I've played with this a bit and found the following:
You can't step-into asm functions (F11), but you
can set breakpoints within the ASM source code as long as it is defined inside a "PROC". Code which lies outside of procs cannot have a breakpoint.
This really isn't too bad a limitation. Before I put my code inside PROCs I was thinking that there was no source level debugging at all.
Larry B.