basic 8080 in many ways, which explains its huge success. It required just one 5-V supply voltage versus the three voltages required by the 8080. Also, it didn’t require a special external clock driver chip.

Although the Z80 had a significantly expanded instruction set, it could run 8080 binaries with no modification whatsoever. Most importantly, it could run the CP/M operating system directly. In addition, it had an on-chip refresh counter, which was driven onto the bus after every instruction fetch. This greatly simplified the design of a memory controller for the newly available DRAM chips. Finally, note that an enhanced interrupt system allowed the processor to handle larger amounts of system I/O. Z80-specific I/O chips that kept the overall system design simple facilitated this.

Contributor: David Tweed