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2 <head>
11 <!--
12 This is the manual for The XORcyst version 1.4.5.
14 Copyright (C) 2004, 2005 Kent Hansen.-->
26 </head>
27 <body>
29 <p>
30 <a name="The-Assembler"></a>Next: <a rel="next" accesskey="n" href="The-Linker.html#The-Linker">The Linker</a>,
33 <hr><br>
34 </div>
38 <p>The XORcyst assembler takes a <dfn>plaintext file</dfn> containing a sequence of 6502 instructions and assembler
39 directives (collectively referred to as assembler statements), and produces from this an <dfn>object file</dfn> (usually referred to as a <dfn>unit</dfn>) that can be fed on to the XORcyst linker.
41 <p>The reason for not producing a plain 6502 binary is largely due to the aim of producing position-independent code- and data-segments. Specifically, in The XORcyst universe code and data labels are not meant to be assigned addresses until the final process of linking. Relocation on the 6502 isn't as simple as just adding an offset to an instruction operand; the 6502 has a special set of <dfn>zero-page instructions</dfn> which can be used when addresses fall in the range 0..255, and we want to utilize these whenever possible. So until we know whether, say, a data label will fall in the zero-page range or not, we don't know whether instructions which refer to this label will have a 1-byte or 2-byte operand. Using the non-zero-page (absolute) version of an instruction would, in the general case, ensure that the address will fit, but is too wasteful in size and processor cycles. So instead of hardcoded addresses the object code contains symbolic links which are up to the linker to resolve and translate. The object file can be thought of as a more compact, linker-ready version of the original assembler file.
43 <p>Another goal is to relieve the programmer of the burden of having to make sure that all variables in a large, complex program have unique addresses, by shifting as much of this responsibility onto the linker as possible. By postponing the mapping of symbol names to addresses until the link phase, variables can be added and moved to any part of the program without risking that it will interfere with the storage allocation of another part of the program.
45 <p>The object file format also enables complex resource sharing between units. An assembler expression can be arbitrarily complex, with references to any number of constants, variables or procedures defined in another unit.
49 <p>The basic usage is
54 If all goes well, this will produce a similarly named file of extension <span class="file">.o</span>.
56 <p>For example,
58 </pre>
59 <p>produces the object file <span class="file">driver.o</span> if no errors are encountered by the assembler.
63 <dl>
64 <dt><code>--define IDENT[=VALUE]</code><dd>Enters the identifier <code>IDENT</code> into the global symbol table, optionally assigning it the value <code>VALUE</code>. The default value is integer <code>0</code>. To assign a string, escape sequences must be used, i.e. <code>--define my_string=\"Have a nice day\"</code>.
66 <br><dt><code>--output FILE</code><dd>Directs output to the file <code>FILE</code> rather than the default file.
68 <br><dt><code>--swap-parens</code><dd>Changes the operators used to specify indirection from <code>[ ]</code> to <code>( )</code>. <code>[ ]</code> takes over <code>( )</code>'s role in arithmetic expressions.
72 <br><dt><code>--verbose</code><dd>Instructs the assembler to print some messages about what it is doing.
74 <br><dt><code>--debug</code><dd>Retains file and line information, so that the linker can produce more descriptive warning and error messages.
76 </dl>
82 <p>(<strong>Note:</strong> This is not meant to be an introductory guide to 6502 assembly. Only the XORcyst-specific features and quirks will be explained. (For readers new to the 6502 and assemblers, <a href="http://www.google.com/search?q=6502+tutorial">http://www.google.com/search?q=6502+tutorial</a> may be a good starting point.)
84 <p>Because the assembler aims to enforce completely position-independent code, it does not allow the <code>.org </code><var>address</var> or <code>.base </code><var>address</var> directives commonly employed by 6502 assemblers. But most other constructs familiar to some people are in place. These and additional features will be explained subsequently. (For a complete list of directives, see <a href="Assembler-Directives.html#Assembler-Directives">Assembler Directives</a>.)
86 <p>In the code templates given in this section, any arguments enclosed in italic square brackets <em>[ ... ]</em> are optional.
