Clem Cole: "IBM allowed the system to be cloned"
I never looked at it that way. To discourage cloning, IBM published and copyrighted the BIOS source code. Most serious programmers were very familiar with it, because one had to know details of the BIOS to program the computer, for many applications (such as ours). That reduced the number of programmers who could work on a BIOS clone, as such people would have to be recruited from outside the world of the IBM PC. A few outfits sprang up to do clean-room BIOS clones, including an outfit called Phoenix, which had the best. Compaq's internal BIOS was also excellent.
As for the computer hardware, it was just Intel parts along with off-the-shelf floppy disk controllers and drives and other such stuff. IBM had built the PC almost entirely from existing parts, and had no exclusive on any of it.
For the clones, no copyrighted code was used, the programmers had never seen the code, and the function of the BIOS wasn't copyrightable. So, IBM really had no way to prevent the clones.
There were a lot of PCs in the early 1980s that weren't clones. They had 8088 or 8086 CPUs, and looked liked PCs, but they weren't identical, so we had to port our software. Sometimes companies gave us machines, including AT&T, whose PC was made by Olivetti. I remember many conversations with computer vendors in which I was just trying to get the memory address and layout for the screen. They never could even understand the question. DEC, which had their own weird version of a PC, was the worst.
Within a few years all these went away, and only identical clones existed, for which we didn't need to develop a special version.
A few people here have said that IBM could have produced a more sophisticated OS. Actually, I would have been against anything that took up more resident memory. Initially, I think the most memory IBM would supply was 384K, and most serious applications needed it all. Multiprogramming or sophisticated system calls of any sort would have sucked up valuable memory. I was able to design the EDIX editor to be entirely memory resident, even with multiple active files, with no floppy swapping at all.
As someone mentioned, we pretty much used MS-DOS only for its file management. Access to all other facilities was through the BIOS or directly to the hardware. With such a completely unprotected system, running more than one application at a time was out of the question.
One might ask why we had such a primitive system with 384K, when UNIX had been developed over 10 years before on a smaller system. Simple: UNIX had swapping. With no hard drive, and floppies being inserted and removed, everything had to be resident in RAM. In addition, as I've mentioned already, screen speed is what distinguished PC software, ever since Apple games and Visicalc. Traditional UNIX screen speed was ridiculously slow, until the workstations came along, but for many times the price of a PC. To get the screen speed on a PC, the application had to own the hardware. UNIX insists on standing between the application and the hardware. In PC land that would be unacceptable.
--Marc