root/arch/nios2/kernel/insnemu.S

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   1 /* SPDX-License-Identifier: GPL-2.0-or-later */
   2 /*
   3  *  Copyright (C) 2003-2013 Altera Corporation
   4  *  All rights reserved.
   5  */
   6 
   7 
   8 #include <linux/linkage.h>
   9 #include <asm/entry.h>
  10 
  11 .set noat
  12 .set nobreak
  13 
  14 /*
  15 * Explicitly allow the use of r1 (the assembler temporary register)
  16 * within this code. This register is normally reserved for the use of
  17 * the compiler.
  18 */
  19 
  20 ENTRY(instruction_trap)
  21         ldw     r1, PT_R1(sp)           // Restore registers
  22         ldw     r2, PT_R2(sp)
  23         ldw     r3, PT_R3(sp)
  24         ldw     r4, PT_R4(sp)
  25         ldw     r5, PT_R5(sp)
  26         ldw     r6, PT_R6(sp)
  27         ldw     r7, PT_R7(sp)
  28         ldw     r8, PT_R8(sp)
  29         ldw     r9, PT_R9(sp)
  30         ldw     r10, PT_R10(sp)
  31         ldw     r11, PT_R11(sp)
  32         ldw     r12, PT_R12(sp)
  33         ldw     r13, PT_R13(sp)
  34         ldw     r14, PT_R14(sp)
  35         ldw     r15, PT_R15(sp)
  36         ldw     ra, PT_RA(sp)
  37         ldw     fp, PT_FP(sp)
  38         ldw     gp, PT_GP(sp)
  39         ldw     et, PT_ESTATUS(sp)
  40         wrctl   estatus, et
  41         ldw     ea, PT_EA(sp)
  42         ldw     et, PT_SP(sp)           /* backup sp in et */
  43 
  44         addi    sp, sp, PT_REGS_SIZE
  45 
  46         /* INSTRUCTION EMULATION
  47         *  ---------------------
  48         *
  49         * Nios II processors generate exceptions for unimplemented instructions.
  50         * The routines below emulate these instructions.  Depending on the
  51         * processor core, the only instructions that might need to be emulated
  52         * are div, divu, mul, muli, mulxss, mulxsu, and mulxuu.
  53         *
  54         * The emulations match the instructions, except for the following
  55         * limitations:
  56         *
  57         * 1) The emulation routines do not emulate the use of the exception
  58         *    temporary register (et) as a source operand because the exception
  59         *    handler already has modified it.
  60         *
  61         * 2) The routines do not emulate the use of the stack pointer (sp) or
  62         *    the exception return address register (ea) as a destination because
  63         *    modifying these registers crashes the exception handler or the
  64         *    interrupted routine.
  65         *
  66         * Detailed Design
  67         * ---------------
  68         *
  69         * The emulation routines expect the contents of integer registers r0-r31
  70         * to be on the stack at addresses sp, 4(sp), 8(sp), ... 124(sp).  The
  71         * routines retrieve source operands from the stack and modify the
  72         * destination register's value on the stack prior to the end of the
  73         * exception handler.  Then all registers except the destination register
  74         * are restored to their previous values.
  75         *
  76         * The instruction that causes the exception is found at address -4(ea).
  77         * The instruction's OP and OPX fields identify the operation to be
  78         * performed.
  79         *
  80         * One instruction, muli, is an I-type instruction that is identified by
  81         * an OP field of 0x24.
  82         *
  83         * muli   AAAAA,BBBBB,IIIIIIIIIIIIIIII,-0x24-
  84         *           27    22                6      0    <-- LSB of field
  85         *
  86         * The remaining emulated instructions are R-type and have an OP field
  87         * of 0x3a.  Their OPX fields identify them.
  88         *
  89         * R-type AAAAA,BBBBB,CCCCC,XXXXXX,NNNNN,-0x3a-
  90         *           27    22    17     11     6      0  <-- LSB of field
  91         *
  92         *
  93         * Opcode Encoding.  muli is identified by its OP value.  Then OPX & 0x02
  94         * is used to differentiate between the division opcodes and the
  95         * remaining multiplication opcodes.
  96         *
  97         * Instruction   OP      OPX    OPX & 0x02
  98         * -----------   ----    ----   ----------
  99         * muli          0x24
 100         * divu          0x3a    0x24         0
 101         * div           0x3a    0x25         0
 102         * mul           0x3a    0x27      != 0
 103         * mulxuu        0x3a    0x07      != 0
 104         * mulxsu        0x3a    0x17      != 0
 105         * mulxss        0x3a    0x1f      != 0
 106         */
 107 
 108 
 109         /*
 110         * Save everything on the stack to make it easy for the emulation
 111         * routines to retrieve the source register operands.
