saving a string of chars in assembly LC-3

Question

I am trying to make a simple program using the LC-3 Architecture. All I am trying to do is read a string from the console, somehow save it in memory, and then print it back out.

This is what I have so Far

; This program attemps to read a string and then output it

        .orig   x3000
        and     r1,r1,0
    lea     r0,prompt 
    puts
loop:
    getc
    putc
    add r4,r4,1
    ld      r7,nlcomp  ; check for 
    add     r7,r7,r0   ; end of line

brz finish

    st  r0,lets
    br      loop

finish:

    lea r0,lets
    puts

    halt

lets:  .blkw   20   
prompt: .stringz "Emter String"
nlcomp  .fill   xfff6        
.end

The output displays only the last char in the string. If I was to enter "steve" it would print out "e"

Obviously my problem is that I need to somehow save each char I read in, into its own memory location. I thought using the .blkw would do this, but apparently all it does it overwrite the bits that are in that position.

MY question is how do I store chars in sequential memory locations and then print them out to the console?

Answer 1

You need to use the STR instruction, which allows you to do base-offset addressing. The syntax for STR is:

STR <src register> <base register> <immediate offset>

So, something like the following would be valid:

    LEA R1,MEMORYSPACE ; saves the address of the storage memory block
loop:
    GETC               ; input character -> r0
    PUTC               ; r0 -> console
    STR R0,R1,#0       ; r0 -> ( memory address stored in r1 + 0 )
    ADD R1,R1,#1       ; increments the memory pointer so that it
                       ; always points at the next available block
    BR loop

MEMORYSPACE .blkw 100  ; declares empty space to store the string