Add a test for incorrect handing of offsets with prefixes

[nasm.git] / assemble.c

/* assemble.c   code generation for the Netwide Assembler
 *
 * The Netwide Assembler is copyright (C) 1996 Simon Tatham and
 * Julian Hall. All rights reserved. The software is
 * redistributable under the licence given in the file "Licence"
 * distributed in the NASM archive.
 *
 * the actual codes (C syntax, i.e. octal):
 * \0            - terminates the code. (Unless it's a literal of course.)
 * \1, \2, \3    - that many literal bytes follow in the code stream
 * \4, \6        - the POP/PUSH (respectively) codes for CS, DS, ES, SS
 *                 (POP is never used for CS) depending on operand 0
 * \5, \7        - the second byte of POP/PUSH codes for FS, GS, depending
 *                 on operand 0
 * \10, \11, \12 - a literal byte follows in the code stream, to be added
 *                 to the register value of operand 0, 1 or 2
 * \17           - encodes the literal byte 0. (Some compilers don't take
 *                 kindly to a zero byte in the _middle_ of a compile time
 *                 string constant, so I had to put this hack in.)
 * \14, \15, \16 - a signed byte immediate operand, from operand 0, 1 or 2
 * \20, \21, \22 - a byte immediate operand, from operand 0, 1 or 2
 * \24, \25, \26 - an unsigned byte immediate operand, from operand 0, 1 or 2
 * \30, \31, \32 - a word immediate operand, from operand 0, 1 or 2
 * \34, \35, \36 - select between \3[012] and \4[012] depending on 16/32 bit
 *                 assembly mode or the operand-size override on the operand
 * \37           - a word constant, from the _segment_ part of operand 0
 * \40, \41, \42 - a long immediate operand, from operand 0, 1 or 2
 * \44, \45, \46 - select between \3[012] and \4[012] depending on 16/32 bit
 *                 assembly mode or the address-size override on the operand
 * \50, \51, \52 - a byte relative operand, from operand 0, 1 or 2
 * \60, \61, \62 - a word relative operand, from operand 0, 1 or 2
 * \64, \65, \66 - select between \6[012] and \7[012] depending on 16/32 bit
 *                 assembly mode or the operand-size override on the operand
 * \70, \71, \72 - a long relative operand, from operand 0, 1 or 2
 * \1ab          - a ModRM, calculated on EA in operand a, with the spare
 *                 field the register value of operand b.
 * \130,\131,\132 - an immediate word or signed byte for operand 0, 1, or 2
 * \133,\134,\135 - or 2 (s-field) into next opcode byte if operand 0, 1, or 2
 *                  is a signed byte rather than a word.
 * \140,\141,\142 - an immediate dword or signed byte for operand 0, 1, or 2
 * \143,\144,\145 - or 2 (s-field) into next opcode byte if operand 0, 1, or 2
 *                  is a signed byte rather than a dword.
 * \2ab          - a ModRM, calculated on EA in operand a, with the spare
 *                 field equal to digit b.
 * \30x          - might be an 0x67 byte, depending on the address size of
 *                 the memory reference in operand x.
 * \310          - indicates fixed 16-bit address size, i.e. optional 0x67.
 * \311          - indicates fixed 32-bit address size, i.e. optional 0x67.
 * \312          - (disassembler only) marker on LOOP, LOOPxx instructions.
 * \320          - indicates fixed 16-bit operand size, i.e. optional 0x66.
 * \321          - indicates fixed 32-bit operand size, i.e. optional 0x66.
 * \322          - indicates that this instruction is only valid when the
 *                 operand size is the default (instruction to disassembler,
 *                 generates no code in the assembler)
 * \330          - a literal byte follows in the code stream, to be added
 *                 to the condition code value of the instruction.
 * \331          - instruction not valid with REP prefix.  Hint for
 *                 disassembler only; for SSE instructions.
 * \332          - disassemble a rep (0xF3 byte) prefix as repe not rep.
 * \333          - REP prefix (0xF3 byte); for SSE instructions.  Not encoded
 *                 as a literal byte in order to aid the disassembler.
 * \340          - reserve <operand 0> bytes of uninitialised storage.
 *                 Operand 0 had better be a segmentless constant.
 * \370,\371,\372 - match only if operand 0 meets byte jump criteria.
 *                 370 is used for Jcc, 371 is used for JMP.
 * \373          - assemble 0x03 if bits==16, 0x05 if bits==32;
 *                 used for conditional jump over longer jump
 */

#include <stdio.h>
#include <string.h>

#include "nasm.h"
#include "nasmlib.h"
#include "assemble.h"
#include "insns.h"

extern struct itemplate *nasm_instructions[];

typedef struct {
    int sib_present;                   /* is a SIB byte necessary? */
    int bytes;                         /* # of bytes of offset needed */
    int size;                          /* lazy - this is sib+bytes+1 */
    unsigned char modrm, sib;          /* the bytes themselves */
} ea;

static unsigned long cpu;               /* cpu level received from nasm.c */
static efunc errfunc;
static struct ofmt *outfmt;
static ListGen *list;

static long calcsize (long, long, int, insn *, const char *);
static void gencode (long, long, int, insn *, const char *, long);
static int  regval (operand *o);
static int  matches (struct itemplate *, insn *);
static ea * process_ea (operand *, ea *, int, int, int);
static int  chsize (operand *, int);

/*
 * This routine wrappers the real output format's output routine,
 * in order to pass a copy of the data off to the listing file
 * generator at the same time.
 */
static void out (long offset, long segto, const void *data, unsigned long type,
                 long segment, long wrt) 
{
    long lineno;
    char *lnfname = NULL;

    if ((type & OUT_TYPMASK) == OUT_ADDRESS) {
        if (segment != NO_SEG || wrt != NO_SEG) {
            /*
             * This address is relocated. We must write it as
             * OUT_ADDRESS, so there's no work to be done here.
             */
            list->output (offset, data, type);
        } 
        else {
            unsigned char p[4], *q = p;
            /*
             * This is a non-relocated address, and we're going to
             * convert it into RAWDATA format.
             */
            if ((type & OUT_SIZMASK) == 4) {
                WRITELONG (q, * (long *) data);
                list->output (offset, p, OUT_RAWDATA+4);
            } 
            else {
                WRITESHORT (q, * (long *) data);
                list->output (offset, p, OUT_RAWDATA+2);
            }
        }
    } 
    else if ((type & OUT_TYPMASK) == OUT_RAWDATA) {
        list->output (offset, data, type);
    } 
    else if ((type & OUT_TYPMASK) == OUT_RESERVE) {
        list->output (offset, NULL, type);
    } 
    else if ((type & OUT_TYPMASK) == OUT_REL2ADR ||
               (type & OUT_TYPMASK) == OUT_REL4ADR) {
        list->output (offset, data, type);
    }

