| 1 | @c Copyright (C) 2001-2017 Free Software Foundation, Inc. |
| 2 | @c This is part of the GAS manual. |
| 3 | @c For copying conditions, see the file as.texinfo. |
| 4 | @c MMIX description by Hans-Peter Nilsson, hp@bitrange.com |
| 5 | @ifset GENERIC |
| 6 | @page |
| 7 | @node MMIX-Dependent |
| 8 | @chapter MMIX Dependent Features |
| 9 | @end ifset |
| 10 | @ifclear GENERIC |
| 11 | @node Machine Dependencies |
| 12 | @chapter MMIX Dependent Features |
| 13 | @end ifclear |
| 14 | |
| 15 | @cindex MMIX support |
| 16 | @menu |
| 17 | * MMIX-Opts:: Command-line Options |
| 18 | * MMIX-Expand:: Instruction expansion |
| 19 | * MMIX-Syntax:: Syntax |
| 20 | * MMIX-mmixal:: Differences to @code{mmixal} syntax and semantics |
| 21 | @end menu |
| 22 | |
| 23 | @node MMIX-Opts |
| 24 | @section Command-line Options |
| 25 | |
| 26 | @cindex options, MMIX |
| 27 | @cindex MMIX options |
| 28 | The MMIX version of @code{@value{AS}} has some machine-dependent options. |
| 29 | |
| 30 | @cindex @samp{--fixed-special-register-names} command line option, MMIX |
| 31 | When @samp{--fixed-special-register-names} is specified, only the register |
| 32 | names specified in @ref{MMIX-Regs} are recognized in the instructions |
| 33 | @code{PUT} and @code{GET}. |
| 34 | |
| 35 | @cindex @samp{--globalize-symbols} command line option, MMIX |
| 36 | You can use the @samp{--globalize-symbols} to make all symbols global. |
| 37 | This option is useful when splitting up a @code{mmixal} program into |
| 38 | several files. |
| 39 | |
| 40 | @cindex @samp{--gnu-syntax} command line option, MMIX |
| 41 | The @samp{--gnu-syntax} turns off most syntax compatibility with |
| 42 | @code{mmixal}. Its usability is currently doubtful. |
| 43 | |
| 44 | @cindex @samp{--relax} command line option, MMIX |
| 45 | The @samp{--relax} option is not fully supported, but will eventually make |
| 46 | the object file prepared for linker relaxation. |
| 47 | |
| 48 | @cindex @samp{--no-predefined-syms} command line option, MMIX |
| 49 | If you want to avoid inadvertently calling a predefined symbol and would |
| 50 | rather get an error, for example when using @code{@value{AS}} with a |
| 51 | compiler or other machine-generated code, specify |
| 52 | @samp{--no-predefined-syms}. This turns off built-in predefined |
| 53 | definitions of all such symbols, including rounding-mode symbols, segment |
| 54 | symbols, @samp{BIT} symbols, and @code{TRAP} symbols used in @code{mmix} |
| 55 | ``system calls''. It also turns off predefined special-register names, |
| 56 | except when used in @code{PUT} and @code{GET} instructions. |
| 57 | |
| 58 | @cindex @samp{--no-expand} command line option, MMIX |
| 59 | By default, some instructions are expanded to fit the size of the operand |
| 60 | or an external symbol (@pxref{MMIX-Expand}). By passing |
| 61 | @samp{--no-expand}, no such expansion will be done, instead causing errors |
| 62 | at link time if the operand does not fit. |
| 63 | |
| 64 | @cindex @samp{--no-merge-gregs} command line option, MMIX |
| 65 | The @code{mmixal} documentation (@pxref{mmixsite}) specifies that global |
| 66 | registers allocated with the @samp{GREG} directive (@pxref{MMIX-greg}) and |
| 67 | initialized to the same non-zero value, will refer to the same global |
| 68 | register. This isn't strictly enforceable in @code{@value{AS}} since the |
| 69 | final addresses aren't known until link-time, but it will do an effort |
| 70 | unless the @samp{--no-merge-gregs} option is specified. (Register merging |
| 71 | isn't yet implemented in @code{@value{LD}}.) |
