11/9/04

Recall the dialogue tonight about Problem 9.5 in the book. A student writes:

     Hi Dr. Patt,
     
          I did what you suggested for the issue we discussed 
     in the review session- in reference to problem 9.5 
     in the book which is also problem 3 on the review 
     (word for word)- which was to look on the errata sheet 
     online to see if it had been considered as an error 
     and I found that it is not on the errata sheet as far as
          I could tell. The problem was you have an instruction 
     ST  R0, x3007 and you cannot write x3007 and there WILL 
     be an assemble time error and the question tells you that 
     there are no errors. Maybe you will have to speak to one 
     of your old TA's after all.

     << name withheld to protect the helpful student >>

Clearly, so I did! It turns out what happened was, as I said in class, that we lifted the exam question from two years ago when we were using the LC-2 and put it at the end of Chapter 9. The LC-2 Assembler was a little more forgiving and allowed ST R0, x3007 to be assembled, recognizing that the programmer meant address x3007, rather than offset x3007.

What we did not realize at the time, however, was that the guy who actually wrote the Assembler for the LC-3 (THREE, not TWO) decided that ST R0, x3007 was just too ugly to let the Assembler let it go. Ergo, the assemble time error on your LC-3 Simulator.

So, in the interest of having an LC-3 assembly language program that does the thing we wanted to illustrate, let me offer:

                .ORIG  x3000
                LEA    R0, LABEL
                STR    R1, R0, #3        
                TRAP   x22     
                TRAP   x25     
        LABEL   .STRINGZ "FUNKY"       
        LABEL2  .STRINGZ "HELLO WORLD" 
                .END

Yale Patt