[dfe]

Are you sure IBM didn't care about 8080 compatibility? The PC was released in August 1981, a full 4 years before any of the other machines you speak of. That XT 5160-078 you speak of is a cost-reduced model with no hard drive from IBM, allowing dealers to install a compatible hard drive from a different supplier.

By 1985, the PC AT (286) had already been out since August 1984. And the original XT was October of 1983. And the XT itself is literally the same as a PC with more expandability: 8 slots instead of 5, beefier PSU, room for a hard drive.

It's unclear if IBM or for that matter anyone would have been able to start designing a 68000-based machine and bring it to market by the fall of 1981. Sun brought the Sun 1 to market in May 1982, so about 9 months later, and with a much higher price.

And what did the Sun run? UNIX. What did IBM run? Quick ports of the last several years of CP/M software made possible because DOS 1 was specifically designed to implement a CP/M-like API, and the 8086 was specifically designed to allow porting of 8080 assembler code directly to small model 8086 code.

[amiga386]

I picked 1985 specifically because of the Amiga/Atari 68000 machines, to show that it's not about price. Had Sun wanted to target the games-machine market, they could've designed for, built and sold a $600 machine based on a 68000 in 1981. Their pricing was because they went for the UNIX workstation market; they also designed and produced their own custom MMU because Motorola's 68851 didn't work correctly for them

Motorola were selling 68000s for $125 each. Steve Jobs talked them down to around $15 each. (Macintosh launch price: $2,495 in 1984) Memory model did not dictate price. Target market and vendor management did.

IBM would have been entirely capable of launching a TMS9900-based PC or a 68000-based PC, had these chips been in full production in 1978. The software and OS for the IBM PC was an afterthought, IBM settled on the CPU first. Had they picked the 68000, they'd have been contracting out for 68000-based OSes.

https://spectrum.ieee.org/the-inside-story-of-texas-instrume...

> Selection of a 16-bit microprocessor by the IBM team couldn’t have been much of a debate. The Motorola 68K, as it was later known, was undoubtedly the hands-down winner. It had the largest logical address space, which was even more important than the minimum 16-bit internal architecture. It was also easily expandable to a full-fledged 32-bit architecture.

> So why aren’t we all using 68K-based computers today? The answer comes back to being first to market. Intel’s 8088 may have been imperfect but at least it was ready, whereas the Motorola 68K was not. And IBM’s thorough component qualification process required that a manufacturer offer up thousands of “production released” samples of any new part so that IBM could perform life tests and other characterizations. IBM had hundreds of engineers doing quality assurance, but component qualifications take time. In the first half of 1978, Intel already had production-released samples of the 8088. By the end of 1978, Motorola’s 68K was still not quite ready for production release.

So it wasn't about architecture, and it wasn't even about price. It was about time-to-market. The 8088 was in full production and the 68000 was not quite there, at precisely the time IBM wanted to create a PC. The 8088 might've been even quicker to market had they left out the memory segmentation, but Intel had their own business reasons (wanting to sell to existing 8080 customers) for adding it.