| 72 | |
| 73 | @cindex @samp{-x} command line option, MMIX |
| 74 | @code{@value{AS}} will warn every time it expands an instruction to fit an |
| 75 | operand unless the option @samp{-x} is specified. It is believed that |
| 76 | this behaviour is more useful than just mimicking @code{mmixal}'s |
| 77 | behaviour, in which instructions are only expanded if the @samp{-x} option |
| 78 | is specified, and assembly fails otherwise, when an instruction needs to |
| 79 | be expanded. It needs to be kept in mind that @code{mmixal} is both an |
| 80 | assembler and linker, while @code{@value{AS}} will expand instructions |
| 81 | that at link stage can be contracted. (Though linker relaxation isn't yet |
| 82 | implemented in @code{@value{LD}}.) The option @samp{-x} also implies |
| 83 | @samp{--linker-allocated-gregs}. |
| 84 | |
| 85 | @cindex @samp{--no-pushj-stubs} command line option, MMIX |
| 86 | @cindex @samp{--no-stubs} command line option, MMIX |
| 87 | If instruction expansion is enabled, @code{@value{AS}} can expand a |
| 88 | @samp{PUSHJ} instruction into a series of instructions. The shortest |
| 89 | expansion is to not expand it, but just mark the call as redirectable to a |
| 90 | stub, which @code{@value{LD}} creates at link-time, but only if the |
| 91 | original @samp{PUSHJ} instruction is found not to reach the target. The |
| 92 | stub consists of the necessary instructions to form a jump to the target. |
| 93 | This happens if @code{@value{AS}} can assert that the @samp{PUSHJ} |
| 94 | instruction can reach such a stub. The option @samp{--no-pushj-stubs} |
| 95 | disables this shorter expansion, and the longer series of instructions is |
| 96 | then created at assembly-time. The option @samp{--no-stubs} is a synonym, |
| 97 | intended for compatibility with future releases, where generation of stubs |
| 98 | for other instructions may be implemented. |
| 99 | |
| 100 | @cindex @samp{--linker-allocated-gregs} command line option, MMIX |
| 101 | Usually a two-operand-expression (@pxref{GREG-base}) without a matching |
| 102 | @samp{GREG} directive is treated as an error by @code{@value{AS}}. When |
| 103 | the option @samp{--linker-allocated-gregs} is in effect, they are instead |
| 104 | passed through to the linker, which will allocate as many global registers |
| 105 | as is needed. |
| 106 | |
| 107 | @node MMIX-Expand |
| 108 | @section Instruction expansion |
| 109 | |
| 110 | @cindex instruction expansion, MMIX |
| 111 | When @code{@value{AS}} encounters an instruction with an operand that is |
| 112 | either not known or does not fit the operand size of the instruction, |
| 113 | @code{@value{AS}} (and @code{@value{LD}}) will expand the instruction into |
| 114 | a sequence of instructions semantically equivalent to the operand fitting |
| 115 | the instruction. Expansion will take place for the following |
| 116 | instructions: |
| 117 | |
| 118 | @table @asis |
| 119 | @item @samp{GETA} |
| 120 | Expands to a sequence of four instructions: @code{SETL}, @code{INCML}, |
| 121 | @code{INCMH} and @code{INCH}. The operand must be a multiple of four. |
| 122 | @item Conditional branches |
| 123 | A branch instruction is turned into a branch with the complemented |
| 124 | condition and prediction bit over five instructions; four instructions |
| 125 | setting @code{$255} to the operand value, which like with @code{GETA} must |
| 126 | be a multiple of four, and a final @code{GO $255,$255,0}. |
| 127 | @item @samp{PUSHJ} |
| 128 | Similar to expansion for conditional branches; four instructions set |
