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1.1 root 1: .th DB I 8/20/73
2: .sh NAME
3: db \*- debug
4: .sh SYNOPSIS
5: .bd db
6: [ core [ namelist ] ] [
7: .bd \*-
8: ]
9: .sh DESCRIPTION
10: Unlike
11: many debugging packages (including DEC's ODT, on
12: which
13: .it db
14: is loosely based),
15: .it db
16: is not loaded as part of the
17: core image which it is used to examine; instead it examines files.
18: Typically, the file will be either a core image produced
19: after a fault or the binary output of
20: the assembler.
21: .it Core
22: is the file being debugged; if omitted \fBcore\fR is assumed.
23: .it Namelist
24: is a file containing a symbol table.
25: If it is omitted,
26: the symbol table is obtained from the
27: file being debugged,
28: or if not there from
29: .bd a.out.
30: If no appropriate name list file
31: can be found,
32: .it db
33: can still be used but some of its symbolic
34: facilities become unavailable.
35: .s3
36: For the meaning of the optional third argument, see
37: the last paragraph below.
38: .s3
39: The format for most
40: .it db
41: requests is an address followed
42: by a one character command.
43: Addresses are expressions built up as follows:
44: .s3
45: .lp +4 3
46: 1. A name has the value assigned to it
47: when the input file was assembled.
48: It may be relocatable or not depending
49: on the use of the name during the assembly.
50: .s3
51: .lp +4 3
52: 2. An octal number is an absolute quantity with the appropriate
53: value.
54: .s3
55: .lp +4 3
56: 3. A decimal number immediately followed by `\fB.\fR' is
57: an absolute quantity with the appropriate value.
58: .s3
59: .lp +4 3
60: 4. An octal number immediately followed by \fBr\fR is a relocatable
61: quantity with the appropriate value.
62: .s3
63: .lp +4 3
64: 5. The symbol \fB.\fR indicates the current pointer
65: of \fIdb\fR.
66: The current pointer is set by many
67: \fIdb\fR
68: requests.
69: .s3
70: .lp +4 3
71: 6. A \fB*\fR before
72: an expression forms an expression whose value is the
73: number in the word addressed by the first expression.
74: A \fB*\fR alone is equivalent to `\fB*.\fR'.
75: .s3
76: .lp +4 3
77: 7. Expressions separated by \fB+\fR or blank are expressions
78: with value equal to the sum of the components. At most
79: one of the components may be relocatable.
80: .s3
81: .lp +4 3
82: 8. Expressions separated by \fB\*-\fR form an expression
83: with value equal to the difference to the components.
84: If the right component is relocatable, the left component
85: must be relocatable.
86: .s3
87: .lp +4 3
88: 9. Expressions are evaluated left to right.
89: .s3
90: .in \n(inu
91: Names for registers are
92: built in:
93: .s3
94: .bd " r0 ... r5"
95: .bd " sp"
96: .bd " pc"
97: .bd " fr0 ... fr5"
98: .s3
99: These may be examined.
100: Their values are deduced from the contents
101: of the stack in a core image file. They are meaningless
102: in a file that is not a core image.
103: .s3
104: If no address is given for a command, the current address
105: (also specified by ``\fB.\fR'') is assumed. In general, ``\fB.\fR''
106: points to the last word or byte printed by
107: .it db.
108: .s3
109: There are
110: .it db
111: commands for examining locations
112: interpreted as numbers, machine instructions,
113: ASCII characters, and addresses.
114: For numbers and characters, either bytes
115: or words may be examined.
116: The following commands are used to examine the specified file.
117: .s3
118: .lp +4 3
119: / The addressed word is printed in octal.
120: .s3
121: .lp +4 3
122: \\ The addressed byte is printed in octal.
123: .s3
124: .lp +4 3
125: " The addressed word is printed as two ASCII characters.
126: .s3
127: .if t .tr '\(aa\`\(ga
128: .lp +4 3
129: .li
130: ' The addressed byte is printed as an ASCII character.
131: .s3
132: .lp +4 3
133: ` The addressed word is printed in decimal.
134: .s3
135: .lp +4 3
136: ? The addressed word is interpreted as a machine
137: instruction and a symbolic form of the instruction,
138: including symbolic addresses, is printed.
139: Often, the result will appear exactly as it was written
140: in the source program.
141: .s3
142: .lp +4 3
143: & The addressed word is interpreted as a symbolic address
144: and is printed as the name of the symbol whose value is closest
145: to the addressed word, possibly followed by a signed offset.
146: .s3
147: .tr ''``
148: .lp +4 3
149: <nl> (i. e., the character ``new line'') This command advances
150: the current location counter ``\fB.\fR'' and prints the resulting
151: location in the mode last specified by
152: one of the above requests.
