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1.1 root 1: .\"
2: .\" Copyright (c) 1982 Regents of the University of California
3: .\" @(#)asdocs2.me 1.6 2/11/83
4: .\"
5: .EQ
6: delim $$
7: .EN
8: .SH 1 "Statements"
9: .pp
10: A source program is composed of a sequence of
11: .i statements .
12: Statements are separated either by new-lines
13: or by semicolons.
14: There are two kinds of statements: null statements
15: and keyword statements.
16: Either kind of statement may be preceded by
17: one or more labels.
18: .SH 2 "Named Global Labels"
19: .pp
20: A global label consists of a name followed
21: by a colon.
22: The effect of a name label is to assign the current
23: value and type of the location counter
24: to the name.
25: An error is indicated in pass 1 if the
26: name is already defined;
27: an error is indicated in pass 2 if the
28: value assigned changes the definition
29: of the label.
30: .pp
31: A global label is referenced by its name.
32: .pp
33: Global labels beginning with a
34: .q "\|L\|"
35: are discarded unless the
36: .b \-L
37: option
38: is in effect.
39: .SH 2 "Numeric Local Labels"
40: .pp
41: A numeric label consists of a digit $0$ to $9$
42: followed by a colon.
43: Such a label serves to define temporary symbols of the form
44: .q "$n roman b$"
45: and
46: .q "$n roman f$" ,
47: where $n$
48: is the digit of the label.
49: As in the case of name labels,
50: a numeric label assigns
51: the current value and type of the location counter
52: to the temporary symbol.
53: However, several numeric labels with the same digit
54: may be used within the same assembly.
55: References to symbols of the form
56: .q "$n roman b$"
57: refer to the first numeric label
58: .q "$n$:"
59: .i b ackwards
60: from the reference;
61: .q "$n roman f$"
62: symbols refer to the first numeric label
63: .q "$n$:"
64: .i f orwards
65: from the reference.
66: Such numeric labels conserve the inventive powers
67: of the human programmer.
68: .pp
69: For various reasons,
70: .i as
71: turns local labels into labels of the form
72: $roman L n roman ".\*(DL" m$.
73: Although unlikely,
74: these generated labels may conflict
75: with programmer defined labels.
76: .SH 2 "Null statements"
77: .pp
78: A null statement is an empty statement ignored by the assembler.
79: A null statement may be labeled, however.
80: .SH 2 "Keyword statements"
81: .pp
82: A keyword statement begins with one of the many predefined
83: keywords known to
84: .i as ;
85: the syntax of the remainder of the statement depends
86: on the keyword.
87: All instruction opcodes are keywords.
88: The remaining keywords are assembler pseudo-operations,
89: also called
90: .i directives .
91: The pseudo-operations are listed in \(sc8,
92: together with the syntax they require.
93: .SH 1 "Expressions"
94: .pp
95: An expression is a sequence of symbols representing a value.
96: Its constituents are identifiers,
97: constants,
98: operators,
99: and parentheses.
100: Each expression has a type.
101: .pp
102: All operators in expressions are fundamentally binary in
103: nature.
104: Arithmetic is two's complement and has 32 bits of precision.
105: .i As
106: can not do arithmetic on floating point numbers, quad or octal
107: precision scalar numbers.
108: There are four levels of precedence,
109: listed here from
110: lowest precedence level to highest:
111: .(b
112: .TS
113: center;
114: c l.
115: precedence operators
116: _
117: binary $+$, $-$
118: binary \fI\(bv\fP\|, $&$, $italic "^"$, $!$
119: binary $*$, $/$, $%$,
120: unary $-$, $italic "~"$
121: .TE
122: .)b
123: .pp
124: All operators of the same precedence are evaluated strictly left to right,
125: except for the evaluation order enforced by parenthesis.
126: .SH 2 "Expression Operators"
127: .pp
128: The operators are:
129: .(b
130: .TS
131: center;
132: c l.
133: operator meaning
134: _
135: $+$ addition
136: $-$ (binary) subtraction
137: $*$ multiplication
138: $/$ division
139: $%$ modulo
140: $-$ (unary) 2's complement
141: $&$ bitwise and
142: $\(bv$ bitwise or
143: $italic "^"$ bitwise exclusive or
144: $!$ bitwise or not
145: $italic "~"$ bitwise 1's complement
146: $>$ logical right shift
147: $>>$ logical right shift
148: $<$ logical left shift
149: $<<$ logical left shift
150: .TE
151: .)b
152: .pp
153: Expressions may be grouped by use of parentheses,
154: .q "("
155: and
156: .q ")".
