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1 .\" Copyright (C) Caldera International Inc. 2001-2002.  All rights reserved.
37 .EH 'PSD:3-%''The UNIX I/O System'
38 .OH 'The UNIX I/O System''PSD:3-%'
61 in the paper ``The UNIX Time-sharing System.''
79 which work, or can work, with addressible 512-byte blocks.
91 Character-type devices have a much
102 in the low-order 8 bits and the major device number
103 in the next-higher 8 bits;
129 system calls is to set up entries in three separate
134 which is stored in the system's per-process
152 which is a per-system data base.
160 This table is per-system because the same instance
170 a count which is used to decide when all processes
173 There is also a 32-bit file offset
180 which contains a copy of the file's i-node.
208 is a special file containing a currently-mounted
214 corresponding i-node as stored on the disk;
218 a count used to determine when it may be
220 and the device and i-number
224 the 3-byte, compressed format used on the disk to full
237 the data-terminal-ready lead of a modem, etc.).
243 that is, when the i-node table entry
247 a count of its users, although it is quite
249 implement an exclusive-use device which cannot
260 table entry are used to set up the
267 of the I/O target area, the byte-count for the transfer,
270 a character-type special file, the appropriate read
273 count and current location appropriately
279 to a physical block number; a block-type
295 open, close, read, write, and special-function
304 on non-exclusive devices that require no setup)
312 on read-only devices)
327 non-zero only if the device is to be written upon.
336 maintain its own count of its users.
338 the second is a flag which is non-zero
347 The per-user variable
371 should copy up to
380 is used to pick up characters
396 .I "iomove(buffer, offset, count, flag)"
399 transfers up to
400 .I count
412 the count is not too large and is non-zero.
416 .I "buffer+offset, count"
427 is guaranteed to be non-zero.
445 The ``special-functions'' routine
463 the device is supposed to place up to 3 words of status information
470 the device should take up to 3 words of
475 Finally, each device should have appropriate interrupt-time
477 When an interrupt occurs, it is turned into a C-compatible call
479 The interrupt-catching mechanism makes
480 the low-order four bits of the ``new PS'' word in the
496 For relatively low data-rate devices, the best mechanism
505 	int	c_cc;	/* character count */
522 which returns either the (non-negative) character
531 care to avoid gobbling up excessive numbers of characters.
534 to device handlers is the sleep-wakeup mechanism.
541 at that time, the process is marked ready-to-run
555 by the sleeper and the waker-up.
566 a higher numerical value indicates a less-favored
595 (for example, a device going on-line, which does not
610 If a device needs to know about real-time intervals,
623 to provide real-time delays after function characters
624 like new-line and tab in typewriter output,
636 is called at clock-interrupt time, so it should
640 The Block-device Interface
651 keeping in-core copies of blocks that are being
658 which maintain a doubly-linked list
663 which generally maintain a doubly-linked list of blocks
672 There is a word count
676 count used to communicate information
716 routine is used to implement read-ahead.
731 There are three subtly-different write routines,
757 it is used, for example, when updating i-nodes.
774 will be given soon which will re-use the same block.
827 if it is non-zero.
834 whereby a more detailed error-reporting
934 the block number, a (negative) word count,
952 is capable, under error-free operation,
954 the device's word-count register should be placed
955 in the residual count slot of
957 otherwise, the residual count should be set to 0.
983 byte to contain an active flag and an error count,
990 except for the buffer-chain pointers.
995 The error count is useful for counting retries
998 in the simplest case it may be maintained as a first-in
999 first-out list.
1027 It may be called only in the non-interrupt
1032 Raw Block-device I/O
1034 A scheme has been set up whereby block device drivers may
1039 The method involves setting up a character-type special file
1045 routines which set up what is usually a private,
1046 non-shared buffer header with the appropriate information
1053 A special-function routine might come in handy, especially for
1070 and a read-write flag
1077 makes sure that the user's base address and count are
1083 and it sets up user error return information.