This article is not part of my series on personal computer operating systems.
In 2005 Apple announced that it would switch from IBM PowerPC CPUs to Intel x86 CPUs.
Two companies made x86 CPUs at the time, Intel and AMD. Apple chose Intel.
25 years earlier IBM pretty much made the same decision, and also chose Intel.
In 1974 Intel started producing the Intel 8080 CPU, an 8 bit CPU with 16 bit memory addressing, which soon became the first standard CPU for the personal computer market. The same year Digital Research released the "Control Program for Microprocessors", CP/M, which became the first standard operating system for personal computers.
In 1976 Zilog produced the Zilog z80, a compatible but better competitor to the 8080 and soon the z80 became the standard CPU for personal computers (except for Apple computers, incidentally).
In the late 1970s both Zilog and Intel were working on the next generation chip that would replace the z80 and 8080. Intel came out with the 8086, which was incompatible with the 8080 but allowed for easy porting even of assembler code; and Zilog came out with the z180, a variant of Hitachi's 64180, which was a binary-compatible new version of the z80. The personal computer world split into two; or might have if IBM had not chosen that moment to enter the market.
IBM chose the Intel 8088 CPU, a cheaper version of the 8086, thereby setting a 16 bit standard based on Intel's architecture and not Zilog's which had ruled the 8 bit world. IBM also chose a new operating system, a competitor to CP/M called 86-DOS made in Seattle (and sold by Microsoft), because CP/M-86 did not exist yet. The rest is history.
The z180 seems to me to be very comparable to the 8086.
This article about z180 memory management goes into some detail, but basically what we have here is a 16 bit CPU with 20 bit memory addressing, just like the 8086, and a similar mechanism for translating 16 bit addresses and 64 KB segments into 1 MB of physical memory.
But it get's better. In 1987, a few years after the Intel 80286, Zilog released the z280, a commercial failure (z180 are still sold, z280 never really have been). Maybe it was supposed to compete against the Intel 80286. Bill Gates famously referred to Intel's 16 bit, 24 bit addressing 80286 as a "braindead" chip (mostly, I think, because of its inability to run common software for the 8086 in its new protected mode), but he sure would have found similar words for the z280; or not. I don't know if the z280, which has a protected mode, is z80-compatible in its protected mode. Let's assume it was similar to the 80286 and it was not.
The z380 was released in 1994, too many years after the Intel 80386, and without support for paging or a "Virtual z80 Mode", but compatible, in "native mode" with the z80 and z180 (but not, apparently, the z280).
I imagine an alternative history where IBM chose the Zilog z180 for the IBM PC instead of the Intel 8088.
In that scenario, IBM would still have needed an operating system to use all the new features of the 16 bit CPU (and Bill Gates would have written one or found one somewhere) but since the 8 bit CP/M would have worked, the IBM PC would have been released in 1981 and IBM PC DOS would have been based on CP/M rather than on Microsoft's 86-DOS. 86-DOS would have been sold with 8086-based mainboards for a while and otherwise not influenced the world.
So yeah, Microsoft would have missed its big chance. But I am not out to bash Microsoft, so I will leave Microsoft in the story. They are still the premier maker of programming languages and they still react to the market faster than DR. So very soon MS-DOS would have existed, like CP/M-86 in 1982, to compete with CP/M. And who knows how soon DR would have produced a version of CP/M that would actually use the z180's full 1 MB of memory?
I imagine that in 1982 both Microsoft and DR would have had their native 16 bit operating systems ready for the IBM PC (and Microsoft would also offer a version of Xenix). MS-DOS would compete against CP/M-180 until the advent of the GUIs, when Microsoft's Windows (on z180) would compete against DR's GEM (on z180).
1987 would have seen the releases of both IBM's new operating system for the z280, written by Microsoft and called OS/2, and Microsoft's new version of Windows that can use some of the new features of the z280, called Windows 2.0.
In 1988 Microsoft would have released Windows/380, since Zilog would have released a new CPU much earlier, and since the new CPU would have supported a protected mode and paging, Microsoft and IBM would have worked on versions 2 and 3 of OS/2, with the latter being based on a new portable operating system a new team at Microsoft had just written, called Windows NT.
The x86 architecture would have been quickly forgotten, and not even the Intel 8080 would have any traction now, because the Zilog z80 would have survived for many years in both worlds.
But IBM and thus the world went for the x86 and that's why I am typing this on a 64 bit Macintosh using an Intel-made CPU which, ironically, uses AMD's new AMD64 (or more neutral: "x64") architecture which supports x86 in a compatibility mode.
It would be interesting to see that other world.