DOS OS/A+ Programmer's Documentation

If I remember correctly, OSS'es DOS A/+ defined a couple of additional service routines indirectly through the DOS vector at $a,$b, and a couple of additional constants, also relatively to this vector - if I remember correctly, to parse off arguments from the command line given to an external command.

I couldn't find a good description of these service routines anywhere, does anyone have more information, specifically on the calling conventions (i.e. input and output arguments)?

Thanks,
Thomas