CBM Studio and old Assembly books

Hi all:

I tried looking this up ahead of time just in case someone had already asked.  But the search term I used is only two characters long and too short to search.

I'm reading the C128 Assembly Language book and I'm on chapter 7 playing with the A.S program.

The book is using Merlin 128 as the assembler, which I think is a great assembler, but I prefer using CBM PRG Studio, it's got a lot of fantastic features and I'm finding it easy to use, which is great because now I can focus on learning machine language programming and not futzing with old school programming applications.

I've hit a bit of a snag though.  I don't know if anyone can help but I figured this was a good place to start.

There's a line of code in the sample I'm playing with that I can't seem to translate from Merlin 128 to CBM Studio.

txtbuf    ds    buflen

I can't figure out exactly what ds does, and I can't find a pseudo op code in CBM Studio to use in it's place.

I've worked out:
ORG    *=
ASC    TEXT
EQU    =

So there are appropriate op codes to use.

Any help would be appreciated.

Thanks.