DOS 3.3 ASM load/save file?

[Lyrical Nanoha <LyricalNanoha@dosius.net>]

Let's say I have a start-of-file and end-of-file address (but not a 
size-of-file), and want to save the file using type S, or I want to read 
the file at the start address and set the end address.  Something like a 
BASIC program. (In fact it is BASIC, just not FP or INT.)  I'm using CA65.

How would you do this?

BTW, this is a slightly tweaked EHBASIC.  I'd like to be able to have the 
LOAD and SAVE commands require filenames, if anyone's familiar with how 
EHBASIC works.

-uso.