comments

1 files changed, 37 insertions, 4 deletions

diff --git a/bio.c b/bio.c
index ab5f41f..dadd1f7 100644
--- a/bio.c
+++ b/bio.c
@@ -1,3 +1,29 @@
+// Buffer cache.
+//
+// The buffer cache is a linked list of buf structures
+// holding cached copies of disk block contents.
+// Each buf has two state bits B_BUSY and B_VALID.
+// If B_BUSY is set, it means that some code is currently
+// editing buf, so other code is not allowed to look at it.
+// To wait for a buffer that is B_BUSY, sleep on buf.
+// (See bget below.)
+// 
+// If B_VALID is set, it means that the memory contents
+// have been initialized by reading them off the disk.
+// (Conversely, if B_VALID is not set, the memory contents
+// of buf must be initialized, often by calling bread,
+// before being used.)
+// 
+// After making changes to a buf's memory, call bwrite to flush
+// the changes out to disk, to keep the disk and memory copies
+// in sync.
+//
+// When finished with a buffer, call brelse to release the buffer
+// (i.e., clear B_BUSY), so that others can access it.
+//
+// Bufs that are not B_BUSY are fair game for reuse for other
+// disk blocks.  It is not allowed to use a buf after calling brelse.
+
 #include "types.h"
 #include "param.h"
 #include "x86.h"
@@ -10,7 +36,7 @@
 struct buf buf[NBUF];
 struct spinlock buf_table_lock;
 
-// linked list of all buffers, through prev/next.
+// Linked list of all buffers, through prev/next.
 // bufhead->next is most recently used.
 // bufhead->tail is least recently used.
 struct buf bufhead;
@@ -22,6 +48,7 @@ binit(void)
 
   initlock(&buf_table_lock, "buf_table");
 
+  // Create linked list of buffers
   bufhead.prev = &bufhead;
   bufhead.next = &bufhead;
   for(b = buf; b < buf+NBUF; b++){
@@ -32,7 +59,10 @@ binit(void)
   }
 }
 
-struct buf*
+// Look through buffer cache for block n on device dev.
+// If not found, allocate fresh block.
+// In either case, return locked buffer.
+static struct buf*
 getblk(uint dev, uint sector)
 {
   struct buf *b;
@@ -63,11 +93,12 @@ getblk(uint dev, uint sector)
           return b;
         }
       }
-      panic("getblk: no buffers");
+      panic("bget: no buffers");
     }
   }
 }
 
+// Read buf's contents from disk.
 struct buf*
 bread(uint dev, uint sector)
 {
@@ -75,7 +106,7 @@ bread(uint dev, uint sector)
   struct buf *b;
   extern struct spinlock ide_lock;
 
-  b = getblk(dev, sector);
+  b = bget(dev, sector);
   if(b->flags & B_VALID)
     return b;
 
@@ -89,6 +120,7 @@ bread(uint dev, uint sector)
   return b;
 }
 
+// Write buf's contents to disk.
 void
 bwrite(struct buf *b, uint sector)
 {
@@ -103,6 +135,7 @@ bwrite(struct buf *b, uint sector)
   release(&ide_lock);
 }
 
+// Release the buffer buf.
 void
 brelse(struct buf *b)
 {