 112         */
 113 
 114         addi sp, sp, -128
 115         stw zero, 0(sp) /* Save zero on stack to avoid special case for r0. */
 116         stw r1, 4(sp)
 117         stw r2,  8(sp)
 118         stw r3, 12(sp)
 119         stw r4, 16(sp)
 120         stw r5, 20(sp)
 121         stw r6, 24(sp)
 122         stw r7, 28(sp)
 123         stw r8, 32(sp)
 124         stw r9, 36(sp)
 125         stw r10, 40(sp)
 126         stw r11, 44(sp)
 127         stw r12, 48(sp)
 128         stw r13, 52(sp)
 129         stw r14, 56(sp)
 130         stw r15, 60(sp)
 131         stw r16, 64(sp)
 132         stw r17, 68(sp)
 133         stw r18, 72(sp)
 134         stw r19, 76(sp)
 135         stw r20, 80(sp)
 136         stw r21, 84(sp)
 137         stw r22, 88(sp)
 138         stw r23, 92(sp)
 139                 /* Don't bother to save et.  It's already been changed. */
 140         rdctl r5, estatus
 141         stw r5,  100(sp)
 142 
 143         stw gp, 104(sp)
 144         stw et, 108(sp) /* et contains previous sp value. */
 145         stw fp, 112(sp)
 146         stw ea, 116(sp)
 147         stw ra, 120(sp)
 148 
 149 
 150         /*
 151         * Split the instruction into its fields.  We need 4*A, 4*B, and 4*C as
 152         * offsets to the stack pointer for access to the stored register values.
 153         */
 154         ldw r2,-4(ea)   /* r2 = AAAAA,BBBBB,IIIIIIIIIIIIIIII,PPPPPP */
 155         roli r3, r2, 7  /* r3 = BBB,IIIIIIIIIIIIIIII,PPPPPP,AAAAA,BB */
 156         roli r4, r3, 3  /* r4 = IIIIIIIIIIIIIIII,PPPPPP,AAAAA,BBBBB */
 157         roli r5, r4, 2  /* r5 = IIIIIIIIIIIIII,PPPPPP,AAAAA,BBBBB,II */
 158         srai r4, r4, 16 /* r4 = (sign-extended) IMM16 */
 159         roli r6, r5, 5  /* r6 = XXXX,NNNNN,PPPPPP,AAAAA,BBBBB,CCCCC,XX */
 160         andi r2, r2, 0x3f       /* r2 = 00000000000000000000000000,PPPPPP */
 161         andi r3, r3, 0x7c       /* r3 = 0000000000000000000000000,AAAAA,00 */
 162         andi r5, r5, 0x7c       /* r5 = 0000000000000000000000000,BBBBB,00 */
 163         andi r6, r6, 0x7c       /* r6 = 0000000000000000000000000,CCCCC,00 */
 164 
 165         /* Now
 166         * r2 = OP
 167         * r3 = 4*A
 168         * r4 = IMM16 (sign extended)
 169         * r5 = 4*B
 170         * r6 = 4*C
 171         */
 172 
 173         /*
 174         * Get the operands.
 175         *
 176         * It is necessary to check for muli because it uses an I-type
 177         * instruction format, while the other instructions are have an R-type
 178         * format.
 179         *
 180         *  Prepare for either multiplication or division loop.
 181         *  They both loop 32 times.
 182         */
 183         movi r14, 32
 184 
 185         add  r3, r3, sp         /* r3 = address of A-operand. */
 186         ldw  r3, 0(r3)          /* r3 = A-operand. */
 187         movi r7, 0x24           /* muli opcode (I-type instruction format) */
 188         beq r2, r7, mul_immed /* muli doesn't use the B register as a source */
 189 
 190         add  r5, r5, sp         /* r5 = address of B-operand. */
 191         ldw  r5, 0(r5)          /* r5 = B-operand. */
 192                                 /* r4 = SSSSSSSSSSSSSSSS,-----IMM16------ */
 193                                 /* IMM16 not needed, align OPX portion */
 194                                 /* r4 = SSSSSSSSSSSSSSSS,CCCCC,-OPX--,00000 */
 195         srli r4, r4, 5          /* r4 = 00000,SSSSSSSSSSSSSSSS,CCCCC,-OPX-- */
 196         andi r4, r4, 0x3f       /* r4 = 00000000000000000000000000,-OPX-- */
 197 
 198         /* Now
 199         * r2 = OP
 200         * r3 = src1
 201         * r5 = src2
 202         * r4 = OPX (no longer can be muli)
 203         * r6 = 4*C
 204         */
 205 
 206 
 207         /*
 208         *  Multiply or Divide?
