78 lines
2.9 KiB
C
78 lines
2.9 KiB
C
/*
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* Each buffer in the pool is usually doubly linked into 2 lists:
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* the device with which it is currently associated (always)
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* and also on a list of blocks available for allocation
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* for other use (usually).
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* The latter list is kept in last-used order, and the two
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* lists are doubly linked to make it easy to remove
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* a buffer from one list when it was found by
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* looking through the other.
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* A buffer is on the available list, and is liable
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* to be reassigned to another disk block, if and only
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* if it is not marked BUSY. When a buffer is busy, the
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* available-list pointers can be used for other purposes.
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* Most drivers use the forward ptr as a link in their I/O
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* active queue.
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* A buffer header contains all the information required
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* to perform I/O.
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* Most of the routines which manipulate these things
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* are in bio.c.
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*/
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struct buf
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{
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int b_flags; /* see defines below */
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struct buf *b_forw; /* headed by devtab of b_dev */
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struct buf *b_back; /* " */
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struct buf *av_forw; /* position on free list, */
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struct buf *av_back; /* if not BUSY*/
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int b_dev; /* major+minor device name */
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int b_wcount; /* transfer count (usu. words) */
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char *b_addr; /* low order core address */
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char *b_xmem; /* high order core address */
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char *b_blkno; /* block # on device */
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char b_error; /* returned after I/O */
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char *b_resid; /* words not transferred after error */
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} buf[NBUF];
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/*
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* Each block device has a devtab, which contains private state stuff
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* and 2 list heads: the b_forw/b_back list, which is doubly linked
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* and has all the buffers currently associated with that major
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* device; and the d_actf/d_actl list, which is private to the
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* device but in fact is always used for the head and tail
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* of the I/O queue for the device.
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* Various routines in bio.c look at b_forw/b_back
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* (notice they are the same as in the buf structure)
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* but the rest is private to each device driver.
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*/
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struct devtab
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{
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char d_active; /* busy flag */
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char d_errcnt; /* error count (for recovery) */
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struct buf *b_forw; /* first buffer for this dev */
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struct buf *b_back; /* last buffer for this dev */
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struct buf *d_actf; /* head of I/O queue */
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struct buf *d_actl; /* tail of I/O queue */
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};
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/*
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* This is the head of the queue of available
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* buffers-- all unused except for the 2 list heads.
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*/
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struct buf bfreelist;
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/*
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* These flags are kept in b_flags.
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*/
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#define B_WRITE 0 /* non-read pseudo-flag */
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#define B_READ 01 /* read when I/O occurs */
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#define B_DONE 02 /* transaction finished */
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#define B_ERROR 04 /* transaction aborted */
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#define B_BUSY 010 /* not on av_forw/back list */
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#define B_PHYS 020 /* Physical IO potentially using UNIBUS map */
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#define B_MAP 040 /* This block has the UNIBUS map allocated */
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#define B_WANTED 0100 /* issue wakeup when BUSY goes off */
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#define B_RELOC 0200 /* no longer used */
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#define B_ASYNC 0400 /* don't wait for I/O completion */
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#define B_DELWRI 01000 /* don't write till block leaves available list */
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