90 <p>Here is a short assembler file which demonstrates basic functionality:
94 my_variable .byte ; define a byte variable
98 .codeseg ; begin code segment
100 .include "config.h" ; include another source file
102 ; conditional definition of constant my_priority
103 .ifdef HAVE_CONFIG_H
104 my_priority = 10
105 .else
106 my_priority = 0
107 .endif
109 ; declare a macro named store_const with parameters value and addr
110 .macro store_const value, addr
111 lda #value
112 sta addr
113 .endm ; end macro
115 ; a subroutine entrypoint is here
116 .proc my_subroutine
118 store_const my_priority, my_variable ; macro invocation
120 lda [$0A], y ; NOTE: [ ] used for indirection, not ( ), unless --swap-parens switch used
121 beq +
122 jsr some_function ; call external function
124 ; produce a short delay
125 + ldx #60
126 @@delay:
127 dex
128 bne @@delay
130 ; exit with my_priority in accumulator
131 lda #my_priority
132 rts
133 .endp ; end of procedure definition
135 .public my_subroutine ; make my_subroutine visible to other units
136 .extrn some_function:proc ; some_function is located in another unit
138 .end ; end of assembler input
139 </pre>
140 <p>While the example itself doesn't do anything useful, it shows how you can.
144 <p>The following kinds of integer literal are understood by the assembler (examples given in parentheses):
146 <ul>
147 <li><strong>Decimal:</strong> Non-zero decimal digit followed by zero or more decimal digits (<code>1234</code>)
149 <li><strong>Hexadecimal:</strong> <code>0x</code> or <code>$</code> followed by one or more hexadecimal digits (<code>0xFACE, $BEEF</code>); one or more hexadecimal digits followed by <code>h</code> (<code>95Ah</code>). In the latter case numbers beginning with A through F must be preceded by a 0 (otherwise, say, <code>BABEh</code> would be interpreted as an identifier).
151 <li><strong>Binary:</strong> String of binary digits either preceded by <code>%</code> or succeeded by <code>b</code> (<code>%010110, 11001100b</code>).
155 </ul>
157 <p>String literals must be enclosed inbetween a pair of <code>"</code> (as in <code>"You are a dweeb"</code>).
159 <p>Character literals must be of the form <code>'A'</code>.
163 <p>Identifiers must conform to the regular expression <code>[[:alpha:]_][[:alnum:]_]*</code>. They are case sensitive.
164 Examples of valid identifiers are
166 </pre>
167 <p>Examples of invalid identifiers are
169 </pre>
172 <p>Operands to assembler statements are expressions. An expression can contain any number of operators, identifiers and literals, and parentheses to group terms. The operators are the familiar arithmetic, binary, shift and relational ones (same as in C, pretty much), plus a few more which are useful when writing code for a machine which has a 16-bit address space but only 8-bit registers:
174 <ul>
175 <li><code>< </code><var>expression</var> : Get low 8 bits of <var>expression</var>
177 <li><code>> </code><var>expression</var> : Get high 8 bits of <var>expression</var>
179 </ul>
181 <p><code>$</code> can be used in an expression to refer to the address where the current instruction is assembled.
183 <p><code>^</code><var>symbol</var> gets the bank number in which <var>symbol</var> is located (determined at link time).
185 <p><code>sizeof(</code><var>symbol</var><code>)</code> gets the size of <var>symbol</var> in bytes.
187 <p>When both operands to an operator are strings, the semantics are as follows: <var>str1</var> + <var>str2</var> concatenates; the relational operators perform string comparison; and all other operators are invalid. When one operand is a string and the other is an integer, the integer is implicitly converted to a string and concatenated with the string operand to produce a string as result.
191 <p>There are two ways to define a global label.
193 <ul>
194 <li><var>identifier</var><strong>:</strong> at the beginning of a source line defines the label <var>identifier</var> and assigns it the address of the current Program Counter. The colon is mandatory.
196 <li>Using the <code>.label</code> directive. It is of the form
198 <pre class="example"> .label <var>identifier</var> <em>[= </em><var>address</var><em>]</em> <em>[ : </em><var>type</var><em>]</em>
199 </pre>
200 <p>The absolute address of the label can be specified. If no address is given, the address is the current Program Counter.
202 <p>The type of data that the label addresses can also be specified. The valid type specifiers are <code>byte</code>, <code>word</code>, <code>dword</code>, or an identifier, which must be the name of a user-defined type.
