1 ;// 2 ;// 3 ;// File Name: armCOMM_BitDec_s.h 4 ;// OpenMAX DL: v1.0.2 5 ;// Revision: 9641 6 ;// Date: Thursday, February 7, 2008 7 ;// 8 ;// (c) Copyright 2007-2008 ARM Limited. All Rights Reserved. 9 ;// 10 ;// 11 ;// 12 ;// OpenMAX optimized bitstream decode module 13 ;// 14 ;// You must include armCOMM_s.h before including this file 15 ;// 16 ;// This module provides macros to perform assembly optimized fixed and 17 ;// variable length decoding from a read-only bitstream. The variable 18 ;// length decode modules take as input a pointer to a table of 16-bit 19 ;// entries of the following format. 20 ;// 21 ;// VLD Table Entry format 22 ;// 23 ;// 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00 24 ;// +------------------------------------------------+ 25 ;// | Len | Symbol | 1 | 26 ;// +------------------------------------------------+ 27 ;// | Offset | 0 | 28 ;// +------------------------------------------------+ 29 ;// 30 ;// If the table entry is a leaf entry then bit 0 set: 31 ;// Len = Number of bits overread (0 to 7) 32 ;// Symbol = Symbol payload (unsigned 12 bits) 33 ;// 34 ;// If the table entry is an internal node then bit 0 is clear: 35 ;// Offset = Number of (16-bit) half words from the table 36 ;// start to the next table node 37 ;// 38 ;// The table is accessed by successive lookup up on the 39 ;// next Step bits of the input bitstream until a leaf node 40 ;// is obtained. The Step sizes are supplied to the VLD macro. 41 ;// 42 ;// USAGE: 43 ;// 44 ;// To use any of the macros in this package, first call: 45 ;// 46 ;// M_BD_INIT ppBitStream, pBitOffset, pBitStream, RBitBuffer, RBitCount, Tmp 47 ;// 48 ;// This caches the current bitstream position and next available 49 ;// bits in registers pBitStream, RBitBuffer, RBitCount. These registers 50 ;// are reserved for use by the bitstream decode package until you 51 ;// call M_BD_FINI. 52 ;// 53 ;// Next call the following macro(s) as many times as you need: 54 ;// 55 ;// M_BD_LOOK8 - Look ahead constant 1<=N<=8 bits into the bitstream 56 ;// M_BD_LOOK16 - Look ahead constant 1<=N<=16 bits into the bitstream 57 ;// M_BD_READ8 - Read constant 1<=N<=8 bits from the bitstream 58 ;// M_BD_READ16 - Read constant 1<=N<=16 bits from the bitstream 59 ;// M_BD_VREAD8 - Read variable 1<=N<=8 bits from the bitstream 60 ;// M_BD_VREAD16 - Read variable 1<=N<=16 bits from the bitstream 61 ;// M_BD_VLD - Perform variable length decode using lookup table 62 ;// 63 ;// Finally call the macro: 64 ;// 65 ;// M_BD_FINI ppBitStream, pBitOffset 66 ;// 67 ;// This writes the bitstream state back to memory. 68 ;// 69 ;// The three bitstream cache register names are assigned to the following global 70 ;// variables: 71 ;// 72 73 GBLS pBitStream ;// Register name for pBitStream 74 GBLS BitBuffer ;// Register name for BitBuffer 75 GBLS BitCount ;// Register name for BitCount 76 77 ;// 78 ;// These register variables must have a certain defined state on entry to every bitstream 79 ;// macro (except M_BD_INIT) and on exit from every bitstream macro (except M_BD_FINI). 