An unexpected perf feature

8086 actually. It was their way of extending the address space of the 8080 to 20 bits.

It was just a slightly more sophisticated take on the way CPU designers have gone about extending the range of addressable memory beyond the natural size of internal registers for eons. It had the advantage you could use separate address spaces for both code and data, giving you 64K for each. So by using a small amount of extra silicon they effectively doubled the amount of address space you had over simple "page extension" hack everybody else was doing. Kudos to them.

So you are saying it was the 80286 is where the rot set it. In the 80286 they had enough silicon to give the programmer true isolation between processes. They could of gone the 32 bits + page table route everybody else did, but no we got segmentation (without page tables instead) and retained 16 bit registers. Why? Well this was also the time the hardware designers had enough silicon to implement microcode - so they could become programmers to! And they decided they could do task switching, ACL's and god know what else better than programmers, so they did. In other words somehow they managed to forget their job was to provide hardware that ran programs quickly, and instead thought their job was to do operating system design.

It was a right royal mess. Fortunately for them the transition from DOS to Windows / OS2 (which is where the extra protections and address space matters) took a while, and by then the i386 was released. It added 32 bits and paging, so we could ignore all that 16 bit segmentation rubbish and get on with life. It turned out the transition to multitasking operating systems wasn't waiting on programmers figuring out had to do it (who would have thunk it?), but rather the price of the RAM needed to hold several tasks at once had to come down.

People here defending the segmentation model should try writing for the 80286, which could only address 64K of code and 64K of data at any one time. There was no reason for it. The 68000 family had a much better memory model at the time, so it wasn't silicon constrains. Well there would have been enough silicon if they hadn't devoted so much of it to creating their own operating system on a chip.

Intel finally came to their senses with Itanium. With it they used all that extra silicon to do what hardware designers should be doing - make programs run fast. Sadly it came along too late.

Back to your point - the time line. The 80286 was released in 1982. The iAPX432 was meant to be released in 1981. The 80286 was the fall back position. As Wikipedia points out, this is a part of its history Intel strives to forget. You will find no mention of the iAPX432 on their web site, for instance.