    if (src_get(&lineno,&lnfname))
    {
        outfmt->current_dfmt->linenum(lnfname,lineno,segto);
        if (lnfname) nasm_free(lnfname);
    }

    outfmt->output (segto, data, type, segment, wrt);
}

static int jmp_match (long segment, long offset, int bits,
                insn *ins, const char *code)
{   long isize;
    unsigned char c = code[0];


    if (c != 0370 && c != 0371) return 0;
    if (ins->oprs[0].opflags & OPFLAG_FORWARD) {
        if ((optimizing<0 || (ins->oprs[0].type & STRICT))
            && c==0370) return 1;
        else return (pass0==0); /* match a forward reference */
    }
    isize = calcsize (segment, offset, bits, ins, code);
    if (ins->oprs[0].segment != segment) return 0;
    isize = ins->oprs[0].offset - offset - isize;       /* isize is now the delta */
    if (isize >= -128L && isize <= 127L) return 1;      /* it is byte size */

    return 0;
}               


long assemble (long segment, long offset, int bits, unsigned long cp,
               insn *instruction, struct ofmt *output, efunc error,
               ListGen *listgen) 
{
    struct itemplate *temp;
    int    j;
    int    size_prob;
    long   insn_end;
    long   itimes;
    long   start = offset;
    long   wsize = 0;                  /* size for DB etc. */

    errfunc = error;                   /* to pass to other functions */
    cpu = cp;
    outfmt = output;                   /* likewise */
    list = listgen;                    /* and again */

    switch (instruction->opcode) 
    {
        case   -1: return 0;
        case I_DB: wsize = 1; break;
        case I_DW: wsize = 2; break;
        case I_DD: wsize = 4; break;
        case I_DQ: wsize = 8; break;
        case I_DT: wsize = 10; break;
    }

    if (wsize) {
        extop  * e;
        long   t = instruction->times;
        if (t < 0)
            errfunc(ERR_PANIC, "instruction->times < 0 (%ld) in assemble()",t);

        while (t--)                    /* repeat TIMES times */
        {
            for (e = instruction->eops; e; e = e->next) 
            {
                if (e->type == EOT_DB_NUMBER) 
                {
                    if (wsize == 1) {
                        if (e->segment != NO_SEG)
                            errfunc (ERR_NONFATAL,
                                     "one-byte relocation attempted");
                        else {
                            unsigned char out_byte = e->offset;
                            out (offset, segment, &out_byte, OUT_RAWDATA+1,
                                 NO_SEG, NO_SEG);
                        }
                    } 
                    else if (wsize > 5) {
                        errfunc (ERR_NONFATAL, "integer supplied to a D%c"
                                 " instruction", wsize==8 ? 'Q' : 'T');
                    } 
                    else
                        out (offset, segment, &e->offset,
                             OUT_ADDRESS+wsize, e->segment,
                             e->wrt);
                    offset += wsize;
                } 
                else if (e->type == EOT_DB_STRING) 
                {
                    int align;

                    out (offset, segment, e->stringval,
                         OUT_RAWDATA+e->stringlen, NO_SEG, NO_SEG);
                    align = e->stringlen % wsize;

                    if (align) {
                        align = wsize - align;
                        out (offset, segment, "\0\0\0\0\0\0\0\0",
                             OUT_RAWDATA+align, NO_SEG, NO_SEG);
                        }
                    offset += e->stringlen + align;
                }
            }
            if (t > 0 && t == instruction->times-1) 
            {
                /*
                 * Dummy call to list->output to give the offset to the
                 * listing module.
                 */
                list->output (offset, NULL, OUT_RAWDATA);
                list->uplevel (LIST_TIMES);
            }
        }
        if (instruction->times > 1)
            list->downlevel (LIST_TIMES);
        return offset - start;
    }

    if (instruction->opcode == I_INCBIN) 
    {
        static char fname[FILENAME_MAX];
        FILE        * fp;
        long        len;

        len = FILENAME_MAX-1;
        if (len > instruction->eops->stringlen)
            len = instruction->eops->stringlen;
        strncpy (fname, instruction->eops->stringval, len);
        fname[len] = '\0';

        if ( (fp = fopen(fname, "rb")) == NULL)
            error (ERR_NONFATAL, "`incbin': unable to open file `%s'", fname);
        else if (fseek(fp, 0L, SEEK_END) < 0)
            error (ERR_NONFATAL, "`incbin': unable to seek on file `%s'",
                   fname);
        else 
        {
            static char buf[2048];
            long t = instruction->times;
            long base = 0;

            len = ftell (fp);
            if (instruction->eops->next) {
                base = instruction->eops->next->offset;
                len -= base;
                if (instruction->eops->next->next &&
                    len > instruction->eops->next->next->offset)
                    len = instruction->eops->next->next->offset;
            }
            /*
             * Dummy call to list->output to give the offset to the
             * listing module.
             */
            list->output (offset, NULL, OUT_RAWDATA);
            list->uplevel(LIST_INCBIN);
            while (t--) 
            {
                long l;

                fseek (fp, base, SEEK_SET);             
                l = len;
                while (l > 0) {
                    long m = fread (buf, 1, (l>sizeof(buf)?sizeof(buf):l),
                                    fp);
                    if (!m) {
                        /*
                         * This shouldn't happen unless the file
                         * actually changes while we are reading
                         * it.
                         */
                        error (ERR_NONFATAL, "`incbin': unexpected EOF while"
                               " reading file `%s'", fname);
                        t=0;  /* Try to exit cleanly */
                        break;
                    }
                    out (offset, segment, buf, OUT_RAWDATA+m,
                         NO_SEG, NO_SEG);
                    l -= m;
                }
            }
            list->downlevel(LIST_INCBIN);
            if (instruction->times > 1) {
                /*
                 * Dummy call to list->output to give the offset to the
                 * listing module.
                 */
                list->output (offset, NULL, OUT_RAWDATA);
                list->uplevel(LIST_TIMES);
                list->downlevel(LIST_TIMES);
            }
            fclose (fp);
            return instruction->times * len;
        }
        return 0;                      /* if we're here, there's an error */
    }

    size_prob = FALSE;
    temp = nasm_instructions[instruction->opcode];
    while (temp->opcode != -1) {
        int m = matches (temp, instruction);
        if (m == 99)
            m += jmp_match(segment, offset, bits, instruction, temp->code);