| 129 | @code{$255} to the operand value, followed by a @code{PUSHGO $255,$255,0}. |
| 130 | @item @samp{JMP} |
| 131 | Similar to conditional branches and @code{PUSHJ}. The final instruction |
| 132 | is @code{GO $255,$255,0}. |
| 133 | @end table |
| 134 | |
| 135 | The linker @code{@value{LD}} is expected to shrink these expansions for |
| 136 | code assembled with @samp{--relax} (though not currently implemented). |
| 137 | |
| 138 | @node MMIX-Syntax |
| 139 | @section Syntax |
| 140 | |
| 141 | The assembly syntax is supposed to be upward compatible with that |
| 142 | described in Sections 1.3 and 1.4 of @samp{The Art of Computer |
| 143 | Programming, Volume 1}. Draft versions of those chapters as well as other |
| 144 | MMIX information is located at |
| 145 | @anchor{mmixsite}@url{http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html}. |
| 146 | Most code examples from the mmixal package located there should work |
| 147 | unmodified when assembled and linked as single files, with a few |
| 148 | noteworthy exceptions (@pxref{MMIX-mmixal}). |
| 149 | |
| 150 | Before an instruction is emitted, the current location is aligned to the |
| 151 | next four-byte boundary. If a label is defined at the beginning of the |
| 152 | line, its value will be the aligned value. |
| 153 | |
| 154 | In addition to the traditional hex-prefix @samp{0x}, a hexadecimal number |
| 155 | can also be specified by the prefix character @samp{#}. |
| 156 | |
| 157 | After all operands to an MMIX instruction or directive have been |
| 158 | specified, the rest of the line is ignored, treated as a comment. |
| 159 | |
| 160 | @menu |
| 161 | * MMIX-Chars:: Special Characters |
| 162 | * MMIX-Symbols:: Symbols |
| 163 | * MMIX-Regs:: Register Names |
| 164 | * MMIX-Pseudos:: Assembler Directives |
| 165 | @end menu |
| 166 | |
| 167 | @node MMIX-Chars |
| 168 | @subsection Special Characters |
| 169 | @cindex line comment characters, MMIX |
| 170 | @cindex MMIX line comment characters |
| 171 | |
| 172 | The characters @samp{*} and @samp{#} are line comment characters; each |
| 173 | start a comment at the beginning of a line, but only at the beginning of a |
| 174 | line. A @samp{#} prefixes a hexadecimal number if found elsewhere on a |
| 175 | line. If a @samp{#} appears at the start of a line the whole line is |
| 176 | treated as a comment, but the line can also act as a logical line |
| 177 | number directive (@pxref{Comments}) or a preprocessor control command |
| 178 | (@pxref{Preprocessing}). |
| 179 | |
| 180 | Two other characters, @samp{%} and @samp{!}, each start a comment anywhere |
| 181 | on the line. Thus you can't use the @samp{modulus} and @samp{not} |
| 182 | operators in expressions normally associated with these two characters. |
| 183 | |
| 184 | A @samp{;} is a line separator, treated as a new-line, so separate |
| 185 | instructions can be specified on a single line. |
| 186 | |
| 187 | @node MMIX-Symbols |
| 188 | @subsection Symbols |
| 189 | The character @samp{:} is permitted in identifiers. There are two |
| 190 | exceptions to it being treated as any other symbol character: if a symbol |
| 191 | begins with @samp{:}, it means that the symbol is in the global namespace |
| 192 | and that the current prefix should not be prepended to that symbol |
| 193 | (@pxref{MMIX-prefix}). The @samp{:} is then not considered part of the |
| 194 | symbol. For a symbol in the label position (first on a line), a @samp{:} |
| 195 | at the end of a symbol is silently stripped off. A label is permitted, |
