From 9c66bb97f1623a05e4705672bdea9db05b7a4560 Mon Sep 17 00:00:00 2001
From: Ken Thompson <ken@research.uucp>
Date: Fri, 18 Jul 1975 03:50:20 -0500
Subject: [PATCH] Research V6 development Work on file usr/sys/buf.h

Co-Authored-By: Dennis Ritchie <dmr@research.uucp>
Synthesized-from: v6
---
 usr/sys/buf.h | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 usr/sys/buf.h

diff --git a/usr/sys/buf.h b/usr/sys/buf.h
new file mode 100644
index 0000000000..e52620522b
--- /dev/null
+++ b/usr/sys/buf.h
@@ -0,0 +1,77 @@
+/*
+ * 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_xmem;		/* high 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_PHYS	020	/* Physical IO potentially using UNIBUS map */
+#define	B_MAP	040	/* This block has the UNIBUS map allocated */
+#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 */
-- 
2.20.1