        if (m == 100)                  /* matches! */
        {
            const char *codes = temp->code;
            long insn_size = calcsize(segment, offset, bits,
                                      instruction, codes);
            itimes = instruction->times;
            if (insn_size < 0)         /* shouldn't be, on pass two */
                error (ERR_PANIC, "errors made it through from pass one");
            else while (itimes--) {
                insn_end = offset + insn_size;
                for (j=0; j<instruction->nprefix; j++) {
                    unsigned char c=0;
                    switch (instruction->prefixes[j]) {
                      case P_LOCK:
                        c = 0xF0; break;
                      case P_REPNE: case P_REPNZ:
                        c = 0xF2; break;
                      case P_REPE: case P_REPZ: case P_REP:
                        c = 0xF3; break;
                      case R_CS: c = 0x2E; break;
                      case R_DS: c = 0x3E; break;
                      case R_ES: c = 0x26; break;
                      case R_FS: c = 0x64; break;
                      case R_GS: c = 0x65; break;
                      case R_SS: c = 0x36; break;
                      case R_SEGR6:
                      case R_SEGR7:
                        error (ERR_NONFATAL, "segr6 and segr7 cannot be used as prefixes");
                        break;
                      case P_A16:
                        if (bits != 16)
                            c = 0x67;
                        break;
                      case P_A32:
                        if (bits != 32)
                            c = 0x67;
                        break;
                      case P_O16:
                        if (bits != 16)
                            c = 0x66;
                        break;
                      case P_O32:
                        if (bits != 32)
                            c = 0x66;
                        break;
                      default:
                        error (ERR_PANIC,
                               "invalid instruction prefix");
                    }
                    if (c != 0) {
                        out (offset, segment, &c, OUT_RAWDATA+1,
                             NO_SEG, NO_SEG);
                        offset++;
                    }
                }
                gencode (segment, offset, bits, instruction, codes, insn_end);
                offset += insn_size;
                if (itimes > 0 && itimes == instruction->times-1) {
                    /*
                     * Dummy call to list->output to give the offset to the
                     * listing module.
                     */
                    list->output (offset, NULL, OUT_RAWDATA);
                    list->uplevel (LIST_TIMES);
                }
            }
            if (instruction->times > 1)
                list->downlevel (LIST_TIMES);
            return offset - start;
        } else if (m > 0  &&  m > size_prob) {
            size_prob = m;
        }
        temp++;
    }

    if (temp->opcode == -1) {          /* didn't match any instruction */
        if (size_prob == 1)            /* would have matched, but for size */
            error (ERR_NONFATAL, "operation size not specified");
        else if (size_prob == 2)
            error (ERR_NONFATAL, "mismatch in operand sizes");
        else if (size_prob == 3)
            error (ERR_NONFATAL, "no instruction for this cpu level");
        else
            error (ERR_NONFATAL,
                   "invalid combination of opcode and operands");
    }
    return 0;
}

long insn_size (long segment, long offset, int bits, unsigned long cp,
                insn *instruction, efunc error) 
{
    struct itemplate *temp;

    errfunc = error;                   /* to pass to other functions */
    cpu = cp;

    if (instruction->opcode == -1)
        return 0;

    if (instruction->opcode == I_DB ||
        instruction->opcode == I_DW ||
        instruction->opcode == I_DD ||
        instruction->opcode == I_DQ ||
        instruction->opcode == I_DT) 
    {
        extop *e;
        long isize, osize, wsize = 0;  /* placate gcc */

        isize = 0;
        switch (instruction->opcode) 
        {
          case I_DB: wsize = 1; break;
          case I_DW: wsize = 2; break;
          case I_DD: wsize = 4; break;
          case I_DQ: wsize = 8; break;
          case I_DT: wsize = 10; break;
        }

        for (e = instruction->eops; e; e = e->next) 
        {
            long align;

            osize = 0;
            if (e->type == EOT_DB_NUMBER)
                osize = 1;
            else if (e->type == EOT_DB_STRING)
                osize = e->stringlen;

            align = (-osize) % wsize;
            if (align < 0)
                align += wsize;
            isize += osize + align;
        }
        return isize * instruction->times;
    }

    if (instruction->opcode == I_INCBIN) 
    {
        char  fname[FILENAME_MAX];
        FILE  * fp;
        long  len;

        len = FILENAME_MAX-1;
        if (len > instruction->eops->stringlen)
            len = instruction->eops->stringlen;
        strncpy (fname, instruction->eops->stringval, len);
        fname[len] = '\0';
        if ( (fp = fopen(fname, "rb")) == NULL )
            error (ERR_NONFATAL, "`incbin': unable to open file `%s'", fname);
        else if (fseek(fp, 0L, SEEK_END) < 0)
            error (ERR_NONFATAL, "`incbin': unable to seek on file `%s'",
                   fname);
        else 
        {
            len = ftell (fp);
            fclose (fp);
            if (instruction->eops->next) 
            {
                len -= instruction->eops->next->offset;
                if (instruction->eops->next->next &&
                    len > instruction->eops->next->next->offset)
                {
                    len = instruction->eops->next->next->offset;
                }
            }
            return instruction->times * len;
        }
        return 0;                      /* if we're here, there's an error */
    }

    temp = nasm_instructions[instruction->opcode];
    while (temp->opcode != -1) {
        int m = matches(temp, instruction);
        if (m == 99)
            m += jmp_match(segment, offset, bits, instruction, temp->code);
        
        if (m == 100) {
            /* we've matched an instruction. */
            long  isize;
            const char  * codes = temp->code;
            int   j;

            isize = calcsize(segment, offset, bits, instruction, codes);
            if (isize < 0)
                return -1;
            for (j = 0; j < instruction->nprefix; j++) 
            {
                if ((instruction->prefixes[j] != P_A16 &&
                     instruction->prefixes[j] != P_O16 && bits==16) ||
                    (instruction->prefixes[j] != P_A32 &&
                     instruction->prefixes[j] != P_O32 && bits==32))
                {
                    isize++;
                }
            }
            return isize * instruction->times;
        }
        temp++;
    }
    return -1;                         /* didn't match any instruction */
}


