Lukand wrote: I mean then there would not be any overlapping segment, it could access full 4GiB of RAM, it may remove confusion to some people and curiosity why did they put *16. If that IBM guys dranked Hennessy (they have big pays) instead of Serbian rakija (Drinking 1l of it leads to 6‰ permil, which could kill almost every human being) then it would be cool!
Here comes the "actually..." for today: First it wasn't IBM who chose segmented memory for the 8086, it was Intel. IBM had made that mistake once before (with the original 360) and probably wouldn't have done it again if it were up to them. The reason that Intel did such a silly thing was because they wanted to make it easy for 8080 assembly programmers to re-write their code for the new chip, which they saw as just a temporary design meant solely for microcontrollers anyway (because home computers weren't here to stay, and the emerging workstation market would
surely want something more powerful such as their spiffy new i432 design - you know, the one that wouldn't hit the market until 1983 and didn't wore right even when it did).
Second, it wasn't IBM that chose the 8086, it was Bill Gates and Gary Kildall (and some awkward timing) that pushed them to it. The Boca Raton design group, who were kind of on the outs with Armonk to begin with (because the management at Big Blue were more interested in quashing home computers for good than in making their own) but were given free rein within their budget constraints, so they did something almost unheard of in IBM circles: they solicited outside advice from other IT companies, specifically Intel, Motorola, Microsoft, and Digital Research.
Initially, their plan was to build another 8080 or Z80 based CP/M system of the type already common in the market, and Intel, who were mainly interested in microcontrollers anyway and didn't want to tie a lot of resources up in producing 8086/8088 chips (which they saw as diverting resources from their 'real' Next Big Thing, the i432), so they recommended that as the course to take. Motorola, naturally, enough, recommended using their own chips, but since the 6800 line was more expensive due to lower sales volumes, and the 68000 was still unproven even after a year on the market, IBM decided on Intel (I don't know if they really considered Zilog a serious option or not; the Z80 was basically a modified 8080, while the Z8000 wasn't much better supported than the 68K was at the time even though it came out before the Motorola chip).
As for the other silicon fabs of the day, TI and Fairchild didn't have any microprocessor yet (AFAIK), National Semi and RCA were on IBM's S*** List for trying break into the minicomputer markets years earlier, and MOS Technologies (Chuck Peddle's company, makers of the 6502, which was used in a huge raft of systems including the Apple ][ and the Atari 2600) had just been bought by Commodore, and both Peddle and Jack Tramiel would sooner have swallowed rat poison than play ball with the Blue Meanies.
They then solicited Digital Research (Gary Kildall's company, who produced CP/M, not to be confused with the Digital Equipment Corporation who built the PDPs and VAXen) and the UCSD p-System team for operating systems, figuring that it would be better to have at least two alternatives for the system, and Microsoft for a BASIC interpreter and an assembler. However, both Kildall and Gates mentioned that the market for 8-bit systems seemed to already be saturated, and brought up the 8086, which (AFAIK) Intel hadn't really discussed for the reasons mentioned earlier. Seeing a chance to leapfrog the competition, the IBM team took the advice despite Intel's misgivings.
It was also around this time the Gates learned that IBM was still looking for bids for alternate operating systems, and as it happened, he knew that another company which Microsoft was negotiating a buyout of, Seattle Computer Products, already had an OS for the the 8086, which they called initially called NDOS but had just renamed SCPDOS. He put in a bid to IBM to have this as their third OS option, and slyly suggested that they could have it ready before Digital Research could modify CPM-86 for the new hardware. This led to IBM signing a deal that had a bit of a sting in its tail: they licensed the re-re-christened MS-DOS under yet another name, PC-DOS, on a non-exclusive basis similar to the one they licensed CP/M-86 and UCSD p-System under. While this was a common practice at the time, it laid the seeds of future PC-compatibles like Compaq, and semi-compatibles such as the DEC Rainbow and the Olivetti M24 (sold in the US as the AT&T 6300), which could run the same OS (if not necessarily the same executable files) as the IBM hardware, and eventually set the stage for Microsoft's rise as more than just a language vendor.
It isn't entirely clear why PC-DOS came to dominate over the better established CP/M-86 and UCSD systems, but the fact that Microsoft took marketing seriously was surely part of it. In the first year, their were several orders of all three, but by the end of 1982, both CP/M and UCSD were basically out of the picture. The old chestnut (which I had repeated myself once only to be roughly corrected) about IBM not selling CP/M because Kildall had blown off a meeting to go sailplaning isn't true; IIRC, there was a delay in getting the contract signed because Kildall was recovering from a sailplane crash which happened several days before the deal was to be finalized, but it didn't actually impact the deal significantly.
But we were talking segments... anyway the TL;DR is, Intel used segments to make it easy to write 8-bit code on their 16-bit CPU, which IBM then used because it seemed like a good idea at the time.
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