diff --git a/src/threads/thread.c b/src/threads/thread.c
index 7102ad2..457f7b9 100644
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -659,6 +659,8 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->recent_cpu = recent_cpu;
   t->priority = t->base_priority;
 
+  t->exit_status = -1;
+
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);
   intr_set_level (old_level);
diff --git a/src/threads/thread.h b/src/threads/thread.h
index c7cc364..1c05030 100644
--- a/src/threads/thread.h
+++ b/src/threads/thread.h
@@ -111,6 +111,7 @@ struct thread
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */
 
+    int exit_status;                    /* Exit Status: 0 = successful exit. */
 
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
diff --git a/src/userprog/process.c b/src/userprog/process.c
index 49b9bd5..6c8ef2d 100644
--- a/src/userprog/process.c
+++ b/src/userprog/process.c
@@ -14,9 +14,21 @@
 #include "threads/flags.h"
 #include "threads/init.h"
 #include "threads/interrupt.h"
+#include "threads/synch.h"
 #include "threads/palloc.h"
+#include "threads/malloc.h"
 #include "threads/thread.h"
 #include "threads/vaddr.h"
+#include "threads/synch.h"
+#include "devices/timer.h"
+
+/* Keeps track of the position of pointers to user program arguments
+   within a linked list. */
+struct arg_elem
+  {
+    char* arg;
+    struct list_elem elem;
+  };
 
 static thread_func start_process NO_RETURN;
 static bool load (const char *cmdline, void (**eip) (void), void **esp);
@@ -36,6 +48,9 @@ process_execute (const char *file_name)
   fn_copy = palloc_get_page (0);
   if (fn_copy == NULL)
     return TID_ERROR;
+
+  /* Imposing implicit limit that the command line arguments
+    including the user program name fit within a single page. */
   strlcpy (fn_copy, file_name, PGSIZE);
 
   /* Create a new thread to execute FILE_NAME. */
@@ -45,15 +60,34 @@ process_execute (const char *file_name)
   return tid;
 }
 
+static void *
+push_to_stack (void **esp, void *data, size_t data_size)
+{
+  *esp -= data_size;
+  memcpy (*esp, data, data_size);
+  return *esp;
+}
+
 /* A thread function that loads a user process and starts it
    running. */
 static void
 start_process (void *file_name_)
 {
-  char *file_name = file_name_;
   struct intr_frame if_;
   bool success;
 
+  /* Retrieve first argument of command, which is the file name
+     of the process. */
+  char *saveptr;
+  char *arg = strtok_r (file_name_, " ", &saveptr);
+  
+  char file_name[15];
+  strlcpy (file_name, arg, 15);
+
+  /* TODO: Move naming of thread to process_execute, so start
+    tokenizing there. */
+  strlcpy (thread_current ()->name, file_name, 15);
+
   /* Initialize interrupt frame and load executable. */
   memset (&if_, 0, sizeof if_);
   if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
@@ -61,8 +95,88 @@ start_process (void *file_name_)
   if_.eflags = FLAG_IF | FLAG_MBS;
   success = load (file_name, &if_.eip, &if_.esp);
 
+  /* Load command line argument *data* to user process stack.
+     This can't cause overflow due to enforcing that the size of
+     command line input must fit in a page. Also keep track
+     of pointers to the argument data within a linked list. */
+  struct list arg_list;
+  list_init (&arg_list);
+
+  int arg_count = 0;
+  while (arg != NULL)
+    {
+      push_to_stack (&if_.esp, arg, (strlen (arg) + 1) * sizeof (char));
+      
+      struct arg_elem *arg_elem = malloc (sizeof (struct arg_elem));
+      if (arg_elem == NULL)
+        {
+          printf("ERROR: Couldn't allocate argument pointer memory for %s!\n",
+                 file_name);
+          palloc_free_page (file_name_);
+          if (success) process_exit ();
+          thread_exit ();
+        }
+
+      arg_elem->arg = if_.esp;
+      list_push_front (&arg_list, &arg_elem->elem);
+
+      arg_count++;
+      arg = strtok_r (NULL, " ", &saveptr);
+    }
+
+  /* Calculate the remaining number of bytes that need to be written
+     to the user process stack in order to check for possible overflow. */
+  size_t align_size = ((unsigned int) if_.esp % 4) * sizeof (uint8_t);
+  size_t argv_data_size = (arg_count + 1) * sizeof (char *);
+  size_t argv_size = sizeof (char **);
+  size_t argc_size = sizeof (int);
+  size_t return_addr_size = sizeof (void *);
+  size_t remaining_size = align_size + argv_data_size + argv_size + argc_size 
+                          + return_addr_size;
+
+  /* If pushing the rest of the data required for the stack would cause
+     overflow, allocate an extra page. */ 
+  if (PHYS_BASE - if_.esp + remaining_size > PGSIZE)
+    {
+      /* TODO: Allocate an extra page for the rest of the process stack. */
+    }
+
+  /* Align stack pointer to word size before pushing argv elements for
+     performance. */
+  if_.esp -= align_size;
+  
+  /* Push a null pointer sentinel inside argv. */
+  if_.esp -= sizeof (char *);
+  *(char *) if_.esp = 0;
+  
+  /* Push pointer to the process file name to the stack. */
+  char **argv;
+
+  /* Push pointers to process arguments from argument linked list */
+  struct list_elem *e = list_begin (&arg_list);
+  struct list_elem *tail = list_tail (&arg_list);
+  while (e != tail)
+    {
+      struct arg_elem *arg_elem = list_entry (e, struct arg_elem, elem);
+      
+      argv = push_to_stack (&if_.esp, &arg_elem->arg, sizeof (arg_elem->arg));
+      
+      e = list_next (e);
+      free (arg_elem);
+    }
+
+  /* Push pointer to the start of argv array. */
+  push_to_stack (&if_.esp, &argv, sizeof(argv));
+
+  /* Push the number of arguments to the stack. */
+  push_to_stack (&if_.esp, &arg_count, sizeof (arg_count));
+
+  /* Push fake return address (null pointer). */
+  if_.esp -= sizeof (char *);
+  *(char *) if_.esp = 0;
+
   /* If load failed, quit. */
-  palloc_free_page (file_name);
+  palloc_free_page (file_name_);
   if (!success) 
     thread_exit ();
 