/* check that  opn[op]  is a signed byte of size 16 or 32,
                                        and return the signed value*/
static int is_sbyte (insn *ins, int op, int size)
{
    signed long v;
    int ret;
    
    ret = !(ins->forw_ref && ins->oprs[op].opflags ) && /* dead in the water on forward reference or External */
        optimizing>=0 &&
        !(ins->oprs[op].type & STRICT) &&
        ins->oprs[op].wrt==NO_SEG && ins->oprs[op].segment==NO_SEG;

    v = ins->oprs[op].offset;
    if (size==16) v = (signed short)v;   /* sign extend if 16 bits */
    
    return ret && v>=-128L && v<=127L;
}

static long calcsize (long segment, long offset, int bits,
                      insn *ins, const char *codes) 
{
    long           length = 0;
    unsigned char  c;

    (void) segment;  /* Don't warn that this parameter is unused */
    (void) offset;   /* Don't warn that this parameter is unused */

    while (*codes) switch (c = *codes++) {
      case 01: case 02: case 03:
        codes += c, length += c; break;
      case 04: case 05: case 06: case 07:
        length++; break;
      case 010: case 011: case 012:
        codes++, length++; break;
      case 017:
        length++; break;
      case 014: case 015: case 016:
        length++; break;
      case 020: case 021: case 022:
        length++; break;
      case 024: case 025: case 026:
        length++; break;
      case 030: case 031: case 032:
        length += 2; break;
      case 034: case 035: case 036:
        if ( ins->oprs[c-034].type & (BITS16|BITS32) )
          length += (ins->oprs[c-034].type & BITS16) ? 2 : 4;
        else
          length += (bits == 16) ? 2 : 4;
        break;
      case 037:
        length += 2; break;
      case 040: case 041: case 042:
        length += 4; break;
      case 044: case 045: case 046:
        length += ((ins->oprs[c-044].addr_size ?
                    ins->oprs[c-044].addr_size : bits) == 16 ? 2 : 4); break;
      case 050: case 051: case 052:
        length++; break;
      case 060: case 061: case 062:
        length += 2; break;
      case 064: case 065: case 066:
        if ( ins->oprs[c-064].type & (BITS16|BITS32) )
          length += (ins->oprs[c-064].type & BITS16) ? 2 : 4;
        else
          length += (bits == 16) ? 2 : 4;
        break;
      case 070: case 071: case 072:
        length += 4; break;
      case 0130: case 0131: case 0132:          
        length += is_sbyte(ins, c-0130, 16) ? 1 : 2; break;
      case 0133: case 0134: case 0135:
        codes+=2; length++; break;
      case 0140: case 0141: case 0142:
        length += is_sbyte(ins, c-0140, 32) ? 1 : 4; break;
      case 0143: case 0144: case 0145:
        codes+=2; length++; break;
      case 0300: case 0301: case 0302:
        length += chsize (&ins->oprs[c-0300], bits);
        break;
      case 0310:
        length += (bits==32);
        break;
      case 0311:
        length += (bits==16);
        break;
      case 0312:
        break;
      case 0320:
        length += (bits==32);
        break;
      case 0321:
        length += (bits==16);
        break;
      case 0322:
        break;
      case 0330:
        codes++, length++; break;
      case 0331:
      case 0332:
        break;
      case 0333:
        length++; break;
      case 0340: case 0341: case 0342:
        if (ins->oprs[0].segment != NO_SEG)
            errfunc (ERR_NONFATAL, "attempt to reserve non-constant"
                     " quantity of BSS space");
        else
            length += ins->oprs[0].offset << (c-0340);
        break;
      case 0370: case 0371: case 0372:
        break;
      case 0373:
        length++; break;
      default:                         /* can't do it by 'case' statements */
        if (c>=0100 && c<=0277) {      /* it's an EA */
            ea ea_data;
            if (!process_ea (&ins->oprs[(c>>3)&7], &ea_data, bits, 0,
                             ins->forw_ref)) {
                errfunc (ERR_NONFATAL, "invalid effective address");
                return -1;
            } else
                length += ea_data.size;
        } else
            errfunc (ERR_PANIC, "internal instruction table corrupt"
                     ": instruction code 0x%02X given", c);
    }
    return length;
}

static void gencode (long segment, long offset, int bits,
                     insn *ins, const char *codes, long insn_end) 
{
    static char condval[] = { /* conditional opcodes */
        0x7, 0x3, 0x2, 0x6, 0x2, 0x4, 0xF, 0xD, 0xC, 0xE, 0x6, 0x2,
        0x3, 0x7, 0x3, 0x5, 0xE, 0xC, 0xD, 0xF, 0x1, 0xB, 0x9, 0x5,
        0x0, 0xA, 0xA, 0xB, 0x8, 0x4
    };
    unsigned char c;
    unsigned char bytes[4];
    long          data, size;

    while (*codes)
        switch (c = *codes++) 
        {
        case 01: case 02: case 03:
            out (offset, segment, codes, OUT_RAWDATA+c, NO_SEG, NO_SEG);
            codes += c;
            offset += c;
            break;

        case 04: case 06:
            switch (ins->oprs[0].basereg) 
            {
            case R_CS: 
                bytes[0] = 0x0E + (c == 0x04 ? 1 : 0); break;
            case R_DS: 
                bytes[0] = 0x1E + (c == 0x04 ? 1 : 0); break;
            case R_ES: 
                bytes[0] = 0x06 + (c == 0x04 ? 1 : 0); break;
            case R_SS: 
                bytes[0] = 0x16 + (c == 0x04 ? 1 : 0); break;
            default:
                errfunc (ERR_PANIC, "bizarre 8086 segment register received");
            }
            out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset++;
            break;

        case 05: case 07:
            switch (ins->oprs[0].basereg) {
            case R_FS: bytes[0] = 0xA0 + (c == 0x05 ? 1 : 0); break;
            case R_GS: bytes[0] = 0xA8 + (c == 0x05 ? 1 : 0); break;
            default:
                errfunc (ERR_PANIC, "bizarre 386 segment register received");
            }
            out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset++;
            break;

        case 010: case 011: case 012:
            bytes[0] = *codes++ + regval(&ins->oprs[c-010]);
            out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset += 1;
            break;

        case 017:
            bytes[0] = 0;
            out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset += 1;
            break;

        case 014: case 015: case 016:
            if (ins->oprs[c-014].offset < -128 
                || ins->oprs[c-014].offset > 127)
            {
                errfunc (ERR_WARNING, "signed byte value exceeds bounds");
            }

            if (ins->oprs[c-014].segment != NO_SEG) 
            {
                data = ins->oprs[c-014].offset;
                out (offset, segment, &data, OUT_ADDRESS+1,
                     ins->oprs[c-014].segment, ins->oprs[c-014].wrt);
            } 
            else {
                bytes[0] = ins->oprs[c-014].offset;
                out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            }
            offset += 1;
            break;