| 196 | but not required, to be followed by a @samp{:}, as with many other |
| 197 | assembly formats. |
| 198 | |
| 199 | The character @samp{@@} in an expression, is a synonym for @samp{.}, the |
| 200 | current location. |
| 201 | |
| 202 | In addition to the common forward and backward local symbol formats |
| 203 | (@pxref{Symbol Names}), they can be specified with upper-case @samp{B} and |
| 204 | @samp{F}, as in @samp{8B} and @samp{9F}. A local label defined for the |
| 205 | current position is written with a @samp{H} appended to the number: |
| 206 | @smallexample |
| 207 | 3H LDB $0,$1,2 |
| 208 | @end smallexample |
| 209 | This and traditional local-label formats cannot be mixed: a label must be |
| 210 | defined and referred to using the same format. |
| 211 | |
| 212 | There's a minor caveat: just as for the ordinary local symbols, the local |
| 213 | symbols are translated into ordinary symbols using control characters are |
| 214 | to hide the ordinal number of the symbol. Unfortunately, these symbols |
| 215 | are not translated back in error messages. Thus you may see confusing |
| 216 | error messages when local symbols are used. Control characters |
| 217 | @samp{\003} (control-C) and @samp{\004} (control-D) are used for the |
| 218 | MMIX-specific local-symbol syntax. |
| 219 | |
| 220 | The symbol @samp{Main} is handled specially; it is always global. |
| 221 | |
| 222 | By defining the symbols @samp{__.MMIX.start..text} and |
| 223 | @samp{__.MMIX.start..data}, the address of respectively the @samp{.text} |
| 224 | and @samp{.data} segments of the final program can be defined, though when |
| 225 | linking more than one object file, the code or data in the object file |
| 226 | containing the symbol is not guaranteed to be start at that position; just |
| 227 | the final executable. @xref{MMIX-loc}. |
| 228 | |
| 229 | @node MMIX-Regs |
| 230 | @subsection Register names |
| 231 | @cindex register names, MMIX |
| 232 | @cindex MMIX register names |
| 233 | |
| 234 | Local and global registers are specified as @samp{$0} to @samp{$255}. |
| 235 | The recognized special register names are @samp{rJ}, @samp{rA}, @samp{rB}, |
| 236 | @samp{rC}, @samp{rD}, @samp{rE}, @samp{rF}, @samp{rG}, @samp{rH}, |
| 237 | @samp{rI}, @samp{rK}, @samp{rL}, @samp{rM}, @samp{rN}, @samp{rO}, |
| 238 | @samp{rP}, @samp{rQ}, @samp{rR}, @samp{rS}, @samp{rT}, @samp{rU}, |
| 239 | @samp{rV}, @samp{rW}, @samp{rX}, @samp{rY}, @samp{rZ}, @samp{rBB}, |
| 240 | @samp{rTT}, @samp{rWW}, @samp{rXX}, @samp{rYY} and @samp{rZZ}. A leading |
| 241 | @samp{:} is optional for special register names. |
| 242 | |
| 243 | Local and global symbols can be equated to register names and used in |
| 244 | place of ordinary registers. |
| 245 | |
| 246 | Similarly for special registers, local and global symbols can be used. |
| 247 | Also, symbols equated from numbers and constant expressions are allowed in |
| 248 | place of a special register, except when either of the options |
| 249 | @code{--no-predefined-syms} and @code{--fixed-special-register-names} are |
| 250 | specified. Then only the special register names above are allowed for the |
| 251 | instructions having a special register operand; @code{GET} and @code{PUT}. |
| 252 | |
| 253 | @node MMIX-Pseudos |
| 254 | @subsection Assembler Directives |
| 255 | @cindex assembler directives, MMIX |
| 256 | @cindex pseudo-ops, MMIX |
| 257 | @cindex MMIX assembler directives |
| 258 | @cindex MMIX pseudo-ops |
| 259 | |
| 260 | @table @code |
| 261 | @item LOC |
| 262 | @cindex assembler directive LOC, MMIX |
| 263 | @cindex pseudo-op LOC, MMIX |
