Hi Glen,
Good luck with learning assembler. Its not too hard. The answer to your question depends a lot on what kind of ADC you have. Is it serially addressed or parallel for example. Is it 8 bit, 10,12,14 16 bits ? How you work the routine depends entirely on how the chip needs to be sequenced to extract its answer. Some you must poll to start a conversion, then wait for it to say "I'm ready", some give you answers all the time. Some need you to say which channel to convert.

For MCS 52 I would use the stack push function to stick the answer on Basic's arithmetic stack and then POP the answer back off in BASIC.

Steve