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				src/fs/buf.h	 	 
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20100	/* Buffer (block) cache.  To acquire a block, a routine calls get_block(),
20101	 * telling which block it wants.  The block is then regarded as "in use"
20102	 * and has its 'b_count' field incremented.  All the blocks that are not
20103	 * in use are chained together in an LRU list, with 'front' pointing
20104	 * to the least recently used block, and 'rear' to the most recently used
20105	 * block.  A reverse chain, using the field b_prev is also maintained.
20106	 * Usage for LRU is measured by the time the put_block() is done.  The second
20107	 * parameter to put_block() can violate the LRU order and put a block on the
20108	 * front of the list, if it will probably not be needed soon.  If a block
20109	 * is modified, the modifying routine must set b_dirt to DIRTY, so the block
20110	 * will eventually be rewritten to the disk.
20111	 */
20112	
20113	#include <sys/dir.h>                    /* need struct direct */
20114	
20115	EXTERN struct buf {
20116	  /* Data portion of the buffer. */
20117	  union {
20118	    char b__data[BLOCK_SIZE];                /* ordinary user data */
20119	    struct direct b__dir[NR_DIR_ENTRIES];    /* directory block */
20120	    zone1_t b__v1_ind[V1_INDIRECTS];         /* V1 indirect block */
20121	    zone_t  b__v2_ind[V2_INDIRECTS];         /* V2 indirect block */
20122	    d1_inode b__v1_ino[V1_INODES_PER_BLOCK]; /* V1 inode block */
20123	    d2_inode b__v2_ino[V2_INODES_PER_BLOCK]; /* V2 inode block */
20124	    bitchunk_t b__bitmap[BITMAP_CHUNKS];     /* bit map block */
20125	  } b;
20126	
20127	  /* Header portion of the buffer. */
20128	  struct buf *b_next;           /* used to link all free bufs in a chain */
20129	  struct buf *b_prev;           /* used to link all free bufs the other way */
20130	  struct buf *b_hash;           /* used to link bufs on hash chains */
20131	  block_t b_blocknr;            /* block number of its (minor) device */
20132	  dev_t b_dev;                  /* major | minor device where block resides */
20133	  char b_dirt;                  /* CLEAN or DIRTY */
20134	  char b_count;                 /* number of users of this buffer */
20135	} buf[NR_BUFS];
20136	
20137	/* A block is free if b_dev == NO_DEV. */
20138	
20139	#define NIL_BUF ((struct buf *) 0)      /* indicates absence of a buffer */
20140	
20141	/* These defs make it possible to use to bp->b_data instead of bp->b.b__data */
20142	#define b_data   b.b__data
20143	#define b_dir    b.b__dir
20144	#define b_v1_ind b.b__v1_ind
20145	#define b_v2_ind b.b__v2_ind
20146	#define b_v1_ino b.b__v1_ino
20147	#define b_v2_ino b.b__v2_ino
20148	#define b_bitmap b.b__bitmap
20149	
20150	EXTERN struct buf *buf_hash[NR_BUF_HASH];       /* the buffer hash table */
20151	
20152	EXTERN struct buf *front;       /* points to least recently used free block */
20153	EXTERN struct buf *rear;        /* points to most recently used free block */
20154	EXTERN int bufs_in_use;         /* # bufs currently in use (not on free list)*/
20155	
20156	/* When a block is released, the type of usage is passed to put_block(). */
20157	#define WRITE_IMMED        0100 /* block should be written to disk now */
20158	#define ONE_SHOT           0200 /* set if block not likely to be needed soon */
20159	
20160	#define INODE_BLOCK       (0 + MAYBE_WRITE_IMMED)        /* inode block */
20161	#define DIRECTORY_BLOCK   (1 + MAYBE_WRITE_IMMED)        /* directory block */
20162	#define INDIRECT_BLOCK    (2 + MAYBE_WRITE_IMMED)        /* pointer block */
20163	#define MAP_BLOCK         (3 + MAYBE_WRITE_IMMED)        /* bit map */
20164	#define ZUPER_BLOCK       (4 + WRITE_IMMED + ONE_SHOT)   /* super block */
20165	#define FULL_DATA_BLOCK    5                             /* data, fully used */
20166	#define PARTIAL_DATA_BLOCK 6                             /* data, partly used*/
20167	
20168	#define HASH_MASK (NR_BUF_HASH - 1)     /* mask for hashing block numbers */