        case 020: case 021: case 022:
            if (ins->oprs[c-020].offset < -256 
                || ins->oprs[c-020].offset > 255)
            {
                errfunc (ERR_WARNING, "byte value exceeds bounds");
            }
            if (ins->oprs[c-020].segment != NO_SEG) {
                data = ins->oprs[c-020].offset;
                out (offset, segment, &data, OUT_ADDRESS+1,
                     ins->oprs[c-020].segment, ins->oprs[c-020].wrt);
            } 
            else {
                bytes[0] = ins->oprs[c-020].offset;
                out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            }
            offset += 1;
            break;
        
        case 024: case 025: case 026:
            if (ins->oprs[c-024].offset < 0 || ins->oprs[c-024].offset > 255)
                errfunc (ERR_WARNING, "unsigned byte value exceeds bounds");
            if (ins->oprs[c-024].segment != NO_SEG) {
                data = ins->oprs[c-024].offset;
                out (offset, segment, &data, OUT_ADDRESS+1,
                     ins->oprs[c-024].segment, ins->oprs[c-024].wrt);
            }
            else {
                bytes[0] = ins->oprs[c-024].offset;
                out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            }
            offset += 1;
            break;

        case 030: case 031: case 032:
            if (ins->oprs[c-030].segment == NO_SEG &&
                ins->oprs[c-030].wrt == NO_SEG &&
                (ins->oprs[c-030].offset < -65536L ||
                 ins->oprs[c-030].offset > 65535L))
            {
                errfunc (ERR_WARNING, "word value exceeds bounds");
            }
            data = ins->oprs[c-030].offset;
            out (offset, segment, &data, OUT_ADDRESS+2,
                 ins->oprs[c-030].segment, ins->oprs[c-030].wrt);
            offset += 2;
            break;

        case 034: case 035: case 036:
            if ( ins->oprs[c-034].type & (BITS16|BITS32) )
              size = (ins->oprs[c-034].type & BITS16) ? 2 : 4;
            else
              size = (bits == 16) ? 2 : 4;
            data = ins->oprs[c-034].offset;
            if (size==2 && (data < -65536L || data > 65535L))
                errfunc (ERR_WARNING, "word value exceeds bounds");
            out (offset, segment, &data, OUT_ADDRESS+size,
                 ins->oprs[c-034].segment, ins->oprs[c-034].wrt);
            offset += size;
            break;

        case 037:
            if (ins->oprs[0].segment == NO_SEG)
                errfunc (ERR_NONFATAL, "value referenced by FAR is not"
                         " relocatable");
            data = 0L;
            out (offset, segment, &data, OUT_ADDRESS+2,
                 outfmt->segbase(1+ins->oprs[0].segment),
                 ins->oprs[0].wrt);
            offset += 2;
                break;

        case 040: case 041: case 042:
            data = ins->oprs[c-040].offset;
            out (offset, segment, &data, OUT_ADDRESS+4,
                 ins->oprs[c-040].segment, ins->oprs[c-040].wrt);
            offset += 4;
            break;

        case 044: case 045: case 046:
            data = ins->oprs[c-044].offset;
            size = ((ins->oprs[c-044].addr_size ?
                     ins->oprs[c-044].addr_size : bits) == 16 ? 2 : 4);
            if (size==2 && (data < -65536L || data > 65535L))
                errfunc (ERR_WARNING, "word value exceeds bounds");
            out (offset, segment, &data, OUT_ADDRESS+size,
                 ins->oprs[c-044].segment, ins->oprs[c-044].wrt);
            offset += size;
            break;

        case 050: case 051: case 052:
            if (ins->oprs[c-050].segment != segment)
                errfunc (ERR_NONFATAL, "short relative jump outside segment");
            data = ins->oprs[c-050].offset - insn_end;
            if (data > 127 || data < -128)
                errfunc (ERR_NONFATAL, "short jump is out of range");
            bytes[0] = data;
            out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset += 1;
            break;

        case 060: case 061: case 062:
            if (ins->oprs[c-060].segment != segment) {
                data = ins->oprs[c-060].offset;
                out (offset, segment, &data, OUT_REL2ADR+insn_end-offset,
                     ins->oprs[c-060].segment, ins->oprs[c-060].wrt);
            } else {
                data = ins->oprs[c-060].offset - insn_end;
                out (offset, segment, &data,
                     OUT_ADDRESS+2, NO_SEG, NO_SEG);
            }
            offset += 2;
            break;

        case 064: case 065: case 066:
            if ( ins->oprs[c-064].type & (BITS16|BITS32) )
              size = (ins->oprs[c-064].type & BITS16) ? 2 : 4;
            else
              size = (bits == 16) ? 2 : 4;
            if (ins->oprs[c-064].segment != segment) {
                long reltype = (size == 2 ? OUT_REL2ADR : OUT_REL4ADR);
                data = ins->oprs[c-064].offset;
                out (offset, segment, &data, reltype+insn_end-offset,
                     ins->oprs[c-064].segment, ins->oprs[c-064].wrt);
            } else {
                data = ins->oprs[c-064].offset - insn_end;
                out (offset, segment, &data,
                     OUT_ADDRESS+size, NO_SEG, NO_SEG);
            }
            offset += size;
            break;

        case 070: case 071: case 072:
            if (ins->oprs[c-070].segment != segment) {
                data = ins->oprs[c-070].offset;
                out (offset, segment, &data, OUT_REL4ADR+insn_end-offset,
                     ins->oprs[c-070].segment, ins->oprs[c-070].wrt);
            } else {
                data = ins->oprs[c-070].offset - insn_end;
                out (offset, segment, &data,
                     OUT_ADDRESS+4, NO_SEG, NO_SEG);
            }
            offset += 4;
            break;

        case 0130: case 0131: case 0132:
            data = ins->oprs[c-0130].offset;
            if (is_sbyte(ins, c-0130, 16)) {
                out (offset, segment, &data, OUT_RAWDATA+1, NO_SEG, NO_SEG);
                offset++;
            } else {
                if (ins->oprs[c-0130].segment == NO_SEG &&
                        ins->oprs[c-0130].wrt == NO_SEG &&
                        (data < -65536L || data > 65535L)) {
                    errfunc (ERR_WARNING, "word value exceeds bounds");
                }    
                out (offset, segment, &data, OUT_ADDRESS+2,
                          ins->oprs[c-0130].segment, ins->oprs[c-0130].wrt);
                offset += 2;
            }
            break;

