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Here's an interesting retweet from ProDOS's author last month.

   @JBrooksBSI
   [59]Aug 17
   Was MS-DOS copied from CP/M? 
   [60]embedded.com/electronics-bl...

   60. https://t.co/mOR5mLBHwC

http://www.embedded.com/electronics-blogs/say-what-/4442498/...

discuss

13of40|9 years ago

"My conclusion is that DOS source code was not copied from CP/M source code."

"The commands were not copied; they were simple, descriptive terms that were common to other operating system such as VMS and Apple DOS."

"The DOS system calls were definitely copied from the CP/M system calls. Given the quantity of identical numbers representing identical functions, it is clear that Tim Paterson referenced the CP/M manual when writing DOS."

The last bit is alluded to in page D-7 of the DOS 1.0 user manual[1], which says "There is an additional mechanism for pre-existing programs that were written with different calling conventions. The function number is placed in the CL register...and an intrasegment call is made to location 5 in the current code segment." That's because in CP/M, "...access to the FDOS functions is accomplished by passing a function number and information address through the primary point at location BOOT+0005H" [2].

[1] https://archive.org/details/bitsavers_ibmpcdos61_7006095

[2] http://www.gaby.de/cpm/manuals/archive/cpm22htm/ch5.htm#Sect...

So, yes, the DOS function calls were designed with CP/M backwards compatibility in mind, and it says so right in the manual. Kind of.