| 264 | @cindex MMIX assembler directive LOC |
| 265 | @cindex MMIX pseudo-op LOC |
| 266 | |
| 267 | @anchor{MMIX-loc} |
| 268 | The @code{LOC} directive sets the current location to the value of the |
| 269 | operand field, which may include changing sections. If the operand is a |
| 270 | constant, the section is set to either @code{.data} if the value is |
| 271 | @code{0x2000000000000000} or larger, else it is set to @code{.text}. |
| 272 | Within a section, the current location may only be changed to |
| 273 | monotonically higher addresses. A LOC expression must be a previously |
| 274 | defined symbol or a ``pure'' constant. |
| 275 | |
| 276 | An example, which sets the label @var{prev} to the current location, and |
| 277 | updates the current location to eight bytes forward: |
| 278 | @smallexample |
| 279 | prev LOC @@+8 |
| 280 | @end smallexample |
| 281 | |
| 282 | When a LOC has a constant as its operand, a symbol |
| 283 | @code{__.MMIX.start..text} or @code{__.MMIX.start..data} is defined |
| 284 | depending on the address as mentioned above. Each such symbol is |
| 285 | interpreted as special by the linker, locating the section at that |
| 286 | address. Note that if multiple files are linked, the first object file |
| 287 | with that section will be mapped to that address (not necessarily the file |
| 288 | with the LOC definition). |
| 289 | |
| 290 | @item LOCAL |
| 291 | @cindex assembler directive LOCAL, MMIX |
| 292 | @cindex pseudo-op LOCAL, MMIX |
| 293 | @cindex MMIX assembler directive LOCAL |
| 294 | @cindex MMIX pseudo-op LOCAL |
| 295 | |
| 296 | @anchor{MMIX-local} |
| 297 | Example: |
| 298 | @smallexample |
| 299 | LOCAL external_symbol |
| 300 | LOCAL 42 |
| 301 | .local asymbol |
| 302 | @end smallexample |
| 303 | |
| 304 | This directive-operation generates a link-time assertion that the operand |
| 305 | does not correspond to a global register. The operand is an expression |
| 306 | that at link-time resolves to a register symbol or a number. A number is |
| 307 | treated as the register having that number. There is one restriction on |
| 308 | the use of this directive: the pseudo-directive must be placed in a |
| 309 | section with contents, code or data. |
| 310 | |
| 311 | @item IS |
| 312 | @cindex assembler directive IS, MMIX |
| 313 | @cindex pseudo-op IS, MMIX |
| 314 | @cindex MMIX assembler directive IS |
| 315 | @cindex MMIX pseudo-op IS |
| 316 | |
| 317 | @anchor{MMIX-is} |
| 318 | The @code{IS} directive: |
| 319 | @smallexample |
| 320 | asymbol IS an_expression |
| 321 | @end smallexample |
| 322 | sets the symbol @samp{asymbol} to @samp{an_expression}. A symbol may not |
| 323 | be set more than once using this directive. Local labels may be set using |
| 324 | this directive, for example: |
| 325 | @smallexample |
| 326 | 5H IS @@+4 |
| 327 | @end smallexample |
| 328 | |
| 329 | @item GREG |
| 330 | @cindex assembler directive GREG, MMIX |
| 331 | @cindex pseudo-op GREG, MMIX |
| 332 | @cindex MMIX assembler directive GREG |
| 333 | @cindex MMIX pseudo-op GREG |
| 334 | |
| 335 | @anchor{MMIX-greg} |
| 336 | This directive reserves a global register, gives it an initial value and |
| 337 | optionally gives it a symbolic name. Some examples: |
| 338 | |
| 339 | @smallexample |
| 340 | areg GREG |
| 341 | breg GREG data_value |
| 342 | GREG data_buffer |
| 343 | .greg creg, another_data_value |
| 344 | @end smallexample |
| 345 | |
| 346 | The symbolic register name can be used in place of a (non-special) |
| 347 | register. If a value isn't provided, it defaults to zero. Unless the |