204 </ul>
208 <p>A <dfn>local label</dfn> is only visible in the scope consisting of the statements between two regular labels; or, for macros, only in the body of the macro. Just as a regular label must be unique in the whole program scope, a local label must be unique in the scope in which it is defined. The big advantage here is that the name of the local label can be reused as long as the definitions exist in different local scopes. Local labels are prefixed by <code>@@</code>. Unlike regular labels the local name itself can start with a digit, so for instance <code>@@10</code> is valid.
209 The following example shows how a local label can exist unambigiously in two scopes.
211 ldx #100
212 @@loop: ; this label exists in my_first_delay's namespace
213 dex
214 bne @@loop
215 rts
217 my_second_delay: ; new local scope begins here
218 ldy #200
219 @@loop: ; this label exists in my_second_delay's namespace
220 dey
221 bne @@loop
222 rts
223 </pre>
224 <p>As mentioned, the same local cannot be redefined within a scope. So having, say, two labels called <code>@@loop</code> in the same scope would produce an assembler error. Also, something like the following would produce an error:
226 bvs @@handle_overflow
227 barrier:
228 rts
229 @@handle_overflow:
230 ; ...
231 </pre>
232 <p>since the branch instruction refers to a local label defined in a different scope (because of the strategic placement of the label <code>barrier</code>).
236 <p>These are “anonymous” labels that can be redefined as many times as you want. A reference to a forward/backward label is resolved to the closest matching definition in the succeeding assembly statements (forward branches) or preceding assembly statements (backward branches).
238 <p>A forward branch consists of one or more (up to eight) consecutive <code>+</code> (plus) symbols. A backward branch consists of one or more (up to eight) consecutive <code>-</code> (minus) symbols. The following examples illustrate use of forward and backward branches.
241 bmi ++
242 lda $40
243 bne + ; branches to first forward label
244 ; do something ...
245 + dex ; first forward label
246 beq + ; branches to second forward label
247 ; do something more ...
248 + sta $40 ; second forward label
249 ++ rts
251 </pre>
253 bmi +
254 - lda $2002 ; first backward label
255 bne - ; branches to first backward label
256 - lda $2002 ; second backward label
257 bne - ; branches to second backward label
258 + rts
259 </pre>
262 <p>There are three ways to define equates.
263 <ul>
264 <li>With the <code>=</code> operator. An equate defined this way can be redefined, and it obeys program order.
267 ldx #i
268 i = i + 1
269 ldy #i
270 </pre>
271 <p>In the example above, the assembler will substitute <code>10</code> for the first occurence of <code>i</code> and <code>11</code> for the last.
273 <li>With the <code>.equ</code> directive. An equate defined this way can only be defined once, and it does not obey program order (that is, it can be defined at a later point from where it is used). An equate of this type can be exported, so that it may be accessed by other units (more on exporting symbols later).
277 </pre>
278 <li>With the <code>.define</code> directive. This directive is semantically equal to <code>.equ</code>, but the value is optional, so you can write CPP-like defines, which is more compact. When no value is given, the symbol is defined as integer 0.
281 .define MYHEADER_H
282 ; ...
283 .endif ; !MYHEADER_H
284 </pre>
285 </ul>
289 <p>There are two ways to go about doing conditional assembly. One way is to test if a certain identifier has been defined (that is, equated) using the <code>.ifdef</code> directive, as shown in the next two templates.
292 <var>statements</var>
293 .endif
294 </pre>
296 <var>true-statements</var>
297 .else
298 <var>false-statements</var>
299 .endif
300 </pre>
301 <p>The other way is to test a full-fledged expression, as shown in the next template.
304 <var>statements</var>
305 .elif <var>expression-II</var>
306 <var>statements-II</var>
307 .else
308 <var>other-statements</var>
309 .endif
310 </pre>
313 <p>Macro definitions are of the form
315 <pre class="example"> .macro <var>name</var> <em>[</em><var>parameter1</var><em>, </em><var>parameter2</var><em>, ...]</em>
316 <var>statements</var>
317 .endm
318 </pre>
319 <p>The parameters must be legal identifiers.
321 <p>To invoke (expand) the statements (body) of a macro in your program, issue the assembler statement <var>name</var>, where <var>name</var> is the macro name, followed by a comma-separated list of actual arguments, if the macro has any. The arguments will be substituted for the respective parameter names in the resulting statements.
