Mercurial > hg > index.cgi
comparison docs/manual/x520.html @ 153:83b7b4ce3bbd
Added prebuilt manual to repository to avoid failures to include it during releases
author | lost@l-w.ca |
---|---|
date | Sat, 27 Aug 2011 00:23:19 -0600 |
parents | |
children | 1e0a0e6cd918 |
comparison
equal
deleted
inserted
replaced
152:a26b045c4e18 | 153:83b7b4ce3bbd |
---|---|
1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd"> | |
2 <HTML | |
3 ><HEAD | |
4 ><TITLE | |
5 >Object Files and Sections</TITLE | |
6 ><META | |
7 NAME="GENERATOR" | |
8 CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK | |
9 REL="HOME" | |
10 TITLE="LW Tool Chain" | |
11 HREF="index.html"><LINK | |
12 REL="UP" | |
13 TITLE="LWASM" | |
14 HREF="c45.html"><LINK | |
15 REL="PREVIOUS" | |
16 TITLE="Structures" | |
17 HREF="x499.html"><LINK | |
18 REL="NEXT" | |
19 TITLE="Assembler Modes and Pragmas" | |
20 HREF="x583.html"></HEAD | |
21 ><BODY | |
22 CLASS="SECTION" | |
23 BGCOLOR="#FFFFFF" | |
24 TEXT="#000000" | |
25 LINK="#0000FF" | |
26 VLINK="#840084" | |
27 ALINK="#0000FF" | |
28 ><DIV | |
29 CLASS="NAVHEADER" | |
30 ><TABLE | |
31 SUMMARY="Header navigation table" | |
32 WIDTH="100%" | |
33 BORDER="0" | |
34 CELLPADDING="0" | |
35 CELLSPACING="0" | |
36 ><TR | |
37 ><TH | |
38 COLSPAN="3" | |
39 ALIGN="center" | |
40 >LW Tool Chain</TH | |
41 ></TR | |
42 ><TR | |
43 ><TD | |
44 WIDTH="10%" | |
45 ALIGN="left" | |
46 VALIGN="bottom" | |
47 ><A | |
48 HREF="x499.html" | |
49 ACCESSKEY="P" | |
50 >Prev</A | |
51 ></TD | |
52 ><TD | |
53 WIDTH="80%" | |
54 ALIGN="center" | |
55 VALIGN="bottom" | |
56 >Chapter 3. LWASM</TD | |
57 ><TD | |
58 WIDTH="10%" | |
59 ALIGN="right" | |
60 VALIGN="bottom" | |
61 ><A | |
62 HREF="x583.html" | |
63 ACCESSKEY="N" | |
64 >Next</A | |
65 ></TD | |
66 ></TR | |
67 ></TABLE | |
68 ><HR | |
69 ALIGN="LEFT" | |
70 WIDTH="100%"></DIV | |
71 ><DIV | |
72 CLASS="SECTION" | |
73 ><H1 | |
74 CLASS="SECTION" | |
75 ><A | |
76 NAME="AEN520" | |
77 >3.9. Object Files and Sections</A | |
78 ></H1 | |
79 ><P | |
80 >The object file target is very useful for large project because it allows | |
81 multiple files to be assembled independently and then linked into the final | |
82 binary at a later time. It allows only the small portion of the project | |
83 that was modified to be re-assembled rather than requiring the entire set | |
84 of source code to be available to the assembler in a single assembly process. | |
85 This can be particularly important if there are a large number of macros, | |
86 symbol definitions, or other metadata that uses resources at assembly time. | |
87 By far the largest benefit, however, is keeping the source files small enough | |
88 for a mere mortal to find things in them.</P | |
89 ><P | |
90 >With multi-file projects, there needs to be a means of resolving references to | |
91 symbols in other source files. These are known as external references. The | |
92 addresses of these symbols cannot be known until the linker joins all the | |
93 object files into a single binary. This means that the assembler must be | |
94 able to output the object code without knowing the value of the symbol. This | |
95 places some restrictions on the code generated by the assembler. For | |
96 example, the assembler cannot generate direct page addressing for instructions | |
97 that reference external symbols because the address of the symbol may not | |
98 be in the direct page. Similarly, relative branches and PC relative addressing | |
99 cannot be used in their eight bit forms. Everything that must be resolved | |
100 by the linker must be assembled to use the largest address size possible to | |
101 allow the linker to fill in the correct value at link time. Note that the | |
102 same problem applies to absolute address references as well, even those in | |
103 the same source file, because the address is not known until link time.</P | |
104 ><P | |
105 >It is often desired in multi-file projects to have code of various types grouped | |
106 together in the final binary generated by the linker as well. The same applies | |