 209         */
 210         andi r7, r4, 0x02       /* For R-type multiply instructions,
 211                                    OPX & 0x02 != 0 */
 212         bne r7, zero, multiply
 213 
 214 
 215         /* DIVISION
 216         *
 217         * Divide an unsigned dividend by an unsigned divisor using
 218         * a shift-and-subtract algorithm.  The example below shows
 219         * 43 div 7 = 6 for 8-bit integers.  This classic algorithm uses a
 220         * single register to store both the dividend and the quotient,
 221         * allowing both values to be shifted with a single instruction.
 222         *
 223         *                               remainder dividend:quotient
 224         *                               --------- -----------------
 225         *   initialize                   00000000     00101011:
 226         *   shift                        00000000     0101011:_
 227         *   remainder >= divisor? no     00000000     0101011:0
 228         *   shift                        00000000     101011:0_
 229         *   remainder >= divisor? no     00000000     101011:00
 230         *   shift                        00000001     01011:00_
 231         *   remainder >= divisor? no     00000001     01011:000
 232         *   shift                        00000010     1011:000_
 233         *   remainder >= divisor? no     00000010     1011:0000
 234         *   shift                        00000101     011:0000_
 235         *   remainder >= divisor? no     00000101     011:00000
 236         *   shift                        00001010     11:00000_
 237         *   remainder >= divisor? yes    00001010     11:000001
 238         *       remainder -= divisor   - 00000111
 239         *                              ----------
 240         *                                00000011     11:000001
 241         *   shift                        00000111     1:000001_
 242         *   remainder >= divisor? yes    00000111     1:0000011
 243         *       remainder -= divisor   - 00000111
 244         *                              ----------
 245         *                                00000000     1:0000011
 246         *   shift                        00000001     :0000011_
 247         *   remainder >= divisor? no     00000001     :00000110
 248         *
 249         * The quotient is 00000110.
 250         */
 251 
 252 divide:
 253         /*
 254         *  Prepare for division by assuming the result
 255         *  is unsigned, and storing its "sign" as 0.
 256         */
 257         movi r17, 0
 258 
 259 
 260         /* Which division opcode? */
 261         xori r7, r4, 0x25               /* OPX of div */
 262         bne r7, zero, unsigned_division
 263 
 264 
 265         /*
 266         *  OPX is div.  Determine and store the sign of the quotient.
 267         *  Then take the absolute value of both operands.
 268         */
 269         xor r17, r3, r5         /* MSB contains sign of quotient */
 270         bge r3,zero,dividend_is_nonnegative
 271         sub r3, zero, r3        /* -r3 */
 272 dividend_is_nonnegative:
 273         bge r5, zero, divisor_is_nonnegative
 274         sub r5, zero, r5        /* -r5 */
 275 divisor_is_nonnegative:
 276 
 277 
 278 unsigned_division:
 279         /* Initialize the unsigned-division loop. */
 280         movi r13, 0     /* remainder = 0 */
 281 
 282         /* Now
 283         * r3 = dividend : quotient
 284         * r4 = 0x25 for div, 0x24 for divu
 285         * r5 = divisor
 286         * r13 = remainder
 287         * r14 = loop counter (already initialized to 32)
 288         * r17 = MSB contains sign of quotient
 289         */
 290 
 291 
 292         /*
 293         *   for (count = 32; count > 0; --count)
 294         *   {
 295         */
 296 divide_loop:
 297 
 298         /*
 299         *       Division:
 300         *
 301         *       (remainder:dividend:quotient) <<= 1;
 302         */
 303         slli r13, r13, 1
 304         cmplt r7, r3, zero      /* r7 = MSB of r3 */
 305         or r13, r13, r7
 306         slli r3, r3, 1
 307 
 308 
 309         /*
 310         *       if (remainder >= divisor)
 311         *       {
 312         *           set LSB of quotient
 313         *           remainder -= divisor;
 314         *       }
 315         */
 316         bltu r13, r5, div_skip
 317         ori r3, r3, 1
 318         sub r13, r13, r5
 319 div_skip:
 320 
 321         /*
 322         *   }
 323         */
 324         subi r14, r14, 1
 325         bne r14, zero, divide_loop
 326 
 327 
 328         /* Now
 329         * r3 = quotient
 330         * r4 = 0x25 for div, 0x24 for divu
 331         * r6 = 4*C
 332         * r17 = MSB contains sign of quotient
 333         */
 334 
 335 
 336         /*
 337         *  Conditionally negate signed quotient.  If quotient is unsigned,
 338         *  the sign already is initialized to 0.