323 <p>You can use local labels in the body of a macro. These labels will be completely local and unique to each expanded macro instance; any local labels defined outside the expanded body are not “seen”. For example, if you have the following macro definition
325 @@loop:
326 dey
327 bne @@loop
328 .endm
329 </pre>
330 <p>and then use the macro as shown in the following
332 my_macro
333 my_macro
334 dex
335 bne @@loop
336 </pre>
337 <p>each expansion of <code>my_macro</code> will have its own local label <code>@@loop</code>, neither of which interfere with the local label <code>@@loop</code> in the scope where the macro is invoked.
339 <p>Macros can be nested to arbitrary depth.
343 <p>An anonymous REPT (REPeaT) macro is of the form
346 <strong>.rept 8</strong>
347 .db i
348 i = i*2
349 <strong>.endm</strong>
350 </pre>
351 <p>The statements between <code>rept</code> and <code>endm</code> will be repeated as many times as specified by the argument to <code>rept</code>. In the preceding example, the resulting expansion is equivalent to
354 </pre>
355 <p>Similarly, an anonymous WHILE macro is of the form
358 <strong>.while i <= 128</strong>
359 .db i
360 i = i*2
361 <strong>.endm</strong>
362 </pre>
363 <p>The statements between <code>while</code> and <code>endm</code> will be repeated while the expression given as argument to <code>while</code> is true (non-zero). The code inside the macro body is responsible for updating the variables involved in the expression, so that it will eventually become false. In the preceding example, the resulting expansion is equivalent to
366 </pre>
369 <p>There are two directives for including files.
371 <ul>
372 <li><code>.incsrc "</code><var>src-file</var><code>"</code> (can also be written <code>.include</code>) interprets the specified file as textual assembler statements.
374 <li><code>.incbin "</code><var>bin-file</var><code>"</code> interprets the specified file as a binary buffer.
376 </ul>
380 <p>There is a class of directives for defining data storage and values.
382 <ul>
383 <li><code>.db</code> <em>[</em><var>expression</var><em>, ...]</em> : Defines a string of bytes
384 <li><code>.dw</code> <em>[</em><var>expression</var><em>, ...]</em> : Defines a string of words
385 <li><code>.dd</code> <em>[</em><var>expression</var><em>, ...]</em> : Defines a string of doublewords
386 <li><code>.char</code> <em>[</em><var>expression</var><em>, ...]</em> : Defines a string of characters (explained later)
387 <li><code>.dsb</code> <em>[</em><var>expression</var><em>]</em> : Defines a storage of size <var>expression</var> bytes
388 <li><code>.dsw</code> <em>[</em><var>expression</var><em>]</em> : Defines a storage of size <var>expression</var> words
389 <li><code>.dsd</code> <em>[</em><var>expression</var><em>]</em> : Defines a storage of size <var>expression</var> doublewords
391 </ul>
393 <p>If no argument is given to the directive, a single item of the respective datatype is allocated, i.e.
395 </pre>
396 <p>is equivalent to
398 </pre>
399 <p>Alternatively, data arrays can be allocated using square brackets [ ] like in C:
402 </pre>
403 <p>which is equivalent to
405 </pre>
406 <p><code>.byte</code>, <code>.word</code> and <code>.dword</code> are more verbose aliases for <code>.db</code>, <code>.dw</code> and <code>.dd</code>, respectively.
408 <p>Note that data cannot be initialized in a data segment; only storage for the data can be allocated there.
412 <p>Use the <code>.charmap</code> directive to specify a map file describing the mapping from regular ASCII-coded characters to your custom set. See <a href="Custom-Character-Maps.html#Custom-Character-Maps">Custom Character Maps</a> for a description of the format of such a custom character map file. Once the character map has been set, you can define your textual data by using the <code>.char</code>-directive. The information in the character map is applied to the given data by the assembler in order to transform it to a regular <code>.db</code> directive internally. The <code>.charmap</code> directive obeys program order, meaning you can use different character maps at different points in your code. If no character map has been set, <code>.char</code> is equivalent to <code>.db</code>. A simple example of the use of <code>.charmap</code> and <code>.char</code> follows.
414 <pre class="example"> .charmap "my_map.tbl" ; set the custom character map to the one defined in my_map.tbl
416 </pre>
419 <p>There are currently four kinds of types that can be defined by the user. For further information on the concepts of their use, consult a C manual.