| 348 | option @samp{--no-merge-gregs} is specified, non-zero registers allocated |
| 349 | with this directive may be eliminated by @code{@value{AS}}; another |
| 350 | register with the same value used in its place. |
| 351 | Any of the instructions |
| 352 | @samp{CSWAP}, |
| 353 | @samp{GO}, |
| 354 | @samp{LDA}, |
| 355 | @samp{LDBU}, |
| 356 | @samp{LDB}, |
| 357 | @samp{LDHT}, |
| 358 | @samp{LDOU}, |
| 359 | @samp{LDO}, |
| 360 | @samp{LDSF}, |
| 361 | @samp{LDTU}, |
| 362 | @samp{LDT}, |
| 363 | @samp{LDUNC}, |
| 364 | @samp{LDVTS}, |
| 365 | @samp{LDWU}, |
| 366 | @samp{LDW}, |
| 367 | @samp{PREGO}, |
| 368 | @samp{PRELD}, |
| 369 | @samp{PREST}, |
| 370 | @samp{PUSHGO}, |
| 371 | @samp{STBU}, |
| 372 | @samp{STB}, |
| 373 | @samp{STCO}, |
| 374 | @samp{STHT}, |
| 375 | @samp{STOU}, |
| 376 | @samp{STSF}, |
| 377 | @samp{STTU}, |
| 378 | @samp{STT}, |
| 379 | @samp{STUNC}, |
| 380 | @samp{SYNCD}, |
| 381 | @samp{SYNCID}, |
| 382 | can have a value nearby @anchor{GREG-base}an initial value in place of its |
| 383 | second and third operands. Here, ``nearby'' is defined as within the |
| 384 | range 0@dots{}255 from the initial value of such an allocated register. |
| 385 | |
| 386 | @smallexample |
| 387 | buffer1 BYTE 0,0,0,0,0 |
| 388 | buffer2 BYTE 0,0,0,0,0 |
| 389 | @dots{} |
| 390 | GREG buffer1 |
| 391 | LDOU $42,buffer2 |
| 392 | @end smallexample |
| 393 | In the example above, the @samp{Y} field of the @code{LDOUI} instruction |
| 394 | (LDOU with a constant Z) will be replaced with the global register |
| 395 | allocated for @samp{buffer1}, and the @samp{Z} field will have the value |
| 396 | 5, the offset from @samp{buffer1} to @samp{buffer2}. The result is |
| 397 | equivalent to this code: |
| 398 | @smallexample |
| 399 | buffer1 BYTE 0,0,0,0,0 |
| 400 | buffer2 BYTE 0,0,0,0,0 |
| 401 | @dots{} |
| 402 | tmpreg GREG buffer1 |
| 403 | LDOU $42,tmpreg,(buffer2-buffer1) |
| 404 | @end smallexample |
| 405 | |
| 406 | Global registers allocated with this directive are allocated in order |
| 407 | higher-to-lower within a file. Other than that, the exact order of |
| 408 | register allocation and elimination is undefined. For example, the order |
| 409 | is undefined when more than one file with such directives are linked |
| 410 | together. With the options @samp{-x} and @samp{--linker-allocated-gregs}, |
| 411 | @samp{GREG} directives for two-operand cases like the one mentioned above |
| 412 | can be omitted. Sufficient global registers will then be allocated by the |
| 413 | linker. |
| 414 | |
| 415 | @item BYTE |
| 416 | @cindex assembler directive BYTE, MMIX |
| 417 | @cindex pseudo-op BYTE, MMIX |
| 418 | @cindex MMIX assembler directive BYTE |
| 419 | @cindex MMIX pseudo-op BYTE |
| 420 | |
| 421 | @anchor{MMIX-byte} |
| 422 | The @samp{BYTE} directive takes a series of operands separated by a comma. |
| 423 | If an operand is a string (@pxref{Strings}), each character of that string |
| 424 | is emitted as a byte. Other operands must be constant expressions without |
| 425 | forward references, in the range 0@dots{}255. If you need operands having |
| 426 | expressions with forward references, use @samp{.byte} (@pxref{Byte}). An |
| 427 | operand can be omitted, defaulting to a zero value. |
| 428 | |
| 429 | @item WYDE |
| 430 | @itemx TETRA |
| 431 | @itemx OCTA |
| 432 | @cindex assembler directive WYDE, MMIX |
| 433 | @cindex pseudo-op WYDE, MMIX |
| 434 | @cindex MMIX assembler directive WYDE |
| 435 | @cindex MMIX pseudo-op WYDE |
| 436 | @cindex assembler directive TETRA, MMIX |