107 to data. In order for the linker to do that, the bits that are to be grouped | |
108 must be tagged in some manner. This is where the concept of sections comes in. | |
109 Each chunk of code or data is part of a section in the object file. Then, | |
110 when the linker reads all the object files, it coalesces all sections of the | |
111 same name into a single section and then considers it as a unit.</P | |
112 ><P | |
113 >The existence of sections, however, raises a problem for symbols even | |
114 within the same source file. Thus, the assembler must treat symbols from | |
115 different sections within the same source file in the same manner as external | |
116 symbols. That is, it must leave them for the linker to resolve at link time, | |
117 with all the limitations that entails.</P | |
118 ><P | |
119 >In the object file target mode, LWASM requires all source lines that | |
120 cause bytes to be output to be inside a section. Any directives that do | |
121 not cause any bytes to be output can appear outside of a section. This includes | |
122 such things as EQU or RMB. Even ORG can appear outside a section. ORG, however, | |
123 makes no sense within a section because it is the linker that determines | |
124 the starting address of the section's code, not the assembler.</P | |
125 ><P | |
126 >All symbols defined globally in the assembly process are local to the | |
127 source file and cannot be exported. All symbols defined within a section are | |
128 considered local to the source file unless otherwise explicitly exported. | |
129 Symbols referenced from external source files must be declared external, | |
130 either explicitly or by asking the assembler to assume that all undefined | |
131 symbols are external.</P | |
132 ><P | |
133 >It is often handy to define a number of memory addresses that will be | |
134 used for data at run-time but which need not be included in the binary file. | |
135 These memory addresses are not initialized until run-time, either by the | |
136 program itself or by the program loader, depending on the operating environment. | |
137 Such sections are often known as BSS sections. LWASM supports generating | |
138 sections with a BSS attribute set which causes the section definition including | |
139 symbols exported from that section and those symbols required to resolve | |
140 references from the local file, but with no actual code in the object file. | |
141 It is illegal for any source lines within a BSS flagged section to cause any | |
142 bytes to be output.</P | |
143 ><P | |
144 >The following directives apply to section handling.</P | |
145 ><P | |
146 ></P | |
147 ><DIV | |
148 CLASS="VARIABLELIST" | |
149 ><DL | |
150 ><DT | |
151 >SECTION <CODE | |
152 CLASS="PARAMETER" | |
153 >name[,flags]</CODE | |
154 >, SECT <CODE | |
155 CLASS="PARAMETER" | |
156 >name[,flags]</CODE | |
157 >, .AREA <CODE | |
158 CLASS="PARAMETER" | |
159 >name[,flags]</CODE | |
160 ></DT | |
161 ><DD | |
162 ><P | |
163 >Instructs the assembler that the code following this directive is to be | |
164 considered part of the section <CODE | |
165 CLASS="PARAMETER" | |
166 >name</CODE | |
167 >. A section name | |
168 may appear multiple times in which case it is as though all the code from | |
169 all the instances of that section appeared adjacent within the source file. | |
170 However, <CODE | |
171 CLASS="PARAMETER" | |
172 >flags</CODE | |
173 > may only be specified on the first | |
174 instance of the section.</P | |
175 ><P | |
176 >There is a single flag supported in <CODE | |
177 CLASS="PARAMETER" | |
178 >flags</CODE | |
179 >. The | |
180 flag <CODE | |
181 CLASS="PARAMETER" | |
182 >bss</CODE | |
183 > will cause the section to be treated as a BSS | |
184 section and, thus, no code will be included in the object file nor will any | |
185 bytes be permitted to be output.</P | |
186 ><P | |
187 >If the section name is "bss" or ".bss" in any combination of upper and | |
188 lower case, the section is assumed to be a BSS section. In that case, | |
189 the flag <CODE | |
