The Z80 was always considered the more &quot;commercial&quot; processor, and businesses ran CP&#x2F;M and various business applications like dBase on it. CP&#x2F;M as an OS was more &quot;OS like&quot; than anything available for the 6502 at the time, I believe that RT-11 inspired CP&#x2F;M which was a DEC operating system (a &quot;real&quot; computer company) and commands like PIP were ported over with almost identical syntax.<p>In contrast there was a number of &quot;proprietary&quot; vendors with 6502 systems, Apple and its OS, SWTP and its OS, Sphere and its OS, Commodore and its OS, all different all with minimal amounts of market share in business. Contrast that to CP&#x2F;M which ran on Altair machines, IMSAI, Morrow, Polywell, Sol20&#x27;s, Heathkits, Etc. Even the Cromemco once my BIOS had gotten reasonable distribution.<p>The I&#x2F;O address space provided another feature on Z80 (and 8080) machines, which was &quot;shadow rom&quot;. Basically you booted with some &quot;massive 8K&quot; ROM and then you could write out a byte which would swap it in memory space for a 1&#x2F;2K very simple jump table&#x2F;switcher. That let you use most of the address space for user programs. Later when MP&#x2F;M came out it let you do wild things with &quot;swapping&quot; processes.<p>Perhaps the saddest part of the Z80 was that the Z800 (which only made it to market in a more limited Z280 form) was still born. It had a sorts of great ideas that folks wanted to use but the momentum of the 8086&#x2F;8088 was unbeatable by that time. Given chip geometries it would be interesting to build a Z800 today but for the same reasons that it is interesting to build the difference engine, to prove that it would have worked and had some nice features that were ahead of its time.