153: .s3
154: .lp +4 3
155: ^ This character decrements ``\fB.\fR'' and prints the
156: resulting location in the mode last selected
157: one of the above requests. It is a converse to <nl>.
158: .s3
159: .lp +4 3
160: % Exit.
161: .s3
162: .in \n(inu
163: Odd addresses to word-oriented commands are rounded
164: down.
165: The incrementing and decrementing
166: of ``\fB.\fR'' done by the
167: .bd <nl>
168: and
169: .bd ^
170: requests is by one or
171: two depending on whether the last command
172: was word or byte oriented.
173: .s3
174: The address portion of any of the above commands
175: may be followed by a comma and then by an
176: expression. In this case that number of sequential
177: words or bytes specified by the expression is printed.
178: ``\fB.\fR'' is advanced so that it points at the
179: last thing printed.
180: .s3
181: There are two commands to interpret the value
182: of expressions.
183: .s3
184: .lp +4 3
185: = When preceded by an expression, the value of the expression
186: is typed in octal.
187: When not preceded by an expression, the value of ``\fB.\fR'' is
188: indicated.
189: This command does not change the value of ``\fB.\fR''.
190: .s3
191: .lp +4 3
192: : An attempt is made to print the given expression
193: as a symbolic address. If the expression is relocatable,
194: that symbol is found whose value is nearest
195: that of the expression, and the symbol is typed, followed by
196: a sign and the appropriate offset.
197: If the value of the expression is absolute, a symbol
198: with exactly the indicated value is sought and
199: printed if found; if no matching symbol is discovered, the
200: octal value of the expression is given.
201: .s3
202: .in \n(inu
203: The following command may be used to patch the file being debugged.
204: .s3
205: .lp +4 3
206: ! This command must be preceded by an expression.
207: The value of the expression is stored at the location
208: addressed by the current value of ``\fB.\fR''.
209: The opcodes do not appear in the symbol
210: table, so the user must assemble them by hand.
211: .s3
212: .in \n(inu
213: The following command is used after a fault has caused
214: a core image file to be produced.
215: .s3
216: .lp +4 3
217: $ causes the fault type and
218: the contents of the general registers and
219: several other registers to be printed both in octal and symbolic
220: format.
221: The values are as they were at the time of the fault.
222: .s3
223: .in \n(inu
224: For some purposes, it is important to know how addresses
225: typed by the user correspond with
226: locations in the file being debugged.
227: The mapping algorithm employed by
228: .it db
229: is non-trivial
230: for two reasons:
231: First, in an
232: .bd a.out
233: file, there is a 20(8) byte header
234: which will not appear when the file is loaded into
235: core for execution.
236: Therefore, apparent location 0 should correspond
237: with actual file offset 20.
238: Second, addresses
239: in core images do not correspond with the
240: addresses used by the program because in a core image
241: there is a 512-byte header containing the
242: system's per-process
243: data for the dumped process, and also because the
244: stack is stored contiguously with the text and data
245: part of the core image rather than at the highest possible
246: locations.
247: .it Db
248: obeys the following rules:
249: .s3
250: If exactly one argument is given, and if it appears
251: to be an
252: .bd a.out
253: file, the 20-byte header is skipped
254: during addressing, i.e., 20 is added to all addresses typed.
255: As a consequence, the header can be examined
256: beginning at location \*-20.
257: .s3
258: If exactly one argument is given and if the file does
259: not appear to be an
260: .bd a.out
261: file, no mapping is done.
262: .s3
263: If zero or two arguments are given,
264: the mapping appropriate to a core image file is employed.
265: This means that locations above the program break
266: and below the stack
267: effectively do not exist (and are not, in fact, recorded
268: in the core file).
269: Locations above the user's stack pointer are mapped,
270: in looking at the core file, to
271: the place where they are really stored.
272: The per-process data kept by the
273: system, which is stored in the first 512(10) bytes
274: of the core file,
275: cannot currently be examined (except by \fB$\fR).
276: .s3
277: If one wants to examine
278: a file which has an associated name list,
279: but is not a core image file, the last argument ``\fB\*-\fR''
280: can be used (actually the only purpose of the
281: last argument is to make the number of
282: arguments not equal to two).
283: This feature is used most frequently in
284: examining the memory file /dev/mem.
285: .sh "SEE ALSO"
286: as(I), core(V), a.out(V), od(I)
287: .sh DIAGNOSTICS
288: ``File not found'' if the first argument
289: cannot be read; otherwise ``\fB?\fR''.
290: .sh BUGS
291: There should be some way to examine the registers
292: and other per-process data in a core image;
293: also there should be some way of specifying
294: double-precision addresses.
295: It does not know yet about shared text segments.
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