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Add documentation for the FILL pseudo operation
author William Astle <lost@l-w.ca>
date Wed, 30 Jan 2013 21:32:14 -0700
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74 CLASS="SECTION"
75 ><A
76 NAME="AEN599"
77 >3.10. Assembler Modes and Pragmas</A
78 ></H1
79 ><P
80 >There are a number of options that affect the way assembly is performed.
81 Some of these options can only be specified on the command line because
82 they determine something absolute about the assembly process. These include
83 such things as the output target. Other things may be switchable during
84 the assembly process. These are known as pragmas and are, by definition,
85 not portable between assemblers.</P
86 ><P
87 >LWASM supports a number of pragmas that affect code generation or
88 otherwise affect the behaviour of the assembler. These may be specified by
89 way of a command line option or by assembler directives. The directives
90 are as follows.</P
91 ><P
92 ></P
93 ><DIV
94 CLASS="VARIABLELIST"
95 ><DL
96 ><DT
97 >PRAGMA <CODE
98 CLASS="PARAMETER"
99 >pragma[,...]</CODE
100 ></DT
101 ><DD
102 ><P
103 >Specifies that the assembler should bring into force all <CODE
104 CLASS="PARAMETER"
105 >pragma</CODE
106 >s
107 specified. Any unrecognized pragma will cause an assembly error. The new
108 pragmas will take effect immediately. This directive should be used when
109 the program will assemble incorrectly if the pragma is ignored or not supported.</P
110 ></DD
111 ><DT
112 >*PRAGMA <CODE
113 CLASS="PARAMETER"
114 >pragma[,...]</CODE
115 ></DT
116 ><DD
117 ><P
118 >This is identical to the PRAGMA directive except no error will occur with
119 unrecognized or unsupported pragmas. This directive, by virtue of starting
120 with a comment character, will also be ignored by assemblers that do not
121 support this directive. Use this variation if the pragma is not required
122 for correct functioning of the code.</P
123 ></DD
124 ><DT
125 >*PRAGMAPUSH <CODE
126 CLASS="PARAMETER"
127 >pragma[,...]</CODE
128 ></DT
129 ><DD
130 ><P
131 >This directive saves the current state of the specified pragma(s) for later retrieval. See discussion below for more information.</P
132 ><P
133 >This directive will not throw any errors for any reason.</P
134 ></DD
135 ><DT
136 >*PRAGMAPOP <CODE
137 CLASS="PARAMETER"
138 >pragma[,...]</CODE
139 ></DT
140 ><DD
141 ><P
142 >This directive restores the previously saved state of the specified pragma(s). See discussion below for more information.</P
143 ><P
144 >This directive will not throw any errors for any reason.</P
145 ></DD
146 ></DL
147 ></DIV
148 ><P
149 >Each pragma supported has a positive version and a negative version.
150 The positive version enables the pragma while the negative version disables
151 it. The negatitve version is simply the positive version with "no" prefixed
152 to it. For instance, "pragma" vs. "nopragma". When only one version is
153 listed below, its opposite can be obtained by prepending "no" if it is not
154 present or removing "no" from the beginning if it is present.</P
155 ><P
156 >Pragmas are not case sensitive.</P
157 ><P
158 ></P
159 ><DIV
160 CLASS="VARIABLELIST"
161 ><DL
162 ><DT
163 >index0tonone</DT
164 ><DD
165 ><P
166 >When in force, this pragma enables an optimization affecting indexed addressing
167 modes. When the offset expression in an indexed mode evaluates to zero but is
168 not explicity written as 0, this will replace the operand with the equivalent
169 no offset mode, thus creating slightly faster code. Because of the advantages
170 of this optimization, it is enabled by default.</P
171 ></DD
172 ><DT
173 >cescapes</DT
174 ><DD
175 ><P
176 >This pragma will cause strings in the FCC, FCS, and FCN pseudo operations to
177 have C-style escape sequences interpreted. The one departure from the official
178 spec is that unrecognized escape sequences will return either the character
179 immediately following the backslash or some undefined value. Do not rely
180 on the behaviour of undefined escape sequences.</P
181 ></DD
182 ><DT
183 >importundefexport</DT
184 ><DD
185 ><P
186 >This pragma is only valid for targets that support external references. When
187 in force, it will cause the EXPORT directive to act as IMPORT if the symbol
188 to be exported is not defined. This is provided for compatibility with the
189 output of gcc6809 and should not be used in hand written code. Because of
190 the confusion this pragma can cause, it is disabled by default.</P
191 ></DD
192 ><DT
193 >undefextern</DT
194 ><DD
195 ><P
196 >This pragma is only valid for targets that support external references. When in
197 force, if the assembler sees an undefined symbol on the second pass, it will
198 automatically define it as an external symbol. This automatic definition will
199 apply for the remainder of the assembly process, even if the pragma is
200 subsequently turned off. Because this behaviour would be potentially surprising,
201 this pragma defaults to off.</P
202 ><P
203 >The primary use for this pragma is for projects that share a large number of
204 symbols between source files. In such cases, it is impractical to enumerate
205 all the external references in every source file. This allows the assembler
206 and linker to do the heavy lifting while not preventing a particular source
207 module from defining a local symbol of the same name as an external symbol
208 if it does not need the external symbol. (This pragma will not cause an
209 automatic external definition if there is already a locally defined symbol.)</P
210 ><P
211 >This pragma will often be specified on the command line for large projects.