 339         */
 340         bge r17, zero, quotient_is_nonnegative
 341         sub r3, zero, r3                /* -r3 */
 342         quotient_is_nonnegative:
 343 
 344 
 345         /*
 346         *  Final quotient is in r3.
 347         */
 348         add r6, r6, sp
 349         stw r3, 0(r6)   /* write quotient to stack */
 350         br restore_registers
 351 
 352 
 353 
 354 
 355         /* MULTIPLICATION
 356         *
 357         * A "product" is the number that one gets by summing a "multiplicand"
 358         * several times.  The "multiplier" specifies the number of copies of the
 359         * multiplicand that are summed.
 360         *
 361         * Actual multiplication algorithms don't use repeated addition, however.
 362         * Shift-and-add algorithms get the same answer as repeated addition, and
 363         * they are faster.  To compute the lower half of a product (pppp below)
 364         * one shifts the product left before adding in each of the partial
 365         * products (a * mmmm) through (d * mmmm).
 366         *
 367         * To compute the upper half of a product (PPPP below), one adds in the
 368         * partial products (d * mmmm) through (a * mmmm), each time following
 369         * the add by a right shift of the product.
 370         *
 371         *     mmmm
 372         *   * abcd
 373         *   ------
 374         *     ####  = d * mmmm
 375         *    ####   = c * mmmm
 376         *   ####    = b * mmmm
 377         *  ####     = a * mmmm
 378         * --------
 379         * PPPPpppp
 380         *
 381         * The example above shows 4 partial products.  Computing actual Nios II
 382         * products requires 32 partials.
 383         *
 384         * It is possible to compute the result of mulxsu from the result of
 385         * mulxuu because the only difference between the results of these two
 386         * opcodes is the value of the partial product associated with the sign
 387         * bit of rA.
 388         *
 389         *   mulxsu = mulxuu - (rA < 0) ? rB : 0;
 390         *
 391         * It is possible to compute the result of mulxss from the result of
 392         * mulxsu because the only difference between the results of these two
 393         * opcodes is the value of the partial product associated with the sign
 394         * bit of rB.
 395         *
 396         *   mulxss = mulxsu - (rB < 0) ? rA : 0;
 397         *
 398         */
 399 
 400 mul_immed:
 401         /* Opcode is muli.  Change it into mul for remainder of algorithm. */
 402         mov r6, r5              /* Field B is dest register, not field C. */
 403         mov r5, r4              /* Field IMM16 is src2, not field B. */
 404         movi r4, 0x27           /* OPX of mul is 0x27 */
 405 
 406 multiply:
 407         /* Initialize the multiplication loop. */
 408         movi r9, 0      /* mul_product    = 0 */
 409         movi r10, 0     /* mulxuu_product = 0 */
 410         mov r11, r5     /* save original multiplier for mulxsu and mulxss */
 411         mov r12, r5     /* mulxuu_multiplier (will be shifted) */
 412         movi r16, 1     /* used to create "rori B,A,1" from "ror B,A,r16" */
 413 
 414         /* Now
 415         * r3 = multiplicand
 416         * r5 = mul_multiplier
 417         * r6 = 4 * dest_register (used later as offset to sp)
 418         * r7 = temp
 419         * r9 = mul_product
 420         * r10 = mulxuu_product
 421         * r11 = original multiplier
 422         * r12 = mulxuu_multiplier
 423         * r14 = loop counter (already initialized)
 424         * r16 = 1
 425         */
 426 
 427 
 428         /*
 429         *   for (count = 32; count > 0; --count)
 430         *   {
 431         */
 432 multiply_loop:
 433 
 434         /*
 435         *       mul_product <<= 1;
 436         *       lsb = multiplier & 1;
 437         */
 438         slli r9, r9, 1
 439         andi r7, r12, 1
 440 
 441         /*
 442         *       if (lsb == 1)
 443         *       {
 444         *           mulxuu_product += multiplicand;
 445         *       }
 446         */
 447         beq r7, zero, mulx_skip
 448         add r10, r10, r3
 449         cmpltu r7, r10, r3 /* Save the carry from the MSB of mulxuu_product. */
 450         ror r7, r7, r16 /* r7 = 0x80000000 on carry, or else 0x00000000 */
 451 mulx_skip:
 452 
 453         /*
 454         *       if (MSB of mul_multiplier == 1)
 455         *       {
 456         *           mul_product += multiplicand;
 457         *       }
 458         */
 459         bge r5, zero, mul_skip
 460         add r9, r9, r3
 461 mul_skip:
 462 
 463         /*
 464         *       mulxuu_product >>= 1;           logical shift
 465         *       mul_multiplier <<= 1;           done with MSB
 466         *       mulx_multiplier >>= 1;          done with LSB
 467         */
 468         srli r10, r10, 1
 469         or r10, r10, r7         /* OR in the saved carry bit. */
 470         slli r5, r5, 1
 471         srli r12, r12, 1
 472 
 473 
 474         /*
 475         *   }
 476         */
 477         subi r14, r14, 1
 478         bne r14, zero, multiply_loop
 479 
 480 
 481         /*
 482         *  Multiply emulation loop done.
