This article is within the scope of WikiProject Software, a collaborative effort to improve the coverage of software on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
In standard DOS, COMMENT and REM do the same thing, there is no redefining of the character for inline comments. Also I really think that the non-standard DOS directives should be put in a separate section from the standard DOS (MS-DOS and PC DOS) directives. Asmpgmr (talk) 00:19, 12 July 2012 (UTC)
Even myself was fooled to believe that COMMENT would be the same as REM for a long while ;-) (probably caused by misleading documentation on Microsoft's side). However, it isn't (and this holds true for at least MS-DOS 6.00 to 6.22, but probably also for older (since 4.0) and newer versions, I just don't know for sure right now - I also assume it is the same for PC DOS, but as I mentioned before there /are/ some very subtle differences even between "same" versions of MS-DOS and PC DOS and if you are not aware of this COMMENT thing in PC DOS, this might be one of these differences, I don't know - but I may look it up). Try:
DEVICE = C:\SYS\MYDRIVER.SYS ; inline comments
COMMENT = //
DEVICE = C:\SYS\MYDRIVER.SYS // inline comments
Since not all characters are accepted, COMMENT may even work as alias to REM in many cases, so that many people never saw a difference. This is also documented in at least two books (DOS Internals by Geoff Chappell and Dissecting DOS by Mike Podanoffsky, whereas in my own publications I erroneously still wrote it would be the same as REM). It is not only supported by MS-DOS (presumably PC DOS as well), but also by RxDOS.
You may be right about the COMMENT thing. With DOS 5 the differences between MS-DOS and PC DOS were minimal. Note Microsoft and IBM still did code sharing for DOS 6 so INTERLNK, INTERSVR and POWER from PC DOS 5.02 appear in MS-DOS 6 while CHOICE, DEFRAG, DELTREE, MOVE, MSCDEX, SMARTDRV.EXE and the CONFIG.SYS menu stuff appear in PC DOS 6.1.
Prior to DOS 5 there were more differences between the two. There is a README.TXT from the DOS 3.2 OAK floating around the web listing the differences.
Regarding SWITCHAR and AVAILDEV. As far as I remember (without looking this up again), IBM was expressively again them, and that's why they were removed again later on. Out of the back of my head, I'm not sure if these directives were only supported by MS-DOS 2.x, not by PC DOS 2.x. If they were supported by PC DOS as well, we could change the "MS-DOS 2.0-2.1 to DOS 2.0-2.1). Do you happen to know for sure? (I can look this up in my archive, but it will take some time.) --Matthiaspaul (talk) 11:42, 12 July 2012 (UTC)
AVAILDEV and SWITCHAR are in MS-DOS 2.0 to 2.11 and are in PC DOS 2.x but not in either version of DOS 3.