        case 0133: case 0134: case 0135:
            codes++;
            bytes[0] = *codes++;
            if (is_sbyte(ins, c-0133, 16)) bytes[0] |= 2;   /* s-bit */
            out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset++;
            break;
                
        case 0140: case 0141: case 0142:
            data = ins->oprs[c-0140].offset;
            if (is_sbyte(ins, c-0140, 32)) {
                out (offset, segment, &data, OUT_RAWDATA+1, NO_SEG, NO_SEG);
                offset++;
            } else {
                out (offset, segment, &data, OUT_ADDRESS+4,
                          ins->oprs[c-0140].segment, ins->oprs[c-0140].wrt);
                offset += 4;
            }
            break;

        case 0143: case 0144: case 0145:
            codes++;
            bytes[0] = *codes++;
            if (is_sbyte(ins, c-0143, 32)) bytes[0] |= 2;   /* s-bit */
            out (offset, segment, bytes, OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset++;
            break;
                    
        case 0300: case 0301: case 0302:
            if (chsize (&ins->oprs[c-0300], bits)) {
                *bytes = 0x67;
                out (offset, segment, bytes,
                     OUT_RAWDATA+1, NO_SEG, NO_SEG);
                offset += 1;
            } else
                offset += 0;
            break;

        case 0310:
            if (bits==32) {
                *bytes = 0x67;
                out (offset, segment, bytes,
                     OUT_RAWDATA+1, NO_SEG, NO_SEG);
                offset += 1;
            } else
                offset += 0;
            break;

        case 0311:
            if (bits==16) {
                *bytes = 0x67;
                out (offset, segment, bytes,
                     OUT_RAWDATA+1, NO_SEG, NO_SEG);
                offset += 1;
            } else
                offset += 0;
            break;

        case 0312:
            break;

        case 0320:
            if (bits==32) {
                *bytes = 0x66;
                out (offset, segment, bytes,
                     OUT_RAWDATA+1, NO_SEG, NO_SEG);
                offset += 1;
            } else
                offset += 0;
            break;

        case 0321:
            if (bits==16) {
                *bytes = 0x66;
                out (offset, segment, bytes,
                     OUT_RAWDATA+1, NO_SEG, NO_SEG);
                offset += 1;
            } else
                offset += 0;
            break;

        case 0322:
            break;

        case 0330:
            *bytes = *codes++ ^ condval[ins->condition];
            out (offset, segment, bytes,
                 OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset += 1;
            break;

        case 0331:
        case 0332:
            break;

        case 0333:
            *bytes = 0xF3;
            out (offset, segment, bytes,
                 OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset += 1;
            break;

        case 0340: case 0341: case 0342:
            if (ins->oprs[0].segment != NO_SEG)
                errfunc (ERR_PANIC, "non-constant BSS size in pass two");
            else {
                long size = ins->oprs[0].offset << (c-0340);
                if (size > 0)
                    out (offset, segment, NULL,
                         OUT_RESERVE+size, NO_SEG, NO_SEG);
                offset += size;
            }
            break;

        case 0370: case 0371: case 0372:
            break;
        
        case 0373:
            *bytes = bits==16 ? 3 : 5;
            out (offset, segment, bytes,
                 OUT_RAWDATA+1, NO_SEG, NO_SEG);
            offset += 1;
            break;

        default:                       /* can't do it by 'case' statements */
            if (c>=0100 && c<=0277) {      /* it's an EA */
                ea ea_data;
                int rfield;
                unsigned char *p;
                long s;

                if (c<=0177)           /* pick rfield from operand b */
                    rfield = regval (&ins->oprs[c&7]);
                else                   /* rfield is constant */
                    rfield = c & 7;

                if (!process_ea (&ins->oprs[(c>>3)&7], &ea_data, bits, rfield,
                                 ins->forw_ref))
                {
                    errfunc (ERR_NONFATAL, "invalid effective address");
                }

                p = bytes;
                *p++ = ea_data.modrm;
                if (ea_data.sib_present)
                    *p++ = ea_data.sib;

                s = p-bytes;
                out (offset, segment, bytes, OUT_RAWDATA + s,
                     NO_SEG, NO_SEG);

                switch (ea_data.bytes) {
                case 0:
                    break;
                case 1:
                    if (ins->oprs[(c>>3)&7].segment != NO_SEG) {
                        data = ins->oprs[(c>>3)&7].offset;
                        out (offset, segment, &data, OUT_ADDRESS+1,
                             ins->oprs[(c>>3)&7].segment,
                             ins->oprs[(c>>3)&7].wrt);
                    } else {
                        *bytes = ins->oprs[(c>>3)&7].offset;
                        out (offset, segment, bytes, OUT_RAWDATA+1,
                             NO_SEG, NO_SEG);
                    }
                    s++;
                    break;
                case 2:
                case 4:
                    data = ins->oprs[(c>>3)&7].offset;
                    out (offset, segment, &data,
                         OUT_ADDRESS+ea_data.bytes,
                         ins->oprs[(c>>3)&7].segment, ins->oprs[(c>>3)&7].wrt);
                    s += ea_data.bytes;
                    break;
                }
                offset += s;
            } else
                errfunc (ERR_PANIC, "internal instruction table corrupt"
                     ": instruction code 0x%02X given", c);
        }
}

#include "regvals.c"

static int regval (operand *o) 
{
    if ( o->basereg < EXPR_REG_START || o->basereg >= REG_ENUM_LIMIT ) {
        errfunc (ERR_PANIC, "invalid operand passed to regval()");
    }
    return regvals[o->basereg];
}

static int matches (struct itemplate *itemp, insn *instruction) 
{
    int i, size[3], asize, oprs, ret;

    ret = 100;

    /*
     * Check the opcode
     */
    if (itemp->opcode != instruction->opcode) return 0;

    /*
     * Count the operands
     */
    if (itemp->operands != instruction->operands) return 0;

    /*
     * Check that no spurious colons or TOs are present
     */
    for (i=0; i<itemp->operands; i++)
        if (instruction->oprs[i].type & ~itemp->opd[i] & (COLON|TO))
            return 0;

    /*
     * Check that the operand flags all match up
     */
    for (i=0; i<itemp->operands; i++)
        if (itemp->opd[i] & ~instruction->oprs[i].type ||
            ((itemp->opd[i] & SIZE_MASK) &&
             ((itemp->opd[i] ^ instruction->oprs[i].type) & SIZE_MASK)))
        {
            if ((itemp->opd[i] & ~instruction->oprs[i].type & NON_SIZE) ||
                (instruction->oprs[i].type & SIZE_MASK))
                return 0;
            else
/*              ret = 1;   */
                return 1;
        }