| 437 | @cindex pseudo-op TETRA, MMIX |
| 438 | @cindex MMIX assembler directive TETRA |
| 439 | @cindex MMIX pseudo-op TETRA |
| 440 | @cindex assembler directive OCTA, MMIX |
| 441 | @cindex pseudo-op OCTA, MMIX |
| 442 | @cindex MMIX assembler directive OCTA |
| 443 | @cindex MMIX pseudo-op OCTA |
| 444 | |
| 445 | @anchor{MMIX-constants} |
| 446 | The directives @samp{WYDE}, @samp{TETRA} and @samp{OCTA} emit constants of |
| 447 | two, four and eight bytes size respectively. Before anything else happens |
| 448 | for the directive, the current location is aligned to the respective |
| 449 | constant-size boundary. If a label is defined at the beginning of the |
| 450 | line, its value will be that after the alignment. A single operand can be |
| 451 | omitted, defaulting to a zero value emitted for the directive. Operands |
| 452 | can be expressed as strings (@pxref{Strings}), in which case each |
| 453 | character in the string is emitted as a separate constant of the size |
| 454 | indicated by the directive. |
| 455 | |
| 456 | @item PREFIX |
| 457 | @cindex assembler directive PREFIX, MMIX |
| 458 | @cindex pseudo-op PREFIX, MMIX |
| 459 | @cindex MMIX assembler directive PREFIX |
| 460 | @cindex MMIX pseudo-op PREFIX |
| 461 | |
| 462 | @anchor{MMIX-prefix} |
| 463 | The @samp{PREFIX} directive sets a symbol name prefix to be prepended to |
| 464 | all symbols (except local symbols, @pxref{MMIX-Symbols}), that are not |
| 465 | prefixed with @samp{:}, until the next @samp{PREFIX} directive. Such |
| 466 | prefixes accumulate. For example, |
| 467 | @smallexample |
| 468 | PREFIX a |
| 469 | PREFIX b |
| 470 | c IS 0 |
| 471 | @end smallexample |
| 472 | defines a symbol @samp{abc} with the value 0. |
| 473 | |
| 474 | @item BSPEC |
| 475 | @itemx ESPEC |
| 476 | @cindex assembler directive BSPEC, MMIX |
| 477 | @cindex pseudo-op BSPEC, MMIX |
| 478 | @cindex MMIX assembler directive BSPEC |
| 479 | @cindex MMIX pseudo-op BSPEC |
| 480 | @cindex assembler directive ESPEC, MMIX |
| 481 | @cindex pseudo-op ESPEC, MMIX |
| 482 | @cindex MMIX assembler directive ESPEC |
| 483 | @cindex MMIX pseudo-op ESPEC |
| 484 | |
| 485 | @anchor{MMIX-spec} |
| 486 | A pair of @samp{BSPEC} and @samp{ESPEC} directives delimit a section of |
| 487 | special contents (without specified semantics). Example: |
| 488 | @smallexample |
| 489 | BSPEC 42 |
| 490 | TETRA 1,2,3 |
| 491 | ESPEC |
| 492 | @end smallexample |
| 493 | The single operand to @samp{BSPEC} must be number in the range |
| 494 | 0@dots{}255. The @samp{BSPEC} number 80 is used by the GNU binutils |
| 495 | implementation. |
| 496 | @end table |
| 497 | |
| 498 | @node MMIX-mmixal |
| 499 | @section Differences to @code{mmixal} |
| 500 | @cindex mmixal differences |
| 501 | @cindex differences, mmixal |
| 502 | |
| 503 | The binutils @code{@value{AS}} and @code{@value{LD}} combination has a few |
| 504 | differences in function compared to @code{mmixal} (@pxref{mmixsite}). |
| 505 | |
| 506 | The replacement of a symbol with a GREG-allocated register |
| 507 | (@pxref{GREG-base}) is not handled the exactly same way in |
| 508 | @code{@value{AS}} as in @code{mmixal}. This is apparent in the |
| 509 | @code{mmixal} example file @code{inout.mms}, where different registers |
| 510 | with different offsets, eventually yielding the same address, are used in |
| 511 | the first instruction. This type of difference should however not affect |
| 512 | the function of any program unless it has specific assumptions about the |
| 513 | allocated register number. |
| 514 | |
