It was instruction compatible with their existing Z80, and differed primarily in having on-chip cache and a memory management unit (MMU) to provide a 16 MB address range.
There were also ambitious provisions for multiprocessing and either loosely or tightly coupled slave processors, with or without shared global memory.
Another change was the addition of an optional 16-bit data bus, which doubled the rate at which it could access memory if set up properly.
Combined with the two address bus sizes this meant that the chip was offered in a total of four versions: The Z800 was, in most ways, a minicomputer-inspired "super Z80" that would run existing, and larger, programs at considerably higher speeds.
Moreover, the plain Z80 were good enough for most applications at the time so the extra computing power was, in many cases, not worth the added complexity.
Unlike the Z800, Z280, and Z380, the eZ80 does not introduce many new instructions or addressing modes, in comparison to the original Z80, but instead primarily extends the 16-bit registers of the Z80 to 24 bits wide.