212 However, depending on the specific dynamics of the project, it may be sufficient
213 for one or two files to use this pragma internally.</P
214 ></DD
215 ><DT
216 >export</DT
217 ><DD
218 ><P
219 >This pragma causes all symbols to be added to the export list
220 automatically. This is useful when a large number of symbols need to be
221 exported but you do not wish to include an EXPORT directive for all of them.
222 This is often useful on the command line but might be useful even inline
223 with the PRAGMA directive if a large number of symbols in a row are to be
224 exported.</P
225 ></DD
226 ><DT
227 >dollarlocal</DT
228 ><DD
229 ><P
230 >When set, a "$" in a symbol makes it local. When not set, "$" does not
231 cause a symbol to be local. It is set by default except when using the OS9
232 target.</P
233 ></DD
234 ><DT
235 >dollarnotlocal</DT
236 ><DD
237 ><P
238 > This is the same as the "dollarlocal" pragma except its sense is
239 reversed. That is, "dollarlocal" and "nodollarnotlocal" are equivalent and
240 "nodollarlocal" and "dollarnotlocal" are equivalent. </P
241 ></DD
242 ><DT
243 >pcaspcr</DT
244 ><DD
245 ><P
246 > Normally, LWASM makes a distinction between PC and PCR in program
247 counter relative addressing. In particular, the use of PC means an absolute
248 offset from PC while PCR causes the assembler to calculate the offset to the
249 specified operand and use that as the offset from PC. By setting this
250 pragma, you can have PC treated the same as PCR. </P
251 ></DD
252 ><DT
253 >shadow</DT
254 ><DD
255 ><P
256 >When this pragma is in effect, it becomes possible to define a macro
257 that matches an internal operation code. Thus, it makes it possible to
258 redefine either CPU instructions or pseudo operations. Because this feature
259 is of dubious utility, it is disabled by default.</P
260 ></DD
261 ><DT
262 >nolist</DT
263 ><DD
264 ><P
265 >Lines where this pragma is in effect will not appear in the assembly
266 listing. Also, any symbols defined under this pragma will not show up in
267 the symbol list. This is most useful in include files to avoid spamming the
268 assembly listing with dozens, hundreds, or thousands of irrelevant
269 symbols.</P
270 ></DD
271 ><DT
272 >autobranchlength</DT
273 ><DD
274 ><P
275 >One of the perennial annoyances for 6809 programmers is that the
276 mneumonics for the short and long branch instructions are different (bxx vs.
277 lbxx), which is at odds with the rest of the instruction set. This pragma
278 is a solution to those annoying byte overflow errors that short branch
279 instructions tend to aquire.</P
280 ><P
281 >When this pragma is in effect, which is not the default, whenever any
282 relative branch instruction is used, its size will be automatically
283 determined based on the actual distance to the destination. In other words,
284 one can write code with long or short branches everywhere and the assembler
285 will choose a size for the branch.</P
286 ><P
287 >Also, while this pragma is in effect, the &#62; and &#60; symbols can be used
288 to force the branch size, analogous to their use for other instructions with
289 &#60; forcing 8 bit offsets and &#62; forcing 16 bit offets.</P
290 ><P
291 >Because this pragma leads to source that is incompatible with other
292 assemblers, it is strongly recommended that it be invoked using the PRAGMA
293 directive within the source code rather than on the command line or via the
294 *PRAGMA directive. This way, an error will be raised if someone tries to
295 assemble the code under a different assembler.</P
296 ></DD
297 ><DT
298 >nosymbolcase, symbolnocase</DT
299 ><DD
300 ><P
301 >Any symbol defined while this pragma is in force will be treated as
302 case insensitive, regardless whether the pragma is in force when the symbol
303 is referenced.</P
304 ><P
305 >It is important to note that this pragma will not work as expected in
306 all cases when using the object file assembly target. It is intended for
307 use only when the assembler will be producing the final binary.</P
308 ></DD
309 ><DT
310 >condundefzero</DT
311 ><DD
312 ><P
313 >This pragma will cause the assembler to change the way it handles
314 symbols in conditional expressions. Ordinarily, any symbol that is not
315 defined prior to the conditional will throw an undefined symbol error. With
316 this pragma in effect, symbols that are not yet defined at the point the
317 conditional is encountered will be treated as zero.</P
318 ><P
319 >This is not the default because it encourages poor code design. One
320 should use the "IFDEF" or "IFNDEF" conditionals to test for the presence of
321 a symbol.</P
322 ><P
323 >It is important to note that if a symbol is defined but it does not
324 yet evaluate to a constant value at the point where the conditional appears,
325 the assembler will still complain about a non constant condition.</P
326 ></DD
327 ></DL
328 ></DIV
329 ><P
330 >As a convenience, each input file has a pragma state stack. This
331 allows, through the use of *PRAGMAPUSH and *PRAGMAPOP, a file to change a
332 pragma state and then restore it to the precise state it had previously.
333 If, at the end of an input file, all pragma states have not been popped,
334 they will be removed from the stack. Thus, it is critical to employ
335 *PRAGMAPOP correctly. Because each input file has its own pragma stack,
336 using *PRAGMAPUSH in one file and *PRAGMAPOP in another file will not
337 work.</P
338 ><P
339 >Pragma stacks are more useful in include files, in particular in
340 conjunction with the nolist pragma. One can push the state of the nolist
341 pragma, engage the nolist pragma, and then pop the state of the nolist
342 pragma at the end of the include file. This will cause the entire include
343 file to operate under the nolist pragma. However, if the file is included
344 while nolist is already engaged, it will not undo that state.</P
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