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1.1 root 1: .\" Copyright (c) 1983 Regents of the University of California.
2: .\" All rights reserved. The Berkeley software License Agreement
3: .\" specifies the terms and conditions for redistribution.
4: .\"
5: .\" @(#)b.t 6.2 (Berkeley) 6/3/86
6: .\"
7: .\".ds RH "Device Defaulting Rules
8: .bp
9: .LG
10: .B
11: .ce
12: APPENDIX B. RULES FOR DEFAULTING SYSTEM DEVICES
13: .sp
14: .R
15: .NL
16: .PP
17: When \fIconfig\fP processes a ``config'' rule which does
18: not fully specify the location of the root file system,
19: paging area(s), device for system dumps, and device for
20: argument list processing it applies a set of rules to
21: define those values left unspecified. The following list
22: of rules are used in defaulting system devices.
23: .IP 1) 3
24: If a root device is not specified, the swap
25: specification must indicate a ``generic'' system is to be built.
26: .IP 2) 3
27: If the root device does not specify a unit number, it
28: defaults to unit 0.
29: .IP 3) 3
30: If the root device does not include a partition specification,
31: it defaults to the ``a'' partition.
32: .IP 4) 3
33: If no swap area is specified, it defaults to the ``b''
34: partition of the root device.
35: .IP 5) 3
36: If no device is specified for processing argument lists, the
37: first swap partition is selected.
38: .IP 6) 3
39: If no device is chosen for system dumps, the first swap
40: partition is selected (see below to find out where dumps are
41: placed within the partition).
42: .PP
43: The following table summarizes the default partitions selected
44: when a device specification is incomplete, e.g. ``hp0''.
45: .DS
46: .TS
47: l l.
48: Type Partition
49: _
50: root ``a''
51: swap ``b''
52: args ``b''
53: dumps ``b''
54: .TE
55: .DE
56: .SH
57: Multiple swap/paging areas
58: .PP
59: When multiple swap partitions are specified, the system treats the
60: first specified as a ``primary'' swap area which is always used.
61: The remaining partitions are then interleaved into the paging
62: system at the time a
63: .IR swapon (2)
64: system call is made. This is normally done at boot time with
65: a call to
66: .IR swapon (8)
67: from the /etc/rc file.
68: .SH
69: System dumps
70: .PP
71: System dumps are automatically taken after a system crash,
72: provided the device driver for the ``dumps'' device supports
73: this. The dump contains the contents of memory, but not
74: the swap areas. Normally the dump device is a disk in
75: which case the information is copied to a location at the
76: back of the partition. The dump is placed in the back of the
77: partition because the primary swap and dump device are commonly
78: the same device and this allows the system to be rebooted without
79: immediately overwriting the saved information. When a dump has
80: occurred, the system variable \fIdumpsize\fP
81: is set to a non-zero value indicating the size (in bytes) of
82: the dump. The \fIsavecore\fP\|(8)
83: program then copies the information from the dump partition to
84: a file in a ``crash'' directory and also makes a copy of the
85: system which was running at the time of the crash (usually
86: ``/vmunix''). The offset to the system dump is defined in the
87: system variable \fIdumplo\fP (a sector offset from
88: the front of the dump partition). The
89: .I savecore
90: program operates by reading the contents of \fIdumplo\fP, \fIdumpdev\fP,
91: and \fIdumpmagic\fP from /dev/kmem, then comparing the value
92: of \fIdumpmagic\fP read from /dev/kmem to that located in
93: corresponding location in the dump area of the dump partition.
94: If a match is found,
95: .I savecore
96: assumes a crash occurred and reads \fIdumpsize\fP from the dump area
97: of the dump partition. This value is then used in copying the
98: system dump. Refer to
99: \fIsavecore\fP\|(8)
100: for more information about its operation.
101: .PP
102: The value \fIdumplo\fP is calculated to be
103: .DS
104: \fIdumpdev-size\fP \- \fImemsize\fP
105: .DE
106: where \fIdumpdev-size\fP is the size of the disk partition
107: where system dumps are to be placed, and
108: \fImemsize\fP is the size of physical memory.
109: If the disk partition is not large enough to hold a full
110: dump, \fIdumplo\fP is set to 0 (the start of the partition).
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