80 ;// The state may depend on implementation. 81 ;// 82 ;// For the default (ARM11) implementation the following hold: 83 ;// pBitStream - points to the first byte not held in the BitBuffer 84 ;// BitBuffer - is a cache of (4 bytes) 32 bits, bit 31 the first bit 85 ;// BitCount - is offset (from the top bit) to the next unused bitstream bit 86 ;// 0<=BitCount<=15 (so BitBuffer holds at least 17 unused bits) 87 ;// 88 ;// 89 90 ;// Bitstream Decode initialise 91 ;// 92 ;// Initialises the bitstream decode global registers from 93 ;// bitstream pointers. This macro is split into 3 parts to enable 94 ;// scheduling. 95 ;// 96 ;// Input Registers: 97 ;// 98 ;// $ppBitStream - pointer to pointer to the next bitstream byte 99 ;// $pBitOffset - pointer to the number of bits used in the current byte (0..7) 100 ;// $RBitStream - register to use for pBitStream (can be $ppBitStream) 101 ;// $RBitBuffer - register to use for BitBuffer 102 ;// $RBitCount - register to use for BitCount (can be $pBitOffset) 103 ;// 104 ;// Output Registers: 105 ;// 106 ;// $T1,$T2,$T3 - registers that must be preserved between calls to 107 ;// M_BD_INIT1 and M_BD_INIT2 108 ;// $pBitStream \ 109 ;// $BitBuffer } See description above. 110 ;// $BitCount / 111 ;// 112 MACRO 113 M_BD_INIT0 $ppBitStream, $pBitOffset, $RBitStream, $RBitBuffer, $RBitCount 114 115 pBitStream SETS "$RBitStream" 116 BitBuffer SETS "$RBitBuffer" 117 BitCount SETS "$RBitCount" 118 119 ;// load inputs 120 LDR $pBitStream, [$ppBitStream] 121 LDR $BitCount, [$pBitOffset] 122 MEND 123 124 MACRO 125 M_BD_INIT1 $T1, $T2, $T3 126 LDRB $T2, [$pBitStream, #2] 127 LDRB $T1, [$pBitStream, #1] 128 LDRB $BitBuffer, [$pBitStream], #3 129 ADD $BitCount, $BitCount, #8 130 MEND 131 132 MACRO 133 M_BD_INIT2 $T1, $T2, $T3 134 ORR $T2, $T2, $T1, LSL #8 135 ORR $BitBuffer, $T2, $BitBuffer, LSL #16 136 MEND 137 138 ;// 139 ;// Look ahead fixed 1<=N<=8 bits without consuming any bits 140 ;// The next bits will be placed at bit 31..24 of destination register 141 ;// 142 ;// Input Registers: 143 ;// 144 ;// $N - number of bits to look 145 ;// $pBitStream \ 146 ;// $BitBuffer } See description above. 147 ;// $BitCount / 148 ;// 149 ;// Output Registers: 150 ;// 151 ;// $Symbol - the next N bits of the bitstream 152 ;// $T1 - corrupted temp/scratch register 153 ;// $pBitStream \ 154 ;// $BitBuffer } See description above. 155 ;// $BitCount / 156 ;// 157 MACRO 158 M_BD_LOOK8 $Symbol, $N 159 ASSERT ($N>=1):LAND:($N<=8) 160 MOV $Symbol, $BitBuffer, LSL $BitCount 161 MEND 162 163 ;// 164 ;// Look ahead fixed 1<=N<=16 bits without consuming any bits 165 ;// The next bits will be placed at bit 31..16 of destination register 166 ;// 167 ;// Input Registers: 168 ;// 169 ;// $N - number of bits to look 170 ;// $pBitStream \ 171 ;// $BitBuffer } See description above. 