| 515 | Line numbers (in the @samp{mmo} object format) are currently not |
| 516 | supported. |
| 517 | |
| 518 | Expression operator precedence is not that of mmixal: operator precedence |
| 519 | is that of the C programming language. It's recommended to use |
| 520 | parentheses to explicitly specify wanted operator precedence whenever more |
| 521 | than one type of operators are used. |
| 522 | |
| 523 | The serialize unary operator @code{&}, the fractional division operator |
| 524 | @samp{//}, the logical not operator @code{!} and the modulus operator |
| 525 | @samp{%} are not available. |
| 526 | |
| 527 | Symbols are not global by default, unless the option |
| 528 | @samp{--globalize-symbols} is passed. Use the @samp{.global} directive to |
| 529 | globalize symbols (@pxref{Global}). |
| 530 | |
| 531 | Operand syntax is a bit stricter with @code{@value{AS}} than |
| 532 | @code{mmixal}. For example, you can't say @code{addu 1,2,3}, instead you |
| 533 | must write @code{addu $1,$2,3}. |
| 534 | |
| 535 | You can't LOC to a lower address than those already visited |
| 536 | (i.e., ``backwards''). |
| 537 | |
| 538 | A LOC directive must come before any emitted code. |
| 539 | |
| 540 | Predefined symbols are visible as file-local symbols after use. (In the |
| 541 | ELF file, that is---the linked mmo file has no notion of a file-local |
| 542 | symbol.) |
| 543 | |
| 544 | Some mapping of constant expressions to sections in LOC expressions is |
| 545 | attempted, but that functionality is easily confused and should be avoided |
| 546 | unless compatibility with @code{mmixal} is required. A LOC expression to |
| 547 | @samp{0x2000000000000000} or higher, maps to the @samp{.data} section and |
| 548 | lower addresses map to the @samp{.text} section (@pxref{MMIX-loc}). |
| 549 | |
| 550 | The code and data areas are each contiguous. Sparse programs with |
| 551 | far-away LOC directives will take up the same amount of space as a |
| 552 | contiguous program with zeros filled in the gaps between the LOC |
| 553 | directives. If you need sparse programs, you might try and get the wanted |
| 554 | effect with a linker script and splitting up the code parts into sections |
| 555 | (@pxref{Section}). Assembly code for this, to be compatible with |
| 556 | @code{mmixal}, would look something like: |
| 557 | @smallexample |
| 558 | .if 0 |
| 559 | LOC away_expression |
| 560 | .else |
| 561 | .section away,"ax" |
| 562 | .fi |
| 563 | @end smallexample |
| 564 | @code{@value{AS}} will not execute the LOC directive and @code{mmixal} |
| 565 | ignores the lines with @code{.}. This construct can be used generally to |
| 566 | help compatibility. |
| 567 | |
| 568 | Symbols can't be defined twice--not even to the same value. |
| 569 | |
| 570 | Instruction mnemonics are recognized case-insensitive, though the |
| 571 | @samp{IS} and @samp{GREG} pseudo-operations must be specified in |
| 572 | upper-case characters. |
| 573 | |
| 574 | There's no unicode support. |
| 575 | |
| 576 | The following is a list of programs in @samp{mmix.tar.gz}, available at |
| 577 | @url{http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html}, last |
| 578 | checked with the version dated 2001-08-25 (md5sum |
| 579 | c393470cfc86fac040487d22d2bf0172) that assemble with @code{mmixal} but do |
| 580 | not assemble with @code{@value{AS}}: |
| 581 | |
| 582 | @table @code |
| 583 | @item silly.mms |
| 584 | LOC to a previous address. |
| 585 | @item sim.mms |
| 586 | Redefines symbol @samp{Done}. |
| 587 | @item test.mms |
| 588 | Uses the serial operator @samp{&}. |
| 589 | @end table |