421 <ul>
422 <li><strong>Structures</strong>.
425 my_1st_field .db
426 my_2nd_field .dw
427 my_3rd_field .type my_other_struc
428 .ends
429 </pre>
430 <p>Using “flat” addressing, structure members are accessed just like in C.
433 </pre>
434 <p>For indirect addressing, the scope operator can be used to get the offset of the field.
437 lda [$00],y ; load ($00).inventory.sword
438 </pre>
439 <li><strong>Unions</strong>.
442 byte_value .db
443 word_value .dw
444 string_value .char[32]
445 .ends
446 </pre>
447 <p>In a union, the fields are “overlaid”; that is, they share the same storage, and in general only one of the fields is used (at a time) for a particular instance of the union. A typical usage is to define a structure with two members: An enumerated type that selects one of the union fields, and the actual union containing the fields.
449 <p>Anonymous unions can be defined “inline” as part of a structure, as shown in the following example:
452 type .byte
453 <strong> .union</strong>
454 <strong> byte_value .byte[4]</strong>
455 <strong> word_value .word[2]</strong>
456 <strong> dword_value .dword</strong>
457 <strong> .ends</strong>
458 .ends
459 </pre>
460 <p><code>byte_value</code>, <code>word_value</code> and <code>dword_value</code> may then be accessed as top-level members of the structure, but do in fact share storage.
462 <li><strong>Records</strong> (bitfields).
465 </pre>
466 <p>A record can be maximum 8 bits (1 byte) wide. The bitfields are arranged from high to low; for example, in the record shown above, <code>top_bits</code> would occupy bits 7:5, <code>middle_bits</code> 4:3 and <code>bottom_bits</code> 2:0. Lower bits are padded if necessary to fill the byte.
468 <p>The scope operator (<code>::</code>) returns the number of right shifts necessary to bring the LSb of a bitfield into the LSb of the accumulator. The <code>MASK</code> operator returns a bitfield's logical AND mask. For example, using the record definition shown above,
471 </pre>
472 <p>returns <code>3</code>, and
474 </pre>
475 <p>returns <code>%00011000</code>. These are the two basic operations necessary to manipulate bitfields. The following macro shows how a field can be extracted:
478 ; rec = record type identifier
479 ; fld = bitfield identifier
480 ; OUT: ACC = field `fld' of `rec' in lower bits; upper bits zero
481 .macro get_field rec, fld
482 and #(mask rec::fld) ; ditch other fields
483 .rept rec::fld ; shift down to bit 0
484 lsr
485 .endm
486 .endm
487 </pre>
488 <li><strong>Enumerations</strong>.
491 option_1 = 1
492 option_2
493 option_3
494 option_4
495 .ende
496 </pre>
497 <p>Note that an enumerated value is encoded as a <code>byte</code>.
499 </ul>
503 <p>The general syntax is
506 </pre>
507 <p>or just
510 </pre>
511 <p>Where <var>identifier</var> is the name of a user-defined type. This allocates <code>sizeof(</code><var>identifier</var><code>)</code> bytes of storage. Optionally, a value initializer can be specified (only in code segments). The form of this initializer depends on the type of data.
513 <ul>
514 <li><strong>Structure</strong>. The initializer is of the form
516 <pre class="example"> { <var>field1-value</var>, <em>[</em><var>field2-value</var><em>, ..., ]</em> }
517 </pre>
518 <p>The field initializers must match the order of the fields in the type definition. To leave a field blank, leave its initializer empty. For example
521 </pre>
522 <p>defines three instances of type <code>my_struc</code>, with various fields explicitly initialized and others implicitly padded by the assembler.
524 <p>Since structures can contain sub-structures, so can a structure initializer. To initialize a sub-structure, simply start a new pair of { } and specify field values, recursively.
526 <li><strong>Union</strong>. The initializer is of the same form as a structure initializer, except only one of the fields in the union can be initialized.
528 <li><strong>Record</strong>. The initializer is of the same form as a structure initializer, but cannot contain sub-structure initializers (each bitfield is a “simple” value).
530 <li><strong>Enum</strong>. The initializer is simply an identifier that must be one of the identifiers appearing in the type definition.