172 ;// $BitCount / 173 ;// 174 ;// Output Registers: 175 ;// 176 ;// $Symbol - the next N bits of the bitstream 177 ;// $T1 - corrupted temp/scratch register 178 ;// $pBitStream \ 179 ;// $BitBuffer } See description above. 180 ;// $BitCount / 181 ;// 182 MACRO 183 M_BD_LOOK16 $Symbol, $N, $T1 184 ASSERT ($N >= 1):LAND:($N <= 16) 185 MOV $Symbol, $BitBuffer, LSL $BitCount 186 MEND 187 188 ;// 189 ;// Skips fixed 1<=N<=8 bits from the bitstream, advancing the bitstream pointer 190 ;// 191 ;// Input Registers: 192 ;// 193 ;// $N - number of bits 194 ;// $pBitStream \ 195 ;// $BitBuffer } See description above. 196 ;// $BitCount / 197 ;// 198 ;// Output Registers: 199 ;// 200 ;// $T1 - corrupted temp/scratch register 201 ;// $pBitStream \ 202 ;// $BitBuffer } See description above. 203 ;// $BitCount / 204 ;// 205 MACRO 206 M_BD_SKIP8 $N, $T1 207 ASSERT ($N>=1):LAND:($N<=8) 208 SUBS $BitCount, $BitCount, #(8-$N) 209 LDRCSB $T1, [$pBitStream], #1 210 ADDCC $BitCount, $BitCount, #8 211 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 212 MEND 213 214 215 ;// 216 ;// Read fixed 1<=N<=8 bits from the bitstream, advancing the bitstream pointer 217 ;// 218 ;// Input Registers: 219 ;// 220 ;// $N - number of bits to read 221 ;// $pBitStream \ 222 ;// $BitBuffer } See description above. 223 ;// $BitCount / 224 ;// 225 ;// Output Registers: 226 ;// 227 ;// $Symbol - the next N bits of the bitstream 228 ;// $T1 - corrupted temp/scratch register 229 ;// $pBitStream \ 230 ;// $BitBuffer } See description above. 231 ;// $BitCount / 232 ;// 233 MACRO 234 M_BD_READ8 $Symbol, $N, $T1 235 ASSERT ($N>=1):LAND:($N<=8) 236 MOVS $Symbol, $BitBuffer, LSL $BitCount 237 SUBS $BitCount, $BitCount, #(8-$N) 238 LDRCSB $T1, [$pBitStream], #1 239 ADDCC $BitCount, $BitCount, #8 240 MOV $Symbol, $Symbol, LSR #(32-$N) 241 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 242 MEND 243 244 ;// 245 ;// Read fixed 1<=N<=16 bits from the bitstream, advancing the bitstream pointer 246 ;// 247 ;// Input Registers: 248 ;// 249 ;// $N - number of bits to read 250 ;// $pBitStream \ 251 ;// $BitBuffer } See description above. 252 ;// $BitCount / 253 ;// 254 ;// Output Registers: 255 ;// 256 ;// $Symbol - the next N bits of the bitstream 257 ;// $T1 - corrupted temp/scratch register 258 ;// $T2 - corrupted temp/scratch register 259 ;// $pBitStream \ 260 ;// $BitBuffer } See description above. 261 ;// $BitCount / 262 ;// 263 MACRO 264 M_BD_READ16 $Symbol, $N, $T1, $T2 265 ASSERT ($N>=1):LAND:($N<=16) 266 ASSERT $Symbol<>$T1 267 IF ($N<=8) 268 M_BD_READ8 $Symbol, $N, $T1 269 ELSE 270 ;// N>8 so we will be able to refill at least one byte 271 LDRB $T1, [$pBitStream], #1 272 MOVS $Symbol, $BitBuffer, LSL $BitCount 273 ORR $BitBuffer, $T1, $BitBuffer, LSL #8 274 SUBS $BitCount, $BitCount, #(16-$N) 275 LDRCSB $T1, [$pBitStream], #1 276 MOV $Symbol, $Symbol, LSR #(32-$N) 277 ADDCC $BitCount, $BitCount, #8 278 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 279 ENDIF 280 MEND 281 282 ;// 283 ;// Skip variable 1<=N<=8 bits from the bitstream, advancing the bitstream pointer. 