2016 Update

Task/Reverse-a-string/MIPS-Assembly/reverse-a-string.mips

@@ -0,0 +1,67 @@
+# First, it gets the length of the original string
+# Then, it allocates memory from the copy
+# Then it copies the pointer to the original string, and adds the strlen
+#     subtract 1, then that new pointer is at the last char.
+# while(strlen)
+#     copy char
+#     decrement strlen
+#     decrement source pointer
+#     increment target pointer
+
+.data
+	ex_msg_og: .asciiz "Original string:\n"
+	ex_msg_cpy: .asciiz "\nCopied string:\n"
+	string: .asciiz "Wow, what a string!"
+
+.text
+	main:
+		la $v1,string #load addr of string into $v0
+		la $t1,($v1)  #copy addr into $t0 for later access
+		lb $a1,($v1)  #load byte from string addr
+	strlen_loop:	
+		beqz $a1,alloc_mem
+		addi $a0,$a0,1 #increment strlen_counter
+		addi $v1,$v1,1 #increment ptr
+		lb $a1,($v1)   #load the byte
+		j strlen_loop
+		
+	alloc_mem:
+		li $v0,9 #alloc memory, $a0 is arg for how many bytes to allocate
+		         #result is stored in $v0
+		syscall
+		la $t0,($v0) #$v0 is static, $t0 is the moving ptr
+		la $v1,($t1) #get a copy we can increment
+		
+		add $t1,$t1,$a0 #add strlen to our original, static addr to equal last char
+		subi $t1,$t1,1  #previous operation is on NULL byte, i.e. off-by-one error.
+		                #this corrects.
+	copy_str:
+		lb $a1,($t1) #copy first byte from source
+
+	strcopy_loop:
+		beq $a0,0,exit_procedure
+		sb $a1,($t0)            #store the byte at the target pointer
+		addi $t0,$t0,1          #increment target ptr
+		subi $t1,$t1,1
+		subi $a0,$a0,1
+		lb $a1,($t1)            #load next byte from source ptr
+		j strcopy_loop
+		
+	exit_procedure:
+		la $a1,($v0) #store our string at $v0 so it doesn't get overwritten
+		li $v0,4 #set syscall to PRINT
+		
+		la $a0,ex_msg_og  #PRINT("original string:")
+		syscall
+		
+		la $a0,($v1)      #PRINT(original string)
+		syscall
+		
+		la $a0,ex_msg_cpy #PRINT("copied string:")
+		syscall
+		
+		la $a0,($a1)      #PRINT(strcopy)
+		syscall
+		
+		li $v0,10         #EXIT(0)
+		syscall