190 CLASS="PARAMETER" | |
191 >!bss</CODE | |
192 > can be used to override this assumption.</P | |
193 ><P | |
194 >If assembly is already happening within a section, the section is implicitly | |
195 ended and the new section started. This is not considered an error although | |
196 it is recommended that all sections be explicitly closed.</P | |
197 ></DD | |
198 ><DT | |
199 >ENDSECTION, ENDSECT</DT | |
200 ><DD | |
201 ><P | |
202 >This directive ends the current section. This puts assembly outside of any | |
203 sections until the next SECTION directive. ENDSECTION is the preferred form. | |
204 Prior to version 3.0 of LWASM, ENDS could also be used to end a section but | |
205 as of version 3.0, it is now an alias for ENDSTRUCT instead.</P | |
206 ></DD | |
207 ><DT | |
208 ><CODE | |
209 CLASS="PARAMETER" | |
210 >sym</CODE | |
211 > EXTERN, <CODE | |
212 CLASS="PARAMETER" | |
213 >sym</CODE | |
214 > EXTERNAL, <CODE | |
215 CLASS="PARAMETER" | |
216 >sym</CODE | |
217 > IMPORT</DT | |
218 ><DD | |
219 ><P | |
220 >This directive defines <CODE | |
221 CLASS="PARAMETER" | |
222 >sym</CODE | |
223 > as an external symbol. | |
224 This directive may occur at any point in the source code. EXTERN definitions | |
225 are resolved on the first pass so an EXTERN definition anywhere in the | |
226 source file is valid for the entire file. The use of this directive is | |
227 optional when the assembler is instructed to assume that all undefined | |
228 symbols are external. In fact, in that mode, if the symbol is referenced | |
229 before the EXTERN directive, an error will occur.</P | |
230 ></DD | |
231 ><DT | |
232 ><CODE | |
233 CLASS="PARAMETER" | |
234 >sym</CODE | |
235 > EXPORT, <CODE | |
236 CLASS="PARAMETER" | |
237 >sym</CODE | |
238 > .GLOBL, EXPORT <CODE | |
239 CLASS="PARAMETER" | |
240 >sym</CODE | |
241 >, .GLOBL <CODE | |
242 CLASS="PARAMETER" | |
243 >sym</CODE | |
244 ></DT | |
245 ><DD | |
246 ><P | |
247 >This directive defines <CODE | |
248 CLASS="PARAMETER" | |
249 >sym</CODE | |
250 > as an exported symbol. | |
251 This directive may occur at any point in the source code, even before the | |
252 definition of the exported symbol.</P | |
253 ><P | |
254 >Note that <CODE | |
255 CLASS="PARAMETER" | |
256 >sym</CODE | |
257 > may appear as the operand or as the | |
258 statement's symbol. If there is a symbol on the statement, that will | |
259 take precedence over any operand that is present.</P | |
260 ></DD | |
261 ><DT | |
262 ><CODE | |
263 CLASS="PARAMETER" | |
264 >sym</CODE | |
265 > EXTDEP</DT | |
266 ><DD | |
267 ><P | |
268 >This directive forces an external dependency on | |
269 <CODE | |
270 CLASS="PARAMETER" | |
271 >sym</CODE | |
272 >, even if it is never referenced anywhere else in | |
273 this file.</P | |
274 ></DD | |
275 ></DL | |
276 ></DIV | |
277 ></DIV | |
278 ><DIV | |
279 CLASS="NAVFOOTER" | |
280 ><HR | |
281 ALIGN="LEFT" | |
282 WIDTH="100%"><TABLE | |
283 SUMMARY="Footer navigation table" | |
284 WIDTH="100%" | |
285 BORDER="0" | |
286 CELLPADDING="0" | |
287 CELLSPACING="0" | |
288 ><TR | |
289 ><TD | |
290 WIDTH="33%" | |
291 ALIGN="left" | |
292 VALIGN="top" | |
293 ><A | |
294 HREF="x499.html" | |
295 ACCESSKEY="P" | |
296 >Prev</A | |
297 ></TD | |
298 ><TD | |
299 WIDTH="34%" | |
300 ALIGN="center" | |
301 VALIGN="top" | |
302 ><A | |
303 HREF="index.html" | |
304 ACCESSKEY="H" | |
305 >Home</A | |
306 ></TD | |
307 ><TD | |
308 WIDTH="33%" | |
309 ALIGN="right" | |
310 VALIGN="top" | |
311 ><A | |
312 HREF="x583.html" | |
313 ACCESSKEY="N" | |
314 >Next</A | |
315 ></TD | |
316 ></TR | |
317 ><TR | |
318 ><TD | |
319 WIDTH="33%" | |
320 ALIGN="left" | |
321 VALIGN="top" | |
322 >Structures</TD | |
323 ><TD | |
324 WIDTH="34%" | |
325 ALIGN="center" | |
326 VALIGN="top" | |
327 ><A | |
328 HREF="c45.html" | |
329 ACCESSKEY="U" | |
330 >Up</A | |
331 ></TD | |
332 ><TD | |
333 WIDTH="33%" | |
334 ALIGN="right" | |
335 VALIGN="top" | |
336 >Assembler Modes and Pragmas</TD | |
337 ></TR | |
338 ></TABLE | |
339 ></DIV | |
340 ></BODY | |
341 ></HTML | |
342 > |