284 ;// 285 ;// Input Registers: 286 ;// 287 ;// $N - number of bits. 1<=N<=8 288 ;// $pBitStream \ 289 ;// $BitBuffer } See description above. 290 ;// $BitCount / 291 ;// 292 ;// Output Registers: 293 ;// 294 ;// $T1 - corrupted temp/scratch register 295 ;// $T2 - corrupted temp/scratch register 296 ;// $pBitStream \ 297 ;// $BitBuffer } See description above. 298 ;// $BitCount / 299 ;// 300 MACRO 301 M_BD_VSKIP8 $N, $T1 302 ADD $BitCount, $BitCount, $N 303 SUBS $BitCount, $BitCount, #8 304 LDRCSB $T1, [$pBitStream], #1 305 ADDCC $BitCount, $BitCount, #8 306 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 307 MEND 308 309 ;// 310 ;// Skip variable 1<=N<=16 bits from the bitstream, advancing the bitstream pointer. 311 ;// 312 ;// Input Registers: 313 ;// 314 ;// $N - number of bits. 1<=N<=16 315 ;// $pBitStream \ 316 ;// $BitBuffer } See description above. 317 ;// $BitCount / 318 ;// 319 ;// Output Registers: 320 ;// 321 ;// $T1 - corrupted temp/scratch register 322 ;// $T2 - corrupted temp/scratch register 323 ;// $pBitStream \ 324 ;// $BitBuffer } See description above. 325 ;// $BitCount / 326 ;// 327 MACRO 328 M_BD_VSKIP16 $N, $T1, $T2 329 ADD $BitCount, $BitCount, $N 330 SUBS $BitCount, $BitCount, #8 331 LDRCSB $T1, [$pBitStream], #1 332 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 333 SUBCSS $BitCount, $BitCount, #8 334 LDRCSB $T1, [$pBitStream], #1 335 ADDCC $BitCount, $BitCount, #8 336 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 337 MEND 338 339 ;// 340 ;// Read variable 1<=N<=8 bits from the bitstream, advancing the bitstream pointer. 341 ;// 342 ;// Input Registers: 343 ;// 344 ;// $N - number of bits to read. 1<=N<=8 345 ;// $pBitStream \ 346 ;// $BitBuffer } See description above. 347 ;// $BitCount / 348 ;// 349 ;// Output Registers: 350 ;// 351 ;// $Symbol - the next N bits of the bitstream 352 ;// $T1 - corrupted temp/scratch register 353 ;// $T2 - corrupted temp/scratch register 354 ;// $pBitStream \ 355 ;// $BitBuffer } See description above. 356 ;// $BitCount / 357 ;// 358 MACRO 359 M_BD_VREAD8 $Symbol, $N, $T1, $T2 360 MOV $Symbol, $BitBuffer, LSL $BitCount 361 ADD $BitCount, $BitCount, $N 362 SUBS $BitCount, $BitCount, #8 363 LDRCSB $T1, [$pBitStream], #1 364 RSB $T2, $N, #32 365 ADDCC $BitCount, $BitCount, #8 366 MOV $Symbol, $Symbol, LSR $T2 367 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 368 MEND 369 370 371 ;// 372 ;// Read variable 1<=N<=16 bits from the bitstream, advancing the bitstream pointer. 373 ;// 374 ;// Input Registers: 375 ;// 376 ;// $N - number of bits to read. 1<=N<=16 377 ;// $pBitStream \ 378 ;// $BitBuffer } See description above. 379 ;// $BitCount / 380 ;// 381 ;// Output Registers: 382 ;// 383 ;// $Symbol - the next N bits of the bitstream 384 ;// $T1 - corrupted temp/scratch register 385 ;// $T2 - corrupted temp/scratch register 386 ;// $pBitStream \ 387 ;// $BitBuffer } See description above. 