@@ -88,7 +202,12 @@ start_process (void *file_name_)
 int
 process_wait (tid_t child_tid UNUSED) 
 {
-  return -1;
+  /* As a temporary wait, waiting will just put the thread to sleep for one
+     second (TIMER_FREQ = 100 ticks ~ 1 second). */
+  /* TODO: Implement process_wait () correctly. Remove the next line. */
+  timer_sleep (TIMER_FREQ);
+
+  return 0; /* TODO: Change this too */
 }
 
 /* Free the current process's resources. */
@@ -98,6 +217,8 @@ process_exit (void)
   struct thread *cur = thread_current ();
   uint32_t *pd;
 
+  printf ("%s: exit(%d)\n", cur->name, cur->exit_status);
+
   /* Destroy the current process's page directory and switch back
      to the kernel-only page directory. */
   pd = cur->pagedir;
diff --git a/src/userprog/syscall.c b/src/userprog/syscall.c
index 370c89b..1dbe73b 100644
--- a/src/userprog/syscall.c
+++ b/src/userprog/syscall.c
@@ -1,11 +1,68 @@
 #include "userprog/syscall.h"
-#include <stdio.h>
-#include <syscall-nr.h>
+#include "devices/shutdown.h"
+#include "devices/input.h"
+#include "threads/vaddr.h"
 #include "threads/interrupt.h"
 #include "threads/thread.h"
+#include "userprog/process.h"
+#include <stdio.h>
+#include <syscall-nr.h>
 
 static void syscall_handler (struct intr_frame *);
 
+/* A syscall_function is a function that receives up to 3 arguments, the
+   arguments to the functions are either ints or pointers taking up to 32 bits
+   in size. */
+typedef uintptr_t (*syscall_function) (uintptr_t, uintptr_t, uintptr_t);
+
+/* System call function prototypes */
+static void syscall_halt (void);
+static void syscall_exit (int status);
+static pid_t syscall_exec (const char *cmd_line);
+static int syscall_wait (pid_t pid);
+static bool syscall_create (const char *file, unsigned initial_size);
+static bool syscall_remove (const char *file);
+static int syscall_open (const char *file);
+static int syscall_filesize (int fd);
+static int syscall_read (int fd, void *buffer, unsigned size);
+static int syscall_write (int fd, const void *buffer, unsigned size);
+static void syscall_seek (int fd, unsigned position);
+static unsigned syscall_tell (int fd);
+static void syscall_close (int fd);
+
+static void *validate_user_pointer (const void *ptr, size_t size);
+
+/* A struct defining a syscall_function pointer along with its arity. */
+typedef struct
+  {
+    syscall_function function;    /* Function pointer. */
+    int arity;                    /* Number of arguments of the function. */
+  } syscall_arguments;
+
+/* A look-up table mapping numbers to system call functions with their number of
+   arguments. */
+static const syscall_arguments syscall_lookup[] =
+{
+  [SYS_HALT] = {(syscall_function) syscall_halt, 0},
+  [SYS_EXIT] = {(syscall_function) syscall_exit, 1},
+  [SYS_EXEC] = {(syscall_function) syscall_exec, 1},
+  [SYS_WAIT] = {(syscall_function) syscall_wait, 1},
+  [SYS_CREATE] = {(syscall_function) syscall_create, 2},
+  [SYS_REMOVE] = {(syscall_function) syscall_remove, 1},
+  [SYS_OPEN] = {(syscall_function) syscall_open, 1},
+  [SYS_FILESIZE] = {(syscall_function) syscall_filesize, 1},
+  [SYS_READ] = {(syscall_function) syscall_read, 3},
+  [SYS_WRITE] = {(syscall_function) syscall_write, 3},
+  [SYS_SEEK] = {(syscall_function) syscall_seek, 2},
+  [SYS_TELL] = {(syscall_function) syscall_tell, 1},
+  [SYS_CLOSE] = {(syscall_function) syscall_close, 1},
+};
+
+/* The number of syscall functions (i.e, number of elements) within the
+   syscall_lookup table. */
+static const int LOOKUP_SIZE
+  = sizeof (syscall_lookup) / sizeof (syscall_arguments);
+
 void
 syscall_init (void) 
 {
@@ -13,8 +70,165 @@ syscall_init (void)
 }
 
