diff --git a/src/threads/synch.c b/src/threads/synch.c
index 09fc71f..d706d19 100644
--- a/src/threads/synch.c
+++ b/src/threads/synch.c
@@ -113,28 +113,33 @@ void
 sema_up (struct semaphore *sema)
 {
   enum intr_level old_level;
+  bool thread_unblocked = false; /* Flag to track if any thread was woken up. */
 
   ASSERT (sema != NULL);
 
   old_level = intr_disable ();
   if (!list_empty (&sema->waiters))
-    {
-      /* Enforces wake-up of the highest priority thread waiting for the
-         semaphore. */
-      struct list_elem *e = list_max (&sema->waiters, priority_less, NULL);
-      list_remove (e);
-      thread_unblock (list_entry (e, struct thread, elem));
-    }
+  {
+    /* Enforces wake-up of the highest priority thread waiting for the
+       semaphore. */
+    struct list_elem *e = list_max (&sema->waiters, priority_less, NULL);
+    list_remove (e);
+    thread_unblock (list_entry (e, struct thread, elem));
+    thread_unblocked = true;
+  }
   sema->value++;
   intr_set_level (old_level);
 
   /* Yields the CPU in case the thread that has been woken up has a higher
      priority that the current running thread, including the case when called
      within an interrupt handler. */
-  if (intr_context ())
-    intr_yield_on_return ();
-  else
-    thread_yield ();
+  if (thread_unblocked)
+  {
+    if (intr_context ())
+      intr_yield_on_return ();
+    else
+      thread_yield ();
+  }
 }
 
 static void sema_test_helper (void *sema_);
@@ -347,7 +352,6 @@ lock_release (struct lock *lock)
 
   lock->holder = NULL;
   sema_up (&lock->semaphore);
-  thread_yield ();
 }
 
 /* Returns true if the current thread holds LOCK, false
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 7f38374..20e2677 100644
--- a/src/userprog/process.c
+++ b/src/userprog/process.c
@@ -14,6 +14,7 @@
 #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"
@@ -201,12 +202,12 @@ start_process (void *file_name_)
 int
 process_wait (tid_t child_tid UNUSED) 
 {
-  /* TODO: Implement correct process waiting behaviour.
-    Currently an infinite loop for testing purposes. */
-  while (1)
-    {
-      barrier ();
-    }
+  /* 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. */
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 */