12/2/04

A student writes (actually several students wrote): 

     Ok, i have a question on the program form rules.

I guess my new name is Ok. 

     First, everything works great.

Even if he does say so, himself. 

     My program is more or less done and submited. But
     there was one thing that was bothering me and I am
     not sure if i did the right thing.  In the Program 5
     page, you guys hint on how one might go about using
     the ROW and COLUMN number to figure out which location
     in memory the user is going to.  well... I did exactly
     what you guys said and put the loacation of ROW0 in R5.  
     I am pretty sure that was the only way to do it but
     there wasnt any info on the registar already having that
     value so i knew i had to put this value in R5 my self.
     Second I am way to far from ROW0 in the starter code to
     use a LEA so i used the LC3 to get its location and
     .Fill it into the LC3. bad form?

Yep. 

     just wrong?

Nope. 

     I apoligize if i am being unclear and would glady clarify
     anything.

     thanks,
     << Not-Ok >>

Actually, it is possible to write the routines in such a way that LEA R5, ROW0 is not too far away from the label ROW0.

However, if you are too far away, you could use the symbolic value in the .FILL instruction. To wit (inside a subroutine), you could put: 

             LD      R5, XYZ
             ...
             ...                ; other code
             ...
             RET
     XYZ        .FILL   ROW0
             ...                ; other data

The Assembler, in its first pass (as you know) will pick up the label ROW0 in creating its Symbol Table. In the second pass, location XYZ will be filled with the address corresponding to ROW0 (which the Assembler will get from the Symbol Table).

Yale Patt