LC-3 Output a string from a .BLKW

Question

The program I am working on takes a 16-digit string of only 1s and 0s and stores that in memory labeled PATTERN and counts the number of 1s in the string and stores that value in NUM_ONES . I need to print the 16-digit string on the screen but am stuck on how to accomplish this.

I am storing the 16-digit string in a .BLKW . I tried loading the address of PATTERN into R0 and using PUTS but that does not print anything out for me.

    .orig x3100

; PROMPTING
            LEA R0, PROMPT_STRING
            PUTS

            LEA R1, PATTERN ; saves the address of the memory
        loop
              GETC
              OUT
              ADD R3, R0, x0 ;
              LD R6, DIFF ;
              ADD R3,R3,R6 ;
              STR R3, R1, #0   ;
              ADD R1, R1, #1  ;
              ADD R5, R0, #-10 ;
              BRZ countOneStart

              BR loop

      countOneStart
            ADD R2 R2 #15 ; Initialize loop count for CountDigits
            AND R7,R7,#0 ; Clear R7
            LEA R0, PATTERN ; Load Memory Space address into R0
            LDR R3,R0,#0 ; Load contents of PATTERN into R3
            AND R4,R3,#1 ; Check if digit is 1 or 0
            BRP addOne ; if positive go to addOne
            BRZ CountDigits ; if zero go to CountDigits

     addOne
          ADD R7,R7,#1 ; Increment R7 to keep count
          BR CountDigits

     CountDigits
          ADD R2 R2 #-1 ;
          BRN EndProgram
          ADD R0,R0,#1 ; Increment address
          LDR R3,R0,#0 ; Load contents
          AND R4,R3,#1 ; Check if digit is 1 or 0
          BRP addOne ; if positive go to addOne
          BRZ CountDigits

      EndProgram
        ST R7, NUM_ONES
    LEA R0 , PATTERN;
        AND R7,R7,#0 ; Clear R7
        STR R7,R0,#0    ; store Null at end of array
        LEA R0, PATTERN ; store  addr in R0
        TRAP x22    ; display output from memory address from R0 untill it founds Null
        
        HALT




 PROMPT_STRING      .STRINGZ "Please enter a 16-character string consisting of only 0s and 1s: "
 PATTERN .blkw #17 ; declares empty space to store the string
 DIFF .fill #-48
 NUM_ONES
 .END

Answer 1

Your algorithm looks ok, but you have missed some details.

If you don't know about debugging, now is the time to learn. You should be able to single step debug this. During single step, the idea is to determine the difference between what you expect to happen and what actually happened. We verify the execution of each instruction, because if any one instruction is wrong, the whole program won't work!

Here's what I did: skipping ahead a bit to the area you described as problematic, I put a breakpoint at EndProgram . The count of 1's in R7 appears correct. Now, look at the string in data memory. You can see it at address x3166, and also you can see that it is a sequence of numeric values consisting of the numbers 1 and 0 — interpreted for character printing, those are control characters and won't print properly using either putc or puts . You need ascii characters for printing.

Next the code nul-terminates the "string", but that is in error, since it doesn't put the nul character at the correct position. You'll see if you look at the string immediately after the nul termination. Though this error is hiding by the fact that the string is not composed of ascii characters, so has many nul's already therein, and a 0 overwritten with a nul won't really show.

You'll have to decide, based on what they want the program to do, whether to use puts with ascii characters (there at PATTERN or copied to somewhere else), or to use a loop with putc where you convert numbers from PATTERN to ascii for output.