/*
 * Each buffer in the pool is usually doubly linked into 2 lists:
 * the device with which it is currently associated (always)
 * and also on a list of blocks available for allocation
 * for other use (usually).
 * The latter list is kept in last-used order, and the two
 * lists are doubly linked to make it easy to remove
 * a buffer from one list when it was found by
 * looking through the other.
 * A buffer is on the available list, and is liable
 * to be reassigned to another disk block, if and only
 * if it is not marked BUSY.  When a buffer is busy, the
 * available-list pointers can be used for other purposes.
 * Most drivers use the forward ptr as a link in their I/O
 * active queue.
 * A buffer header contains all the information required
 * to perform I/O.
 * Most of the routines which manipulate these things
 * are in bio.c.
 */
struct buf
{
	int	b_flags;		/* see defines below */
	struct	buf *b_forw;		/* headed by devtab of b_dev */
	struct	buf *b_back;		/*  "  */
	struct	buf *av_forw;		/* position on free list, */
	struct	buf *av_back;		/*     if not BUSY*/
	int	b_dev;			/* major+minor device name */
	int	b_wcount;		/* transfer count (usu. words) */
	char	*b_addr;		/* low order core address */
	char	*b_blkno;		/* block # on device */
	char	b_error;		/* returned after I/O */
	char	*b_resid;		/* words not transferred after error */
} buf[NBUF];

/*
 * Each block device has a devtab, which contains private state stuff
 * and 2 list heads: the b_forw/b_back list, which is doubly linked
 * and has all the buffers currently associated with that major
 * device; and the d_actf/d_actl list, which is private to the
 * device but in fact is always used for the head and tail
 * of the I/O queue for the device.
 * Various routines in bio.c look at b_forw/b_back
 * (notice they are the same as in the buf structure)
 * but the rest is private to each device driver.
 */
struct devtab
{
	char	d_active;		/* busy flag */
	char	d_errcnt;		/* error count (for recovery) */
	struct	buf *b_forw;		/* first buffer for this dev */
	struct	buf *b_back;		/* last buffer for this dev */
	struct	buf *d_actf;		/* head of I/O queue */
	struct 	buf *d_actl;		/* tail of I/O queue */
};

/*
 * This is the head of the queue of available
 * buffers-- all unused except for the 2 list heads.
 */
struct	buf bfreelist;

/*
 * These flags are kept in b_flags.
 */
#define	B_WRITE	0	/* non-read pseudo-flag */
#define	B_READ	01	/* read when I/O occurs */
#define	B_DONE	02	/* transaction finished */
#define	B_ERROR	04	/* transaction aborted */
#define	B_BUSY	010	/* not on av_forw/back list */
#define	B_WANTED 0100	/* issue wakeup when BUSY goes off */
#define	B_RELOC	0200	/* no longer used */
#define	B_ASYNC	0400	/* don't wait for I/O completion */
#define	B_DELWRI 01000	/* don't write till block leaves available list */
