Previous posts examined the question why IBM implemented the A20 hardware in the PC/AT, causing endless headaches to future PC hardware and software developers. WordStar emerged as a possible culprit, but no one would quite point the finger at it.
The OS/2 Museum is now in possession of the original MS-DOS Encyclopedia (1986) which identifies WordStar in print. This is the same book that Larry Osterman claimed to have been subject to a publisher recall; either Microsoft Press didn’t do a very good job of pulling the book from the shelves or Larry Osterman’s recollection is inaccurate, but the book isn’t very difficult to find. (However, at about 8 pounds, or nearly 4 kilograms, it is quite difficult to carry around!)
On page 34, the MS-DOS Encyclopedia says: “For example, in CP/M-80, programmers would call address 5 in order to request a function. In MS-DOS, Interrupt 21 was the function call. But to support old programs, the first version of MS-DOS also allowed a program to request functions by calling address 5. Although this feature was not documented, a number of existing programs continued to use it when they were converted to MS-DOS. One of these programs was WordStar. Microsoft could not afford to make changes in the operating system that would make it impossible to run a program as popular as WordStar. So each new version had to continue supporting CALL 5, even though it was never documented.”
It is worth pointing out that the Encyclopedia contradicts itself somewhat. On page 14, it talks about how one of the objectives of MS-DOS (or 86-DOS) was to support mechanical translation of CP/M-80 programs. Such programs naturally used the CALL 5 interface and of course DOS had to support that, officially documented or not. It is sufficiently different from the INT 21h interface that automatic translation would be likely to fail.
In the second edition of the MS-DOS Encyclopedia, CP/M compatibility and WordStar are mentioned, but without explicitly naming the CALL 5 interface. At any rate, if the early WordStar editions for the PC used the CALL 5 interface, they would have certainly (indirectly) required address wraparound. WordStar may well have been enough reason for IBM to worry about backwards compatibility in the PC/AT. Examining a copy of WordStar for the IBM PC from 1982-1983 would be the only way to tell with 100% certainty, but at this point it seems beyond reasonable doubt that WordStar did use the CALL 5 interface.
It should be noted that years later, when DOS 5.0 might run with the A20 line permanently enabled, the most common source of problems related to address wraparound was Microsoft’s EXEPACK utility (later built into Microsoft’s LINK), or rather executables processed with EXEPACK. The problem manifested itself with the infamous “Packed file corrupt” message. However, there is no evidence that EXEPACK was available before the release of the IBM PC/AT in August 1984; the first known EXEPACK version is from early 1985 (shipped with Microsoft C 3.0). On the other hand, WordStar most certainly predated the PC/AT and was quite popular at the time.