532 </ul>
534 <p>To define an array of (uninitialized) values of a user-defined type, use the C-style method, for example:
536 <pre class="example"> my_array .my_struc<strong>[100]</strong> ; array of 100 values of type my_struc
537 </pre>
540 <p>A symbol can be indexed statically using the C-style syntax
542 <pre class="example"> <var>identifier</var><strong>[</strong><var>expression</var><strong>]</strong>
543 </pre>
544 <p>For byte arrays, this is simply equivalent to the expression
547 </pre>
548 <p>In general, it is equivalent to
550 <pre class="example"> <var>identifier</var> + <var>expression</var> * sizeof <var>identifier-type</var>
551 </pre>
552 <p>where <var>identifier-type</var> is the type of <var>identifier</var>.
554 <p>An example:
557 lda #1
558 i = 0
559 .while i < 10
560 sta my_array[i].my_field ; initialize my_field to 1
561 i = i + 1
562 .endm
564 </pre>
567 <p>A procedure is of the form
570 <var>statements</var>
571 .endp
572 </pre>
573 <p>Currently, there is no internal differentiation between a procedure and a label, but <code>.proc</code> is more specific than a label, so it improves the semantics.
577 <p>To specify that a symbol used in your code is defined in a different unit, use the <code>.extrn</code> directive. This way you can call procedures or access constants exported by that unit. When you use the linker to create a final executable you also have to link in the unit(s) where the external symbols you use are defined.
579 <p>The <code>extrn</code> directive takes as arguments a comma-separated list of identifiers, followed by a colon (:), followed by a <var>symbol type</var>. The symbol type must be one of <code>BYTE</code>, <code>WORD</code>, <code>DWORD</code>, <code>LABEL</code>, <code>PROC</code>, or the name of a user-defined type, such as a structure or union.
581 <p>To export a symbol defined in your own code, thereby making it accessible to other units, use the <code>.public</code> directive. The next example shows how both directives may be used.
584 my_proc:
585 jsr proc1
586 jsr proc2
587 jsr proc3
588 rts
589 .public my_proc ; make my_proc accessible to the outside world
591 </pre>
592 <p>You can also specify the <code>.public</code> keyword directly when defining a variable, so you don't need a separate directive to make it public:
595 </pre>
598 <p>By default, the linker takes the members of data segments and maps them to the best free RAM locations it finds. However, there are times when you want to specify some constraints on the mapping. For example, you want the variable to always be mapped to the 6502's zero page. Or, you have a large array and want it to be aligned to a proper boundary so you don't risk suffering page cross penalties on indexed accesses.
600 <p>The XORcyst assembler provides the following ways to communicate mapping constraints to the linker.
602 <ul>
603 <li>To specify that a data segment variable should always be mapped to zero page, precede its definition by the <code>.zeropage</code> keyword:
606 </pre>
607 <p>Alternatively, specify the <code>.zeropage</code> keyword as argument to the <code>.dataseg</code> directive:
610 my_1st_var .byte ; .zeropage constraint will be set automatically
611 my_2nd_var .word ; ditto
612 .dataseg ; turn off .zeropage constraint
613 </pre>
614 <li>To specify that one or more data variables should be aligned, use the <code>.align</code> directive. It takes a list of identifiers followed by the alignment boundary, for example
617 my_array .byte[64]
618 .align my_array 64 ; my_array should be aligned on a 64-byte boundary
619 </pre>
620 </ul>
624 <p>If you're familiar with 6502 assembly, you know that parentheses ( ) are normally used to indicate indirect addressing modes. Unfortunately, this clashes with the use of parentheses in operand expressions. I couldn't get Bison (the parser generator) to deal with this context dependency. As I'm used to coding Intel X86 assembly, which uses brackets for indirection, I opted for [ ] as the default indirection operators. This could be a source of bugs, since if you type it the “old” way, <code>LDA ($FA),Y</code> is equivalent to <code>LDA $FA,Y</code> – which probably isn't what you wanted. However, by specifying the switch
627 </pre>
628 <p>upon invoking the assembler, the behaviour of [ ] and ( ) will be reversed. That is, the “normal” way of specifying indirection, i.e. <code>LDA ($00),Y</code> is used, while expression operands are grouped with [ ], i.e. <code>A/[B+C]</code>.
630 </body></html>