A student writes:

        Mighty Creator of LC-3,

I respond:

Enthusiastic programmer of the LC-3,

        How may I subtract one register from another register 
        by using existing LC-3 opcodes?

Shall I tell you, or should I give you a hint?

Aha, you prefer a hint!  Good.  The hint is:

                A-B = A+(-B).

Got it?  If not, please recall the two-step short-cut 
for creating -B if you know B?  If you can't, let me
know and I will try again.

        Please help me...I forgot to ask the TAs this question 
        during my 5.5 hours spent in Lab 507 tonight (Tues. night)...

        Thank you very much!

        Yours sincerely,
        <<name withheld to protect one who is spacy after 5.5 hours in ENS507>>

My pleasure,
Yale Patt