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1 | .\" -*-nroff-*- |
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2 | .TH bits 3 "20 June 1999" "Straylight/Edgeware" "mLib utilities library" |
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3 | .SH NAME |
4 | bits \- portable bit manipulation macros |
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5 | .\" @U8 |
6 | .\" @U16 |
7 | .\" @U32 |
8 | .\" |
9 | .\" @LSL8 |
10 | .\" @LSR8 |
11 | .\" @LSL16 |
12 | .\" @LSR16 |
13 | .\" @LSL32 |
14 | .\" @LSR32 |
15 | .\" |
16 | .\" @ROL8 |
17 | .\" @ROR8 |
18 | .\" @ROL16 |
19 | .\" @ROR16 |
20 | .\" @ROL32 |
21 | .\" @ROR32 |
22 | .\" |
23 | .\" @GETBYTE |
24 | .\" @PUTBYTE |
25 | .\" |
26 | .\" @LOAD8 |
27 | .\" @STORE8 |
28 | .\" |
29 | .\" @LOAD16_L |
30 | .\" @LOAD16_B |
31 | .\" @LOAD16 |
32 | .\" @STORE16_L |
33 | .\" @STORE16_B |
34 | .\" @STORE16 |
35 | .\" |
36 | .\" @LOAD32_L |
37 | .\" @LOAD32_B |
38 | .\" @LOAD32 |
39 | .\" @STORE32_L |
40 | .\" @STORE32_B |
41 | .\" @STORE32 |
42 | .\" |
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43 | .SH SYNOPSIS |
44 | .nf |
45 | .B "#include <mLib/bits.h>" |
46 | |
47 | .BI "octet U8(" v ); |
48 | .BI "uint16 U16(" v ); |
49 | .BI "uint32 U32(" v ); |
50 | |
51 | .BI "octet LSL8(" v ", " s ); |
52 | .BI "octet LSR8(" v ", " s ); |
53 | .BI "uint16 LSL16(" v ", " s ); |
54 | .BI "uint16 LSR16(" v ", " s ); |
55 | .BI "uint32 LSL32(" v ", " s ); |
56 | .BI "uint32 LSR32(" v ", " s ); |
57 | |
58 | .BI "octet ROL8(" v ", " s ); |
59 | .BI "octet ROR8(" v ", " s ); |
60 | .BI "uint16 ROL16(" v ", " s ); |
61 | .BI "uint16 ROR16(" v ", " s ); |
62 | .BI "uint32 ROL32(" v ", " s ); |
63 | .BI "uint32 ROR32(" v ", " s ); |
64 | |
65 | .BI "octet GETBYTE(" p ", " o ); |
66 | .BI "void PUTBYTE(" p ", " o ", " v ); |
67 | |
68 | .BI "octet LOAD8(" p ); |
69 | .BI "void STORE8(" p ", " v ); |
70 | |
71 | .BI "uint16 LOAD16_B(" p ); |
72 | .BI "uint16 LOAD16_L(" p ); |
73 | .BI "uint16 LOAD16(" p ); |
74 | .BI "void STORE16_B(" p ", " v ); |
75 | .BI "void STORE16_L(" p ", " v ); |
76 | .BI "void STORE16(" p ", " v ); |
77 | |
78 | .BI "uint32 LOAD32_B(" p ); |
79 | .BI "uint32 LOAD32_L(" p ); |
80 | .BI "uint32 LOAD32(" p ); |
81 | .BI "void STORE32_B(" p ", " v ); |
82 | .BI "void STORE32_L(" p ", " v ); |
83 | .BI "void STORE32(" p ", " v ); |
84 | .fi |
85 | .SH DESCRIPTION |
86 | The header file |
87 | .B <mLib/bits.h> |
88 | contains a number of useful definitions for portably dealing with bit- |
89 | and byte-level manipulation of larger quantities. It declares three |
90 | types: |
91 | .TP |
92 | .B octet |
93 | Equivalent to |
94 | .BR "unsigned char" . |
95 | This is intended to be used when a character array is used to represent |
96 | the octets of some external data format. Note that on some |
97 | architectures the |
98 | .B "unsigned char" |
99 | type may occupy more than 8 bits. |
100 | .TP |
101 | .B uint16 |
102 | Equivalent to |
103 | .BR "unsigned short" . |
104 | Intended to be used when a 16-bit value is required. This type is |
105 | always capable of representing any 16-bit unsigned value, but the actual |
106 | type may be wider than 16 bits and will require masking. |
107 | .TP |
108 | .B uint32 |
109 | Equivalent to some (architecture-dependent) standard type. Capable of |