 static void
-syscall_handler (struct intr_frame *f UNUSED) 
+syscall_handler (struct intr_frame *f)
 {
-  printf ("system call!\n");
+  /* First, read the system call number from the stack. */
+  validate_user_pointer (f->esp, 1);
+  unsigned syscall_number = *(int *) f->esp;
+
+  /* Ensures the number corresponds to a system call that can be handled. */
+  if (syscall_number >= LOOKUP_SIZE)
+    thread_exit ();
+
+  syscall_arguments syscall = syscall_lookup[syscall_number];
+
+  /* Next, read and copy the arguments from the stack pointer. */
+  validate_user_pointer (f->esp + sizeof (uintptr_t),
+                         syscall.arity * sizeof (uintptr_t));
+  uintptr_t args[3] = {0};
+  for (int i=0; i < syscall.arity; i++)
+      args[i] = *(uintptr_t *) (f->esp + sizeof (uintptr_t) * (i + 1));
+
+  /* Call the function that handles this system call with the arguments. When
+     there is a return value it is stored in f->eax. */
+  f->eax = syscall.function (args[0], args[1], args[2]);
+}
+
+static void
+syscall_halt (void)
+{
+  shutdown_power_off ();
+}
+
+static void
+syscall_exit (int status)
+{
+  /* Sets exit_status of the thread to status. thread_exit () will call
+     process_exit () if user programs are allowed. */
+  thread_current ()->exit_status = status;
   thread_exit ();
 }
+
+static pid_t
+syscall_exec (const char *cmd_line UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_wait (pid_t pid)
+{
+  return process_wait (pid);
+}
+
+static bool
+syscall_create (const char *file UNUSED, unsigned initial_size UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static bool
+syscall_remove (const char *file UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_open (const char *file UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_filesize (int fd UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_read (int fd, void *buffer, unsigned size)
+{
+  /* Only console (fd = 0) or other files, not including STDOUT, (fd > 1) are
+     allowed. */
+  if (fd < 0 && fd != STDOUT_FILENO)
+    return -1;
+
+  validate_user_pointer (buffer, size);
+
+  if (fd == STDIN_FILENO)
+    {
+      /* Reading from the console. */
+      char *write_buffer = buffer;
+      for (int i = 0; i < size; i++)
+        write_buffer[i] = input_getc ();
+
+      return size;
+    }
+  else
+    {
+      /* Reading from a file. */
+      return 0; // TODO: Implement Write to Files
+    }
+}
+
+static int
+syscall_write (int fd, const void *buffer, unsigned size)
+{
+  /* Only console (fd = 1) or other files, not including STDIN, (fd > 1) are
+     allowed. */
+  if (fd <= 0)
+    return 0;
+
+  validate_user_pointer (buffer, size);
+
+  if (fd == STDOUT_FILENO)
+    {
+      /* Writing to the console. */
+      putbuf (buffer, size);
+      return size;
+    }
+  else
+    {
+      /* Writing to a file. */
+      return 0; // TODO: Implement Write to Files
+    }
+}
+
+static void
+syscall_seek (int fd UNUSED, unsigned position UNUSED)
+{
+  //TODO
+}
+
+static unsigned
+syscall_tell (int fd UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static void
+syscall_close (int fd UNUSED)
+{
+  //TODO
+}
+
+/* Validates if a block of memory starting at PTR and of size SIZE bytes is
+   fully contained within user virtual memory. Kills the thread (by calling
+   thread_exit) if the memory is invalid. Otherwise, returns the PTR given.
+   If the size is 0, the function does no checks and returns PTR.*/
+static void *
+validate_user_pointer (const void *ptr, size_t size)
+{
+  if (size > 0 && (ptr == NULL ||
+                   !is_user_vaddr (ptr) ||
+                   !is_user_vaddr (ptr + size - 1)))
+    thread_exit ();
+
+  return ptr;
+}
diff --git a/src/userprog/syscall.h b/src/userprog/syscall.h
index 9059096..702a6c7 100644
--- a/src/userprog/syscall.h
+++ b/src/userprog/syscall.h
@@ -1,6 +1,8 @@
 #ifndef USERPROG_SYSCALL_H
 #define USERPROG_SYSCALL_H
 
+typedef int pid_t;
+
 void syscall_init (void);
 
 #endif /* userprog/syscall.h */