Merge pull request MARIE-js#287 from kishorliv/fix-multiply-example

(MARIE-js#286) Fix multiplication when the second input(Y) is negative.

src/code/multiply.mas

@@ -3,30 +3,67 @@
 / Copyright (C) 2016. Licensed under the MIT License
 
 / Prompt user to type in integers
-INPUT
+Clear
+Store result
+Input
 Store X
-INPUT
+Input
 Store Y
 
+/ check if Y is negative, if -ve negate Y and set negative flag
+Load Y
+Skipcond 000
+Jump nonneg
+
+Subt Y
+Subt Y
+Store Y
+Clear
+Add one
+Store negflag
+Clear
+Jump loop
+
+nonneg,  Clear
+         Store negflag
+         / check if Y is zero, if it is, then we jump to halt
+         Load Y
+         Skipcond 400
+         Jump loop / false
+         Jump halt / true
+
 / Loop for performing iterative addition
-loop,    Load num
+loop,    Load result
          Add X
-         Store num
+         Store result
 
          Load Y
          Subt one
          Store Y
-
+	
          Skipcond 400 / have we completed the multiplication?
          Jump loop / no; repeat loop
          / yes, so exit the loop
 
+/ check for negative flag, if it is set, negate the result
+Load negflag
+Skipcond 800
+Jump halt
+
+/ negate result
+Load result
+Subt result
+Subt result
+Store result
+/ run the next three instructions, which halts the program
+
 / Output result to user then halt program
-Load num
-Output
-Halt
+halt,    Load result
+         Output	
+         Halt
 
 X, DEC 0
 Y, DEC 0
-num, DEC 0
 one, DEC 1
+negflag, DEC 0
+result, DEC 0