2 files changed, 173 insertions, 0 deletions
diff --git a/user/uthread.c b/user/uthread.c
new file mode 100644
index 0000000..06349f5
--- /dev/null
+++ b/user/uthread.c
@@ -0,0 +1,162 @@
+#include "kernel/types.h"
+#include "kernel/stat.h"
+#include "user/user.h"
+
+/* Possible states of a thread: */
+#define FREE        0x0
+#define RUNNING     0x1
+#define RUNNABLE    0x2
+
+#define STACK_SIZE  8192
+#define MAX_THREAD  4
+
+
+struct thread {
+  char       stack[STACK_SIZE]; /* the thread's stack */
+  int        state;             /* FREE, RUNNING, RUNNABLE */
+};
+struct thread all_thread[MAX_THREAD];
+struct thread *current_thread;
+extern void thread_switch(uint64, uint64);
+              
+void 
+thread_init(void)
+{
+  // main() is thread 0, which will make the first invocation to
+  // thread_schedule().  it needs a stack so that the first thread_switch() can
+  // save thread 0's state.  thread_schedule() won't run the main thread ever
+  // again, because its state is set to RUNNING, and thread_schedule() selects
+  // a RUNNABLE thread.
+  current_thread = &all_thread[0];
+  current_thread->state = RUNNING;
+}
+
+void 
+thread_schedule(void)
+{
+  struct thread *t, *next_thread;
+
+  /* Find another runnable thread. */
+  next_thread = 0;
+  t = current_thread + 1;
+  for(int i = 0; i < MAX_THREAD; i++){
+    if(t >= all_thread + MAX_THREAD)
+      t = all_thread;
+    if(t->state == RUNNABLE) {
+      next_thread = t;
+      break;
+    }
+    t = t + 1;
+  }
+
+  if (next_thread == 0) {
+    printf("thread_schedule: no runnable threads\n");
+    exit(-1);
+  }
+
+  if (current_thread != next_thread) {         /* switch threads?  */
+    next_thread->state = RUNNING;
+    t = current_thread;
+    current_thread = next_thread;
+    /* YOUR CODE HERE
+     * Invoke thread_switch to switch from t to next_thread:
+     * thread_switch(??, ??);
+     */
+  } else
+    next_thread = 0;
+}
+
+void 
+thread_create(void (*func)())
+{
+  struct thread *t;
+
+  for (t = all_thread; t < all_thread + MAX_THREAD; t++) {
+    if (t->state == FREE) break;
+  }
+  t->state = RUNNABLE;
+  // YOUR CODE HERE
+}
+
+void 
+thread_yield(void)
+{
+  current_thread->state = RUNNABLE;
+  thread_schedule();
+}
+
+volatile int a_started, b_started, c_started;
+volatile int a_n, b_n, c_n;
+
+void 
+thread_a(void)
+{
+  int i;
+  printf("thread_a started\n");
+  a_started = 1;
+  while(b_started == 0 || c_started == 0)
+    thread_yield();
+  
+  for (i = 0; i < 100; i++) {
+    printf("thread_a %d\n", i);
+    a_n += 1;
+    thread_yield();
+  }
+  printf("thread_a: exit after %d\n", a_n);
+
+  current_thread->state = FREE;
+  thread_schedule();
+}
+
+void 
+thread_b(void)
+{
+  int i;
+  printf("thread_b started\n");
+  b_started = 1;
+  while(a_started == 0 || c_started == 0)
+    thread_yield();
+  
+  for (i = 0; i < 100; i++) {
+    printf("thread_b %d\n", i);
+    b_n += 1;
+    thread_yield();
+  }
+  printf("thread_b: exit after %d\n", b_n);
+
+  current_thread->state = FREE;
+  thread_schedule();
+}
+
+void 
+thread_c(void)
+{
+  int i;
+  printf("thread_c started\n");
+  c_started = 1;
+  while(a_started == 0 || b_started == 0)
+    thread_yield();
+  
+  for (i = 0; i < 100; i++) {
+    printf("thread_c %d\n", i);
+    c_n += 1;
+    thread_yield();
+  }
+  printf("thread_c: exit after %d\n", c_n);
+
+  current_thread->state = FREE;
+  thread_schedule();
+}
+
+int 
+main(int argc, char *argv[]) 
+{
+  a_started = b_started = c_started = 0;
+  a_n = b_n = c_n = 0;
+  thread_init();
+  thread_create(thread_a);
+  thread_create(thread_b);
+  thread_create(thread_c);
+  thread_schedule();
+  exit(0);
+}
diff --git a/user/uthread_switch.S b/user/uthread_switch.S
new file mode 100644
index 0000000..5defb12
--- /dev/null
+++ b/user/uthread_switch.S
@@ -0,0 +1,11 @@
+	.text
+
+	/*
+         * save the old thread's registers,
+         * restore the new thread's registers.
+         */
+
+	.globl thread_switch
+thread_switch:
+	/* YOUR CODE HERE */
+	ret    /* return to ra */