110 | representing any unsigned 32-bit value, although the the actual type may |
111 | be wider than 32 bits. |
112 | .PP |
113 | The constants |
114 | .BR MASK8 , |
115 | .B MASK16 |
116 | and |
117 | .B MASK32 |
118 | contain bitmasks appropriate for discarding additional bits from a value |
119 | before use as an 8-, 16- or 32-bit quantity. The macros |
120 | .BR U8 , |
121 | .B U16 |
122 | and |
123 | .B U32 |
124 | perform masking and type-coercion on a value, and may be more useful in |
125 | general. For example, |
126 | .B U16(x) |
127 | yields a value of type |
128 | .B uint16 |
129 | which contains (only) the least-significant 16 bits of |
130 | .BR x . |
131 | .PP |
132 | The macros |
133 | .BI LSL n |
134 | and |
135 | .BI LSR n |
136 | perform left and right logical shift operations on values of width |
137 | .IR n , |
138 | where |
139 | .I n |
140 | is one of |
141 | .BR 8 , |
142 | .B 16 |
143 | or |
144 | .BR 32 . |
145 | The first argument, written |
146 | .IR v , |
147 | is the value to shift, and the second, written |
148 | .IR s , |
149 | is the number of bits to shift. The value |
150 | .I s |
151 | is reduced modulo |
152 | .I n |
153 | before use. Similarly, the macros |
154 | .BI ROL n |
155 | and |
156 | .BI ROR n |
157 | perform left and right rotations (cyclic shifts) on values of width |
158 | .IR n . |
159 | These macros are all written in such a way as to maximize portability. |
160 | A compiler may be able to spot that simple machine instructions will |
161 | suffice in many cases, although that's not the main objective. |
162 | .PP |
163 | The macros |
164 | .BI LOAD n |
165 | and |
166 | .BI STORE n |
167 | (where again |
168 | .I n |
169 | is one of |
170 | .BR 8 , |
171 | .B 16 |
172 | or |
173 | .BR 32 ) |
174 | perform transformations between |
175 | .IR n -bit |
176 | quantities and arrays of octets. For example, |
177 | .B LOAD32(q) |
178 | returns the 32-bit value stored in the four octets starting at address |
179 | .BR q , |
180 | and |
181 | .B STORE16(q, x) |
182 | stores the 16-bit value |
183 | .B x |
184 | in the 2 octets starting at address |
185 | .BR q . |
186 | Values are loaded and stored such that the most significant octet is |
187 | assigned the lowest address (i.e., they use network, or big-endian byte |
188 | order). Macros |
189 | .BI LOAD n _L |
190 | and |
191 | .BI STORE n _L |
192 | are also provided for |
193 | .I n |
194 | either |
195 | .B 16 |
196 | or |
197 | .BR 32 : |
198 | they use little-endian byte order. There are |
199 | explicit big-endian macros |
200 | .BI LOAD n _B |
201 | and |
202 | .BI STORE n _B |
203 | too. The pointer arguments don't have to be pointers to octets; the |
204 | value arguments don't have to be of the right type. The macros perform |
205 | all appropriate type casting and masking. Again, these macros are |
206 | written with portability foremost in mind, although it seems they don't |
207 | actually perform at all badly in real use. |
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208 | .SH "SEE ALSO" |
209 | .BR mLib (3). |
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210 | .SH AUTHOR |
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211 | Mark Wooding, <mdw@distorted.org.uk> |
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212 | |