388 ;// $BitCount / 389 ;// 390 MACRO 391 M_BD_VREAD16 $Symbol, $N, $T1, $T2 392 MOV $Symbol, $BitBuffer, LSL $BitCount 393 ADD $BitCount, $BitCount, $N 394 SUBS $BitCount, $BitCount, #8 395 LDRCSB $T1, [$pBitStream], #1 396 RSB $T2, $N, #32 397 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 398 SUBCSS $BitCount, $BitCount, #8 399 LDRCSB $T1, [$pBitStream], #1 400 ADDCC $BitCount, $BitCount, #8 401 MOV $Symbol, $Symbol, LSR $T2 402 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 403 MEND 404 405 406 ;// 407 ;// Decode a code of the form 0000...001 where there 408 ;// are N zeros before the 1 and N<=15 (code length<=16) 409 ;// 410 ;// Input Registers: 411 ;// 412 ;// $pBitStream \ 413 ;// $BitBuffer } See description above. 414 ;// $BitCount / 415 ;// 416 ;// Output Registers: 417 ;// 418 ;// $Symbol - the number of zeros before the next 1 419 ;// >=16 is an illegal code 420 ;// $T1 - corrupted temp/scratch register 421 ;// $T2 - corrupted temp/scratch register 422 ;// $pBitStream \ 423 ;// $BitBuffer } See description above. 424 ;// $BitCount / 425 ;// 426 MACRO 427 M_BD_CLZ16 $Symbol, $T1, $T2 428 MOVS $Symbol, $BitBuffer, LSL $BitCount 429 CLZ $Symbol, $Symbol 430 ADD $BitCount, $BitCount, $Symbol 431 SUBS $BitCount, $BitCount, #7 ;// length is Symbol+1 432 LDRCSB $T1, [$pBitStream], #1 433 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 434 SUBCSS $BitCount, $BitCount, #8 435 LDRCSB $T1, [$pBitStream], #1 436 ADDCC $BitCount, $BitCount, #8 437 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 438 MEND 439 440 ;// 441 ;// Decode a code of the form 1111...110 where there 442 ;// are N ones before the 0 and N<=15 (code length<=16) 443 ;// 444 ;// Input Registers: 445 ;// 446 ;// $pBitStream \ 447 ;// $BitBuffer } See description above. 448 ;// $BitCount / 449 ;// 450 ;// Output Registers: 451 ;// 452 ;// $Symbol - the number of zeros before the next 1 453 ;// >=16 is an illegal code 454 ;// $T1 - corrupted temp/scratch register 455 ;// $T2 - corrupted temp/scratch register 456 ;// $pBitStream \ 457 ;// $BitBuffer } See description above. 458 ;// $BitCount / 459 ;// 460 MACRO 461 M_BD_CLO16 $Symbol, $T1, $T2 462 MOV $Symbol, $BitBuffer, LSL $BitCount 463 MVN $Symbol, $Symbol 464 CLZ $Symbol, $Symbol 465 ADD $BitCount, $BitCount, $Symbol 466 SUBS $BitCount, $BitCount, #7 ;// length is Symbol+1 467 LDRCSB $T1, [$pBitStream], #1 468 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 469 SUBCSS $BitCount, $BitCount, #8 470 LDRCSB $T1, [$pBitStream], #1 471 ADDCC $BitCount, $BitCount, #8 472 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 473 MEND 474 475 476 ;// 477 ;// Variable Length Decode module 478 ;// 479 ;// Decodes one VLD Symbol from a bitstream and refill the bitstream 480 ;// buffer. 481 ;// 482 ;// Input Registers: 483 ;// 484 ;// $pVLDTable - pointer to VLD decode table of 16-bit entries. 