How can I write a simple LC-3 program

Question

How can I write a simple LC-3 program that compares the two numbers in R1 and R2 and puts the value 0 in R0 if R1 = R2, 1 if R1 > R2 and -1 if R1 < R2.

Answer 1

The comparison is done using simple arithmetic.

In my example we compare 2 and 6, you know what the result is.

           LD R1, NUMBER1    ;load NUMBER1 into R1
           LD R2, NUMBER2    ;load NUMBER1 into R2

           AND R6,R6,#0      ;initialize R0 with 0

           NOT R3, R2        ;R3 = -R2  (we negate NUMBER2)

           ADD R4, R3, R1    ;R4 = R1 - R2

           BRz Equals        ;we jump to Equals if NUMBER1 = NUMBER2  (we can just jump directly to END)
           BRn GreaterR2     ;we jump to GreaterR2 if NUMBER1 < NUMBER2
           BRp GreaterR1     ;we jump to GreaterR2 if NUMBER1 > NUMBER2

Equals     BRnzp End         ;nothing to do, because R0=0 (THIS IS NOT NECCESARY)

GreaterR2  ADD R0, R0, #-1   ;R0 = -1
           BRnzp End

GreaterR1  ADD R0, R0, #1    ;R0 = 1
           BRnzp End

Done       HALT               ;THE END


NUMBER1 .FILL #2              ;/ Here we declare the numbers we want to compare
NUMBER1 .FILL #6              ;\