LC-3 Decimal to Binary Converter

Question

I have to create a program that converts the given user input (decimal) into its binary match when the user presses enter or exit when they press X. Can anyone give a pointer on how to start this assignment. We are required to use masks and loops, but I do not know where to start with this.

Answer 1

I wrote this because there aren't a whole lot of examples of binary masks.

A binary mask is useful in assembly because it gives us the power to examine a single bit in a decimal or hex value.

Example: If we look at the decimal number 5 we know its binary value is 0101, when we AND R4, R4, R5 on our first loop we compare 1000 with 0101. Since bit[3] is a zero in 0101 we then tell the LC3 simulator to print out an ASCII char "0". We then repeat the process with the next binary mask 0100.

This can be a tricky concept to get at first so I would first look at a few examples of loops and bit masks.

LC3 Bit Counter

How do I write a program that prints out “Hello World”, 5 times using a loop in LC3?

.ORIG x3000

    LEA R0, PROMPT
    PUTs                ; TRAP x22
    LD R0, ENTER
    OUT                 ; TRAP x21
    IN                  ; TRAP x23

    AND R5, R5, #0      ; clear R5
    ADD R5, R5, R0      ; Store the user input into R5

    AND R1, R1, #0      ; clear R1, R1 is our loop count
    LD R2, MASK_COUNT   ; load our mask limit into R2
    NOT R2, R2          ; Invert the bits in R2
    ADD R2, R2, #1      ; because of 2's compliment we have
                        ; to add 1 to R2 to get -4
WHILE_LOOP
    ADD R3, R1, R2      ; Adding R1, and R2 to see if they'll
                        ; will equal zero
    BRz LOOP_END        ; If R1+R2=0 then we've looped 4
                        ; times and need to exit

    LEA R3, BINARY      ; load the first memory location 
                        ; in our binary mask array
    ADD R3, R3, R1      ; use R1 as our array index and
                        ; add that to the first array location
    LDR R4, R3, #0      ; load the next binary mask into R4

    AND R4, R4, R5      ; AND the user input with the 
                        ; binary mask
    BRz NO_BIT
    LD R0, ASCII_ONE
    OUT                 ; TRAP x21
    ADD R1, R1, #1      ; add one to our loop counter
    BRnzp WHILE_LOOP    ; loop again
NO_BIT
    LD R0, ASCII_ZERO
    OUT                 ; TRAP x21

    ADD R1, R1, #1      ; add one to our loop counter
    BRnzp WHILE_LOOP    ; loop again
LOOP_END

    LD R0, ENTER
    OUT                 ; TRAP x21
    HALT                ; TRAP x25

; Binary Maps
BINARY  .FILL   b0000000000001000
        .FILL   b0000000000000100
        .FILL   b0000000000000010
        .FILL   b0000000000000001
        .FILL   b0000000000000000

; Stored Values
ENTER       .FILL   x000A
ASCII_ZERO  .FILL   x0030
ASCII_ONE   .FILL   x0031
MASK_COUNT  .FILL   x04     ; loop limit = 4
PROMPT      .STRINGZ "Enter a number from 0-9"

.END