485 ;// The format is described above at the start of 486 ;// this file. 487 ;// $S0 - The number of bits to look up for the first step 488 ;// 1<=$S0<=8 489 ;// $S1 - The number of bits to look up for each subsequent 490 ;// step 1<=$S1<=$S0. 491 ;// 492 ;// $pBitStream \ 493 ;// $BitBuffer } See description above. 494 ;// $BitCount / 495 ;// 496 ;// Output Registers: 497 ;// 498 ;// $Symbol - decoded VLD symbol value 499 ;// $T1 - corrupted temp/scratch register 500 ;// $T2 - corrupted temp/scratch register 501 ;// $pBitStream \ 502 ;// $BitBuffer } See description above. 503 ;// $BitCount / 504 ;// 505 MACRO 506 M_BD_VLD $Symbol, $T1, $T2, $pVLDTable, $S0, $S1 507 ASSERT (1<=$S0):LAND:($S0<=8) 508 ASSERT (1<=$S1):LAND:($S1<=$S0) 509 510 ;// Note 0<=BitCount<=15 on entry and exit 511 512 MOVS $T1, $BitBuffer, LSL $BitCount ;// left align next bits 513 MOVS $Symbol, #(2<<$S0)-2 ;// create mask 514 AND $Symbol, $Symbol, $T1, LSR #(31-$S0) ;// 2*(next $S0 bits) 515 SUBS $BitCount, $BitCount, #8 ;// CS if buffer can be filled 516 01 517 LDRCSB $T1, [$pBitStream], #1 ;// load refill byte 518 LDRH $Symbol, [$pVLDTable, $Symbol] ;// load table entry 519 ADDCC $BitCount, $BitCount, #8 ;// refill not possible 520 ADD $BitCount, $BitCount, #$S0 ;// assume $S0 bits used 521 ORRCS $BitBuffer, $T1, $BitBuffer, LSL #8 ;// merge in refill byte 522 MOVS $T1, $Symbol, LSR #1 ;// CS=leaf entry 523 BCS %FT02 524 525 MOVS $T1, $BitBuffer, LSL $BitCount ;// left align next bit 526 IF (2*$S0-$S1<=8) 527 ;// Can combine refill check and -S0+S1 and keep $BitCount<=15 528 SUBS $BitCount, $BitCount, #8+($S0-$S1) 529 ELSE 530 ;// Separate refill check and -S0+S1 offset 531 SUBS $BitCount, $BitCount, #8 532 SUB $BitCount, $BitCount, #($S0-$S1) 533 ENDIF 534 ADD $Symbol, $Symbol, $T1, LSR #(31-$S1) ;// add 2*(next $S1 bits) to 535 BIC $Symbol, $Symbol, #1 ;// table offset 536 B %BT01 ;// load next table entry 537 02 538 ;// BitCount range now depend on the route here 539 ;// if (first step) S0 <= BitCount <= 7+S0 <=15 540 ;// else if (2*S0-S1<=8) S0 <= BitCount <= 7+(2*S0-S1) <=15 541 ;// else S1 <= BitCount <= 7+S1 <=15 542 543 SUB $BitCount, $BitCount, $Symbol, LSR#13 544 BIC $Symbol, $T1, #0xF000 545 MEND 546 547 548 ;// Add an offset number of bits 549 ;// 550 ;// Outputs destination byte and bit index values which corresponds to an offset number of bits 551 ;// from the current location. This is used to compare bitstream positions using. M_BD_CMP. 552 ;// 553 ;// Input Registers: 554 ;// 555 ;// $Offset - Offset to be added in bits. 556 ;// $pBitStream \ 557 ;// $BitBuffer } See description above. 558 ;// $BitCount / 559 ;// 560 ;// Output Registers: 561 ;// 562 ;// $ByteIndex - Destination pBitStream pointer after adding the Offset. 563 ;// This value will be 4 byte ahead and needs to subtract by 4 to get exact 564 ;// pointer (as in M_BD_FINI). But for using with M_BD_CMP subtract is not needed. 