157: .SH 2 "Data Types"
158: .pp
159: The assembler manipulates several different
160: types of expressions.
161: The types likely to be met explicitly are:
162: .nr ii \w'undefined\ \ 'u
163: .ip "undefined"
164: Upon first encounter,
165: each symbol is undefined.
166: It may become undefined if it is assigned an undefined expression.
167: It is an error to attempt to assemble an undefined
168: expression in pass 2;
169: in pass 1,
170: it is not (except that
171: certain keywords require operands which are not undefined).
172: .ip "undefined\ external"
173: A symbol which is declared
174: .b .globl
175: but not defined
176: in the current assembly is an undefined external.
177: If such a symbol is declared,
178: the link editor
179: .i ld
180: must be used to load the assembler's output with
181: another routine that defines the undefined reference.
182: .ip "absolute"
183: An absolute symbol is defined ultimately from a constant.
184: Its value is unaffected by any possible future applications
185: of the link-editor to the output file.
186: .ip "text"
187: The value of a text symbol is measured
188: with respect to the beginning of the text segment of the program.
189: If the assembler output is link-edited,
190: its text symbols may change in value
191: since the program need
192: not be the first in the link editor's output.
193: Most text symbols are defined by appearing as labels.
194: At the start of an assembly,
195: the value of
196: .q "\|\fB.\fP\|"
197: is
198: .q "text 0" .
199: .ip "data"
200: The value of a data symbol is measured
201: with respect to the origin of the data segment of a program.
202: Like text symbols,
203: the value of a data symbol may change
204: during a subsequent link-editor run since previously
205: loaded programs may have data segments.
206: After the first
207: .b .data
208: statement,
209: the value of
210: .q "\|\fB.\fP\|"
211: is
212: .q "data 0".
213: .ip "bss"
214: The value of a bss symbol is measured from
215: the beginning of the bss segment of a program.
216: Like text and data symbols,
217: the value of a bss symbol
218: may change during a subsequent link-editor run,
219: since previously loaded programs may have bss segments.
220: .ip "external\ absolute,\ text,\ data,\ or\ bss"
221: Symbols declared
222: .b .globl
223: but defined within an assembly as absolute,
224: text,
225: data,
226: or bss
227: symbols may be used exactly as if they were not
228: declared
229: .b .globl ;
230: however, their value and type are available
231: to the link editor so that the program may be loaded with others
232: that reference these symbols.
233: .ip "register"
234: The symbols
235: .(q
236: .b "r0 r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12 r13 r14 r15"
237: .b "ap fp sp pc"
238: .)q
239: are predefined
240: as register symbols.
241: In addition, the
242: .q "%"
243: operator converts the following absolute expression whose
244: value is between 0 and 15 into a register reference.
245: .ip "other\ types\ \ "
246: Each keyword known to the assembler has a type which
247: is used to select the routine which processes
248: the associated keyword statement.
249: The behavior of such symbols
250: when not used as keywords is the same as if they were absolute.
251: .SH 2 "Type Propagation in Expressions"
252: .pp
253: When operands are combined by expression operators,
254: the result has a type which depends on the types
255: of the operands and on the operator.
256: The rules involved are complex to state but
257: were intended to be sensible and predictable.
258: For purposes of expression evaluation the
259: important types are
260: .(b
261: .TS
262: center;
263: l.
264: undefined
265: absolute
266: text
267: data
268: bss
269: undefined external
270: other
271: .TE
272: .)b
273: .pp
274: The combination rules are then
275: .nr ii 5n
276: .np
277: If one of the operands is undefined,
278: the result is undefined.
279: .np
280: If both operands are absolute,
281: the result is absolute.
282: .np
283: If an absolute is combined with one of the
284: .q "other types"
285: mentioned above,
286: the result has the other type.
287: An
288: .q "other type"
289: combined with an explicitly
290: discussed type other than absolute
291: it acts like an absolute.
292: .pp
293: Further rules applying to particular operators are:
294: .ip "$+$"
295: If one operand is text-, data-, or bss-segment
296: relocatable, or is an undefined external,
297: the result has the postulated type and the other operand
298: must be absolute.
299: .ip $-$
300: If the first operand is a relocatable
301: text-, data-, or bss-segment symbol, the second operand
302: may be absolute (in which case the result has the
303: type of the first operand);
304: or the second operand may have the same type
305: as the first (in which case the result is absolute).
306: If the first operand is external undefined,
307: the second must be absolute.
308: All other combinations are illegal.
309: .ip others
310: It is illegal to apply these operators to any but absolute symbols.
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