 483         */
 484 
 485         /* Now
 486         * r3 = multiplicand
 487         * r4 = OPX
 488         * r6 = 4 * dest_register (used later as offset to sp)
 489         * r7 = temp
 490         * r9 = mul_product
 491         * r10 = mulxuu_product
 492         * r11 = original multiplier
 493         */
 494 
 495 
 496         /* Calculate address for result from 4 * dest_register */
 497         add r6, r6, sp
 498 
 499 
 500         /*
 501         * Select/compute the result based on OPX.
 502         */
 503 
 504 
 505         /* OPX == mul?  Then store. */
 506         xori r7, r4, 0x27
 507         beq r7, zero, store_product
 508 
 509         /* It's one of the mulx.. opcodes.  Move over the result. */
 510         mov r9, r10
 511 
 512         /* OPX == mulxuu?  Then store. */
 513         xori r7, r4, 0x07
 514         beq r7, zero, store_product
 515 
 516         /* Compute mulxsu
 517          *
 518          * mulxsu = mulxuu - (rA < 0) ? rB : 0;
 519          */
 520         bge r3, zero, mulxsu_skip
 521         sub r9, r9, r11
 522 mulxsu_skip:
 523 
 524         /* OPX == mulxsu?  Then store. */
 525         xori r7, r4, 0x17
 526         beq r7, zero, store_product
 527 
 528         /* Compute mulxss
 529          *
 530          * mulxss = mulxsu - (rB < 0) ? rA : 0;
 531          */
 532         bge r11,zero,mulxss_skip
 533         sub r9, r9, r3
 534 mulxss_skip:
 535         /* At this point, assume that OPX is mulxss, so store*/
 536 
 537 
 538 store_product:
 539         stw r9, 0(r6)
 540 
 541 
 542 restore_registers:
 543                         /* No need to restore r0. */
 544         ldw r5, 100(sp)
 545         wrctl estatus, r5
 546 
 547         ldw r1, 4(sp)
 548         ldw r2, 8(sp)
 549         ldw r3, 12(sp)
 550         ldw r4, 16(sp)
 551         ldw r5, 20(sp)
 552         ldw r6, 24(sp)
 553         ldw r7, 28(sp)
 554         ldw r8, 32(sp)
 555         ldw r9, 36(sp)
 556         ldw r10, 40(sp)
 557         ldw r11, 44(sp)
 558         ldw r12, 48(sp)
 559         ldw r13, 52(sp)
 560         ldw r14, 56(sp)
 561         ldw r15, 60(sp)
 562         ldw r16, 64(sp)
 563         ldw r17, 68(sp)
 564         ldw r18, 72(sp)
 565         ldw r19, 76(sp)
 566         ldw r20, 80(sp)
 567         ldw r21, 84(sp)
 568         ldw r22, 88(sp)
 569         ldw r23, 92(sp)
 570                         /* Does not need to restore et */
 571         ldw gp, 104(sp)
 572 
 573         ldw fp, 112(sp)
 574         ldw ea, 116(sp)
 575         ldw ra, 120(sp)
 576         ldw sp, 108(sp) /* last restore sp */
 577         eret
 578 
 579 .set at
 580 .set break

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