    /*
     * Check operand sizes
     */
    if (itemp->flags & IF_ARMASK) {
      size[0] = size[1] = size[2] = 0;

      switch (itemp->flags & IF_ARMASK) {
      case IF_AR0: i = 0; break;
      case IF_AR1: i = 1; break;
      case IF_AR2: i = 2; break;
      default:     break;       /* Shouldn't happen */
      }
      if (itemp->flags & IF_SB) {
        size[i] = BITS8;
      } else if (itemp->flags & IF_SW) {
        size[i] = BITS16;
      } else if (itemp->flags & IF_SD) {
        size[i] = BITS32;
      }
    } else {
      asize = 0;
      if (itemp->flags & IF_SB) {
        asize = BITS8;
        oprs = itemp->operands;
      } else if (itemp->flags & IF_SW) {
        asize = BITS16;
        oprs = itemp->operands;
      } else if (itemp->flags & IF_SD) {
        asize = BITS32;
        oprs = itemp->operands;
      }
      size[0] = size[1] = size[2] = asize;
    }

    if (itemp->flags & (IF_SM | IF_SM2)) {
      oprs = (itemp->flags & IF_SM2 ? 2 : itemp->operands);
      asize = 0;
      for (i=0; i<oprs; i++) {
        if ( (asize = itemp->opd[i] & SIZE_MASK) != 0) {
          int j;
          for (j=0; j<oprs; j++)
            size[j] = asize;
          break;
        }
      }
    } else {
      oprs = itemp->operands;
    }

    for (i=0; i<itemp->operands; i++)
        if (!(itemp->opd[i] & SIZE_MASK) &&
            (instruction->oprs[i].type & SIZE_MASK & ~size[i]))
/*          ret = 2;  */
            return 2;

    /*
     * Check template is okay at the set cpu level
     */
    if ((itemp->flags & IF_PLEVEL) > cpu) return 3;
    
    /*
     * Check if special handling needed for Jumps
     */
    if ((unsigned char)(itemp->code[0]) >= 0370) return 99;
     
    return ret;
}

static ea *process_ea (operand *input, ea *output, int addrbits, int rfield,
                       int forw_ref) 
{
    if (!(REGISTER & ~input->type)) {  /* it's a single register */
        static int regs[] = {
          R_AL,   R_CL,   R_DL,   R_BL,   R_AH,   R_CH,   R_DH,   R_BH,
          R_AX,   R_CX,   R_DX,   R_BX,   R_SP,   R_BP,   R_SI,   R_DI,
          R_EAX,  R_ECX,  R_EDX,  R_EBX,  R_ESP,  R_EBP,  R_ESI,  R_EDI,
          R_MM0,  R_MM1,  R_MM2,  R_MM3,  R_MM4,  R_MM5,  R_MM6,  R_MM7,
          R_XMM0, R_XMM1, R_XMM2, R_XMM3, R_XMM4, R_XMM5, R_XMM6, R_XMM7
        };
        int i;

        for (i=0; i<elements(regs); i++)
            if (input->basereg == regs[i]) break;
        if (i<elements(regs)) {
            output->sib_present = FALSE;/* no SIB necessary */
            output->bytes = 0;         /* no offset necessary either */
            output->modrm = 0xC0 | (rfield << 3) | (i & 7);
        } 
        else
            return NULL;
    } else {                           /* it's a memory reference */
        if (input->basereg==-1 && (input->indexreg==-1 || input->scale==0)) {
            /* it's a pure offset */
            if (input->addr_size)
                addrbits = input->addr_size;
            output->sib_present = FALSE;
            output->bytes = (addrbits==32 ? 4 : 2);
            output->modrm = (addrbits==32 ? 5 : 6) | (rfield << 3);
        } 
        else {                 /* it's an indirection */
            int i=input->indexreg, b=input->basereg, s=input->scale;
            long o=input->offset, seg=input->segment;
            int hb=input->hintbase, ht=input->hinttype;
            int t;

            if (s==0) i = -1;          /* make this easy, at least */

            if (i==R_EAX || i==R_EBX || i==R_ECX || i==R_EDX
                || i==R_EBP || i==R_ESP || i==R_ESI || i==R_EDI
                || b==R_EAX || b==R_EBX || b==R_ECX || b==R_EDX
                || b==R_EBP || b==R_ESP || b==R_ESI || b==R_EDI) {
                /* it must be a 32-bit memory reference. Firstly we have
                 * to check that all registers involved are type Exx. */
                if (i!=-1 && i!=R_EAX && i!=R_EBX && i!=R_ECX && i!=R_EDX
                    && i!=R_EBP && i!=R_ESP && i!=R_ESI && i!=R_EDI)
                    return NULL;
                if (b!=-1 && b!=R_EAX && b!=R_EBX && b!=R_ECX && b!=R_EDX
                    && b!=R_EBP && b!=R_ESP && b!=R_ESI && b!=R_EDI)
                    return NULL;

                /* While we're here, ensure the user didn't specify WORD. */
                if (input->addr_size == 16)
                    return NULL;

                /* now reorganise base/index */
                if (s == 1 && b != i && b != -1 && i != -1 &&
                    ((hb==b&&ht==EAH_NOTBASE) || (hb==i&&ht==EAH_MAKEBASE)))
                    t = b, b = i, i = t;   /* swap if hints say so */
                if (b==i)              /* convert EAX+2*EAX to 3*EAX */
                    b = -1, s++;
                if (b==-1 && s==1 && !(hb == i && ht == EAH_NOTBASE))
                    b = i, i = -1;     /* make single reg base, unless hint */
                if (((s==2 && i!=R_ESP && !(input->eaflags & EAF_TIMESTWO)) ||
                     s==3 || s==5 || s==9) && b==-1)
                    b = i, s--;       /* convert 3*EAX to EAX+2*EAX */
                if (i==-1 && b!=R_ESP && (input->eaflags & EAF_TIMESTWO))
                    i = b, b = -1, s = 1;
                        /* convert [NOSPLIT EAX] to sib format with 0x0 displacement */
                if (s==1 && i==R_ESP)  /* swap ESP into base if scale is 1 */
                    i = b, b = R_ESP;
                if (i==R_ESP || (s!=1 && s!=2 && s!=4 && s!=8 && i!=-1))
                    return NULL;      /* wrong, for various reasons */

