--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/source/bio.c	Sun Oct 22 18:25:39 2017 +0900
@@ -0,0 +1,141 @@
+// Buffer cache.
+//
+// The buffer cache is a linked list of buf structures holding
+// cached copies of disk block contents.  Caching disk blocks
+// in memory reduces the number of disk reads and also provides
+// a synchronization point for disk blocks used by multiple processes.
+// 
+// Interface:
+// * To get a buffer for a particular disk block, call bread.
+// * After changing buffer data, call bwrite to write it to disk.
+// * When done with the buffer, call brelse.
+// * Do not use the buffer after calling brelse.
+// * Only one process at a time can use a buffer,
+//     so do not keep them longer than necessary.
+// 
+// The implementation uses three state flags internally:
+// * B_BUSY: the block has been returned from bread
+//     and has not been passed back to brelse.  
+// * B_VALID: the buffer data has been read from the disk.
+// * B_DIRTY: the buffer data has been modified
+//     and needs to be written to disk.
+
+#include "types.h"
+#include "defs.h"
+#include "param.h"
+#include "spinlock.h"
+#include "buf.h"
+
+struct {
+  struct spinlock lock;
+  struct buf buf[NBUF];
+
+  // Linked list of all buffers, through prev/next.
+  // head.next is most recently used.
+  struct buf head;
+} bcache;
+
+void
+binit(void)
+{
+  struct buf *b;
+
+  memset(&bcache, 0, sizeof(bcache));
+  initlock(&bcache.lock, "bcache");
+
+//PAGEBREAK!
+  // Create linked list of buffers
+  bcache.head.prev = &bcache.head;
+  bcache.head.next = &bcache.head;
+  for(b = bcache.buf; b < bcache.buf+NBUF; b++){
+    b->next = bcache.head.next;
+    b->prev = &bcache.head;
+    b->dev = -1;
+    bcache.head.next->prev = b;
+    bcache.head.next = b;
+  }
+}
+
+// Look through buffer cache for sector on device dev.
+// If not found, allocate fresh block.
+// In either case, return B_BUSY buffer.
+static struct buf*
+bget(uint dev, uint sector)
+{
+  struct buf *b;
+
+  acquire(&bcache.lock);
+
+ loop:
+  // Is the sector already cached?
+  for(b = bcache.head.next; b != &bcache.head; b = b->next){
+    if(b->dev == dev && b->sector == sector){
+      if(!(b->flags & B_BUSY)){
+        b->flags |= B_BUSY;
+        release(&bcache.lock);
+        return b;
+      }
+      sleep(b, &bcache.lock);
+      goto loop;
+    }
+  }
+
+  // Not cached; recycle some non-busy and clean buffer.
+  for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
+    if((b->flags & B_BUSY) == 0 && (b->flags & B_DIRTY) == 0){
+      b->dev = dev;
+      b->sector = sector;
+      b->flags = B_BUSY;
+      release(&bcache.lock);
+      return b;
+    }
+  }
+  panic("bget: no buffers");
+  return 0;
+}
+
+// Return a B_BUSY buf with the contents of the indicated disk sector.
+struct buf*
+bread(uint dev, uint sector)
+{
+  struct buf *b;
+
+  b = bget(dev, sector);
+  if(!(b->flags & B_VALID))
+    iderw(b);
+  return b;
+}
+
+// Write b's contents to disk.  Must be B_BUSY.
+void
+bwrite(struct buf *b)
+{
+  if((b->flags & B_BUSY) == 0)
+    panic("bwrite");
+  b->flags |= B_DIRTY;
+  iderw(b);
+}
+
+// Release a B_BUSY buffer.
+// Move to the head of the MRU list.
+void
+brelse(struct buf *b)
+{
+  if((b->flags & B_BUSY) == 0)
+    panic("brelse");
+
+  acquire(&bcache.lock);
+
+  b->next->prev = b->prev;
+  b->prev->next = b->next;
+  b->next = bcache.head.next;
+  b->prev = &bcache.head;
+  bcache.head.next->prev = b;
+  bcache.head.next = b;
+
+  b->flags &= ~B_BUSY;
+  wakeup(b);
+
+  release(&bcache.lock);
+}
+