Zilog Z80 processors
Introduction: July 1976
In July of 1976, it released the Z80 processor, which was a vastly improved version of the 8080. It was not pin compatible, but instead combined functions such as the memory interface and RAM refresh circuitry, which allowed cheaper and simpler systems to be designed. The Z80 also incorporated a superset of 8080 instructions, meaning it could run all 8080 programs. It also included new instructions and new internal registers, so software that was designed for the Z80 would not necessarily run on the older 8080. The Z80 ran initially at 2.5MHz (later versions ran up to 10MHz), and contained 8,500 transistors. The Z80 could access 64KB of memory.
Zilog's follow-up designs, however, were over-ambitious, and the 16 and 32-bit Z800 and Z8000 were very late to market and suffered from teething troubles. The Z80, by the way, introduced the idea of SIMD (Single Instruction, Multiple Data) with its block move and copy instructions. These were considered very powerful at the time: real mainframe stuff. Modern SSE instructions work on highly advanced versions of this same basic principle.
Source: Upgrading and Repairing PCs (13th Edition) by Scott Mueller.