diff --git a/csim.c b/csim.c
index 8b0f0c7..733381f 100644
--- a/csim.c
+++ b/csim.c
@@ -1,8 +1,23 @@
 #include <stdlib.h>
 #include <stdio.h>
+#include <getopt.h>
+#include <stdint.h>
 #include "cachelab.h"
 
-int VERBOSE = 0;
+#define mprintf(...) if (VERBOSE) printf(__VA_ARGS__)
+
+/**
+   If VERBOSE is non-zero, mprintf will not print. Otherwise, it will.
+   Set in main().
+ */
+int8_t VERBOSE = 0;
+
+/* As defined in conference. */
+struct line_s {
+	int8_t validity;
+	int tag;
+};
+typedef struct line_s line;
 
 void print_usage()
 {
@@ -10,58 +25,86 @@ void print_usage()
 		   "Options:\n"
 		   "  -h  Print this help.\n"
 		   "  -v  Display trace info.\n"
-		   "  -s <number> The number of sets is the base-2 logarithm of this argument.\n"
-		   "  -E <number> Associativity (lines per set).\n"
-		   "  -b <number> Block size in bits is the base-2 logarithm of this argument.\n"
+		   "  -s <number> The number of set index bits. Must be a positive integer.\n"
+		   "  -E <number> Associativity (lines per set). Must be a positive integer.\n"
+		   "  -b <number> The number of block bits. Must be a positive integer.\n"
 		   "  -t <file>   Name of the file to read a valgrind trace from.");
-	exit(0);
 }
 
-int parse_int_arg(char* arg)
+/* TODO: Move the exit call from parse_int_arg into the getopt cases. */
+/**
+   Parses a string
+   @param arg The string to parse.
+   @param opt The option being parsed.
+   @return The parsed integer if the argument was an integer.
+   @note Exits if argument is invalid
+ */
+size_t parse_int_arg(char* arg, char opt)
 {
 	char* end;
-	int i = strtol(arg, &end, 0);
-	if (!i || *end != '\0')
-	{
-		printf("Invalid argument \"%s\"\n", arg);
-		print_usage();
+	long i = strtol(arg, &end, 0);
+
+	if (!i || *end != '\0') {
+		printf("Invalid argument \"%s\" for option -%c\n", arg, opt);
+		printf("Usage: ./csim [-hv] -s <number> -E <number> -b <number> -t <file>\n");
+		exit(1);
+	}
+	else if (i < 0) {
+		printf("Argument %ld for option %c too small (must be greater than 0)", i, opt);
+		exit(1);
+	}
+	else {
+	return (size_t)i;
 	}
-	return i;
 }
 
+
 int main(int argc, char* argv[])
 {
-	int set_bits = 0, lines = 0, block_bits = 0;
-	char* filename;
-	int ii;
-	for (ii = 0; ii < argc; ii++)
-	{
-		switch (argv[ii][1])
-		{
-		case 'h':
-			//print_usage();
-			break;
+	size_t index_bits = 1, lines = ~0, block_bits = 1; // Useful variables.
+	char *filename;
+	int opt = 0, argflags = 0;
+	while ((opt=getopt(argc, argv, "hvs:E:b:t:")) != -1) {
+		switch (opt) {
+			/* TODO: maybe add checking for optarg swallowing actual arguments. */
 		case 'v':
 			VERBOSE = 1;
 			break;
-		case 's':
-			set_bits = parse_int_arg(argv[ii+1]);
+		case 's': // 2^s = number of sets
+			index_bits = parse_int_arg(optarg, 's');
+			argflags |= 8; // 0b1000
 			break;
-		case 'E':
-			lines = parse_int_arg(argv[ii+1]);
+		case 'E': // associativity - lines per set
+			lines = parse_int_arg(optarg, 'E');
+			argflags |= 4; // 0b0100
 			break;
-		case 'b':
-			block_bits = parse_int_arg(argv[ii+1]);
+		case 'b': // 2^b = block size
+			block_bits = parse_int_arg(optarg, 'b');
+			argflags |= 2; // 0b0010
 			break;
 		case 't':
-			filename = argv[ii+1];
+			filename = optarg;
+			argflags |= 1; // 0b0001
 			break;
+		case 'h':
+			print_usage();
+			return 0;
+		default:
+			printf("Usage: ./csim [-hv] -s <number> -E <number> -b <number> -t <file>\n");
+			return 1;
 		}
 	}
+	/* If any required arguments were not provided. (argflags != 0b1111) */
+	if (argflags != 15) {
+		printf("Usage: ./csim [-hv] -s <number> -E <number> -b <number> -t <file>\n");
+		return 1;
+	}
+	/* End of argument parsing. */
 
-	printf("Arguments: %d, %d, %d, %d, %s\n", set_bits, lines, block_bits, VERBOSE, filename);
-	if(!set_bits|| !lines || !block_bits || !filename)
+	printf("Arguments: %zd, %zd, %zd, %hd, %s\n", index_bits, lines, block_bits, VERBOSE, filename);
+	if(!index_bits || !lines || !block_bits || !filename) {
 		print_usage();
+	}
 
 	printSummary(0, 0, 0);
 	return 0;