565 ;// $BitIndex - Destination BitCount after the addition of Offset number of bits 566 ;// 567 MACRO 568 M_BD_ADD $ByteIndex, $BitIndex, $Offset 569 570 ;// ($ByteIndex,$BitIndex) = Current position + $Offset bits 571 ADD $Offset, $Offset, $BitCount 572 AND $BitIndex, $Offset, #7 573 ADD $ByteIndex, $pBitStream, $Offset, ASR #3 574 MEND 575 576 ;// Move bitstream pointers to the location given 577 ;// 578 ;// Outputs destination byte and bit index values which corresponds to 579 ;// the current location given (calculated using M_BD_ADD). 580 ;// 581 ;// Input Registers: 582 ;// 583 ;// $pBitStream \ 584 ;// $BitBuffer } See description above. 585 ;// $BitCount / 586 ;// $ByteIndex - Destination pBitStream pointer after move. 587 ;// This value will be 4 byte ahead and needs to subtract by 4 to get exact 588 ;// pointer (as in M_BD_FINI). 589 ;// $BitIndex - Destination BitCount after the move 590 ;// 591 ;// Output Registers: 592 ;// 593 ;// $pBitStream \ 594 ;// } See description above. 595 ;// $BitCount / 596 ;// 597 MACRO 598 M_BD_MOV $ByteIndex, $BitIndex 599 600 ;// ($pBitStream, $Offset) = ($ByteIndex,$BitIndex) 601 MOV $BitCount, $BitIndex 602 MOV $pBitStream, $ByteIndex 603 MEND 604 605 ;// Bitstream Compare 606 ;// 607 ;// Compares bitstream position with that of a destination position. Destination position 608 ;// is held in two input registers which are calculated using M_BD_ADD macro 609 ;// 610 ;// Input Registers: 611 ;// 612 ;// $ByteIndex - Destination pBitStream pointer, (4 byte ahead as described in M_BD_ADD) 613 ;// $BitIndex - Destination BitCount 614 ;// $pBitStream \ 615 ;// $BitBuffer } See description above. 616 ;// $BitCount / 617 ;// 618 ;// Output Registers: 619 ;// 620 ;// FLAGS - GE if destination is reached, LT = is destination is ahead 621 ;// $T1 - corrupted temp/scratch register 622 ;// 623 MACRO 624 M_BD_CMP $ByteIndex, $BitIndex, $T1 625 626 ;// Return flags set by (current positon)-($ByteIndex,$BitIndex) 627 ;// so GE means that we have reached the indicated position 628 629 ADD $T1, $pBitStream, $BitCount, LSR #3 630 CMP $T1, $ByteIndex 631 AND $T1, $BitCount, #7 632 CMPEQ $T1, $BitIndex 633 MEND 634 635 636 ;// Bitstream Decode finalise 637 ;// 638 ;// Writes back the bitstream state to the bitstream pointers 639 ;// 640 ;// Input Registers: 641 ;// 642 ;// $pBitStream \ 643 ;// $BitBuffer } See description above. 644 ;// $BitCount / 645 ;// 646 ;// Output Registers: 647 ;// 648 ;// $ppBitStream - pointer to pointer to the next bitstream byte 649 ;// $pBitOffset - pointer to the number of bits used in the current byte (0..7) 650 ;// $pBitStream \ 651 ;// $BitBuffer } these register are corrupted 652 ;// $BitCount / 653 ;// 654 MACRO 655 M_BD_FINI $ppBitStream, $pBitOffset 656 657 ;// Advance pointer by the number of free bits in the buffer 658 ADD $pBitStream, $pBitStream, $BitCount, LSR#3 659 AND $BitCount, $BitCount, #7 660 661 ;// Now move back 32 bits to reach the first usued bit 662 SUB $pBitStream, $pBitStream, #4 663 664 ;// Store out bitstream state 665 STR $BitCount, [$pBitOffset] 666 STR $pBitStream, [$ppBitStream] 667 MEND 668 669 END 670
