The 8086 had NO CONCEPT OF MEMORY MANAGEMENT. It had the concept of an EXTERNAL floating point unit -- so boards were built with provisions for external FPU's (even though not sold with those installed).
The 68000 had mechanisms to rerun a bus cycle -- essential for a demand paged MMU. So, you could make provisions in your hardware and software to add support for the PMMU when it became available.
The same was true of the 32k (you could actually design a system that would "short out" the MMU if it was not present and "switch it in" if installed -- no settings or jumpers necessary!)
In Eggebrecht's book (architect and team leader for the IBM PC), he explained his reasons for selecting the 8086 family over other alternatives available at the time of the design.
The choices were essentially:
- 6502
- z80
- z8000
- 8086
- 68k
- "proprietary" chip (i.e., an "IBM special") (Amusing that he didn't even consider the DEC parts -- perhaps fear that DEC might not be able to produce in the volumes they envisioned? Or, maybe just "out-of-the-question" to patronize a competitor?)
The 6502 and z80 were ruled out because they feared being seen as "followers" in the industry (instead of LEADERS) -- the z80 and
6502 (apple!) having already carved out markets.The z8000 was too different from the z80 so no simple migration path from the HUGE z80 code base to that architecture. (amusing considering how hard it is/was to port code *to* the PC)
The proprietary solution was ruled out because the only tools available ran on IBM mainframes ("Buy one of our PC's! Then, buy one of our mainframes so you can write code for it!!")
The 68k saw a lot of attention. It was recognized as a much nicer architecture (more like an '11 -- OhMiGosh!). The reasons against adopting it boiled down to the fact that it had a 16b bus. They were more interested in pinching pennies than designing a real computer!
I.e., "We'll fix it in version 2"
In his words: "In summary the 8088 was selected because it allowed the lowest cost implementation of an architecture that provided a migration path to a larger address space and higher performance implementations. Because it was a unique choice relative to competitive system implementations, IBM could be viewed as a leader, rather than a follower. It had a feasible software migration path that allowed access to the large base of existing 8080 software. The 8088 was a comfortable solution for IBM. Was it the best processor architecture available at the time? Probably not, but history seems to have been kind to the decision."
I think he underestimates the "kindness" aspect! History has been TOLERANT of the decision. How many millions (?) of man years of discarded software have come and gone because of the endless contortions as Intel keeps trying to make an antique architecture work in a modern world! (each contortion causing software to become obsolete or extraordinary measures taken to allow it to live on for a short while longer...)