                if (i==-1 && b!=R_ESP) {/* no SIB needed */
                    int mod, rm;
                    switch(b) {
                      case R_EAX: rm = 0; break;
                      case R_ECX: rm = 1; break;
                      case R_EDX: rm = 2; break;
                      case R_EBX: rm = 3; break;
                      case R_EBP: rm = 5; break;
                      case R_ESI: rm = 6; break;
                      case R_EDI: rm = 7; break;
                      case -1: rm = 5; break;
                      default:         /* should never happen */
                        return NULL;
                    }
                    if (b==-1 || (b!=R_EBP && o==0 &&
                                  seg==NO_SEG && !forw_ref &&
                                  !(input->eaflags &
                                    (EAF_BYTEOFFS|EAF_WORDOFFS))))
                        mod = 0;
                    else if (input->eaflags & EAF_BYTEOFFS ||
                             (o>=-128 && o<=127 && seg==NO_SEG && !forw_ref &&
                              !(input->eaflags & EAF_WORDOFFS))) {
                        mod = 1;
                    } 
                    else
                        mod = 2;

                    output->sib_present = FALSE;
                    output->bytes = (b==-1 || mod==2 ? 4 : mod);
                    output->modrm = (mod<<6) | (rfield<<3) | rm;
                } 
                else {         /* we need a SIB */
                    int mod, scale, index, base;

                    switch (b) {
                      case R_EAX: base = 0; break;
                      case R_ECX: base = 1; break;
                      case R_EDX: base = 2; break;
                      case R_EBX: base = 3; break;
                      case R_ESP: base = 4; break;
                      case R_EBP: case -1: base = 5; break;
                      case R_ESI: base = 6; break;
                      case R_EDI: base = 7; break;
                      default:         /* then what the smeg is it? */
                        return NULL;  /* panic */
                    }

                    switch (i) {
                      case R_EAX: index = 0; break;
                      case R_ECX: index = 1; break;
                      case R_EDX: index = 2; break;
                      case R_EBX: index = 3; break;
                      case -1: index = 4; break;
                      case R_EBP: index = 5; break;
                      case R_ESI: index = 6; break;
                      case R_EDI: index = 7; break;
                      default:         /* then what the smeg is it? */
                        return NULL;  /* panic */
                    }

                    if (i==-1) s = 1;
                    switch (s) {
                      case 1: scale = 0; break;
                      case 2: scale = 1; break;
                      case 4: scale = 2; break;
                      case 8: scale = 3; break;
                      default:         /* then what the smeg is it? */
                        return NULL;  /* panic */
                    }

                    if (b==-1 || (b!=R_EBP && o==0 &&
                                  seg==NO_SEG && !forw_ref &&
                                  !(input->eaflags &
                                    (EAF_BYTEOFFS|EAF_WORDOFFS))))
                        mod = 0;
                    else if (input->eaflags & EAF_BYTEOFFS ||
                             (o>=-128 && o<=127 && seg==NO_SEG && !forw_ref &&
                              !(input->eaflags & EAF_WORDOFFS)))
                        mod = 1;
                    else
                        mod = 2;

                    output->sib_present = TRUE;
                    output->bytes = (b==-1 || mod==2 ? 4 : mod);
                    output->modrm = (mod<<6) | (rfield<<3) | 4;
                    output->sib = (scale<<6) | (index<<3) | base;
                }
            } 
            else {                     /* it's 16-bit */
                int mod, rm;

                /* check all registers are BX, BP, SI or DI */
                if ((b!=-1 && b!=R_BP && b!=R_BX && b!=R_SI && b!=R_DI) ||
                    (i!=-1 && i!=R_BP && i!=R_BX && i!=R_SI && i!=R_DI))
                    return NULL;

                /* ensure the user didn't specify DWORD */
                if (input->addr_size == 32)
                    return NULL;

                if (s!=1 && i!=-1) return NULL;/* no can do, in 16-bit EA */
                if (b==-1 && i!=-1) { int tmp = b; b = i; i = tmp; } /* swap */
                if ((b==R_SI || b==R_DI) && i!=-1)
                  { int tmp = b; b = i; i = tmp; }
                                      /* have BX/BP as base, SI/DI index */
                if (b==i) return NULL;/* shouldn't ever happen, in theory */
                if (i!=-1 && b!=-1 &&
                    (i==R_BP || i==R_BX || b==R_SI || b==R_DI))
                    return NULL;      /* invalid combinations */
                if (b==-1)            /* pure offset: handled above */
                    return NULL;      /* so if it gets to here, panic! */

                rm = -1;
                if (i!=-1)
                    switch (i*256 + b) {
                      case R_SI*256+R_BX: rm=0; break;
                      case R_DI*256+R_BX: rm=1; break;
                      case R_SI*256+R_BP: rm=2; break;
                      case R_DI*256+R_BP: rm=3; break;
                    }
                else
                    switch (b) {
                      case R_SI: rm=4; break;
                      case R_DI: rm=5; break;
                      case R_BP: rm=6; break;
                      case R_BX: rm=7; break;
                    }
                if (rm==-1)            /* can't happen, in theory */
                    return NULL;      /* so panic if it does */

                if (o==0 && seg==NO_SEG && !forw_ref && rm!=6 &&
                    !(input->eaflags & (EAF_BYTEOFFS|EAF_WORDOFFS)))
                    mod = 0;
                else if (input->eaflags & EAF_BYTEOFFS ||
                         (o>=-128 && o<=127 && seg==NO_SEG && !forw_ref &&
                          !(input->eaflags & EAF_WORDOFFS)))
                    mod = 1;
                else
                    mod = 2;

                output->sib_present = FALSE;  /* no SIB - it's 16-bit */
                output->bytes = mod;  /* bytes of offset needed */
                output->modrm = (mod<<6) | (rfield<<3) | rm;
            }
        }
    }
    output->size = 1 + output->sib_present + output->bytes;
    return output;
}

static int chsize (operand *input, int addrbits) 
{
    if (!(MEMORY & ~input->type)) {
        int i=input->indexreg, b=input->basereg;

        if (input->scale==0) i = -1;

        if (i == -1 && b == -1) /* pure offset */
            return (input->addr_size != 0 && input->addr_size != addrbits);

        if (i==R_EAX || i==R_EBX || i==R_ECX || i==R_EDX
            || i==R_EBP || i==R_ESP || i==R_ESI || i==R_EDI
            || b==R_EAX || b==R_EBX || b==R_ECX || b==R_EDX
            || b==R_EBP || b==R_ESP || b==R_ESI || b==R_EDI)
            return (addrbits==16);
        else
            return (addrbits==32);
    } 
    else
        return 0;
}