XED command interface

The command line tool called xed or xed.exe is built when you build the examples (Examples of using XED) that come with XED. The xed-ex3 is just encode portion of the xed command line tool.

This tool is useful for encoding and decoding or even decoding-then-re-encoding a single instruction or all the instructions in the text segment of an ELF binary (32 or 64b). For decoding, just jump to the examples.

This section also explains a little language for writing the instructions for encode requests (-e option). I am constantly using this tool and updating it. The xed-ex3 (xed-ex3.exe) example is just the encoder portion of the xed command line tool.

The SUPPRESSED operands emitted by the decoder are not used when encoding. They are ignored. They are not required to select an encoding.

The syntax for encodable strings is as follows:

             Opcode[/width]   [operand [operand]]

The width is a 8, 16, 32 or 64, indicating the effective operand width if it differs from the default. 8b operations generally require this. Or since most operations that default to 32b widths in 64b mode, it is required for 64b operation widths in 64b mode.

The operand specifier is one of the following.

• A register name such as EAX or R8B, etc. Case does not matter.

• An immediate specifier such as IMM(12ff) . Note: most shells require quotes because of the parens.

• A displacement specifier such as DISP(0000001f) . Note: need the quotes.

• A memory specifier that indicates the base register, index register, scale value, and displacement value. If one of the fields is not required, a - is necessary. The displacement is omittable. For example: MEM4(ESI,EAX,8,ff) or MEM4(EBX,-,-). The first one specifies that the memory address 4 bytes and should be ESI + EAX * 8 + 0xff. The second one specifies that EBX should be used to access 4 bytes of memory; note the displacement is omitted. A segment override can be specified as follows: MEM4(GS:EAX,-,-,-), by using a segment-name followed by a ":" before the base register. If there is no base register, you can use a "-", for example: MEM4(GS:-,-,11223344). One also needs to specify a memory operation width. This can be accomplished by indicating a number of bytes just after the MEM specifier. For example: MEM2(EAX,-,-) indicates a 2 byte memory operation.

• An address generation specifer that has the same syntax as the above MEM() specifier, but is only used for LEA instructions. Example: AGEN(EAX,EBX,2,-)

Here is the help message:

% obj/xed -h
Usage: obj/xed [options]
One of the following is required:
  -i input_file             (decode file)
  -ide input_file           (decode/encode file)
  -d hex-string             (decode one instruction)
  -e instruction            (encode, must be last)
  -de hex-string            (decode-then-encode)
 
Optional arguments:
  -v verbosity  (0=quiet, 1=errors, 2=useful-info, 3=trace, 5=very verbose)
  -n number-of-instructions-to-decode (default 10,000, accepts K/M/G qualifiers)
  -I            (Intel SYSV syntax for disassembly)
  -A            (ATT SYSV syntax for disassembly)
  -16           (for LEGACY_16 mode)
  -32           (for LEGACY_32 mode, default)
  -64           (for LONG_64 mode w/64b addressing)
  -a32          (32b addressing, default, not in LONG_64 mode)
  -a16          (16b addressing, not in LONG_64 mode)
  -s32          (32b stack addressing, default, not in LONG_64 mode)
  -s16          (16b stack addressing, not in LONG_64 mode)

Here are a couple of examples:

% xed -d 0000
ADD INT_ALU BASE  Opcode: 00  MODRM: 00 Bytes: 2
        Eb/EXPLICIT/RW Gb/EXPLICIT/R 
        ADD EffWidth: 8b
        MachineMode: LEGACY_32 AddrWidth: 32b StackAddrWidth: 32b
        MEM/EXPLICIT/RW REG/AL(REG8)/EXPLICIT/R 
        Read Write BASE= EAX(REG32) MemopLength = 1

        rFLAGS: of-mod sf-mod zf-mod af-mod pf-mod cf-mod Read:  Written: of sf zf af pf cf             writes

% xed -e ADD EAX EBX
Encodable! 01d8

xed -e ADD EAX 'MEM4(ESP,EBX,4)'
Encodable! 03049c

% xed -d 6a00
PUSH INT_ALU BASE  Opcode: 6a  Immed: 00 Bytes: 2
        Ib/EXPLICIT/R STACKPUSH/SUPPRESSED/R 
        PUSH EffWidth: 32b
        MachineMode: LEGACY_32 AddrWidth: 32b StackAddrWidth: 32b
        MEM/SUPPRESSED/W REG/ESP(REG32)/SUPPRESSED/RW IMM/EXPLICIT/R 
        Write SEG= SS BASE= ESP(REG32) MemopLength = 4
        IMMED: 00

        Does not use rFLAGS

% xed -e MOV EAX 'MEM4(SS:ESP,-,-)'
Encodable! 8b0424

Or using the xed-ex3 example tool:

% obj/xed-ex3
Usage: obj/xed-ex3 [-16|-32|-64] [-a16|-a32] [-s16|-s32] encode-string

The -16, -32 or -64 are for specifying the major mode of the machine. The -a16 and -a32 are for specifying 16 or 32 bit addressing. The default addressing mode in 64b mode is 64b addressing. The -s16 and -a32 are for specifying 16 or 32 bit stack addressing in 16 or 32 bit modes.

% obj/xed-ex3 -64 PUSH/64 RAX
Encode request:
PUSH Prefixes:  EffOpWidth: 64b EffAddrWidth: 64b
        MachineMode: LONG_64 AddrWidth: 64b StackAddrWidth: 32b
        REG/RAX(REG64)/EXPLICIT/RW 
        MemopLength = 0

Encodable! 50

% obj/xed-ex3 MOV 'MEM4(EAX,-,-)' 'IMM(11223344)'
Encode request:
MOV Prefixes:  EffOpWidth: 32b EffAddrWidth: 32b
        MachineMode: LEGACY_32 AddrWidth: 32b StackAddrWidth: 32b
        MEM0/EXPLICIT/RW IMM/EXPLICIT/RW 
        TmpltIdx=0 BASE= EAX(REG32) MemopLength = 0
        IMMED: 0x11223344 signed: 1144201745 starts@byte: 1

Encodable! c70011223344

An example of using the encoder

The encoder language file which is part of the xed command line tool shows how to build up instructions from scratch. The example uses a string to drive the creation of the instruction, but that is just an example. Look at the parse_encode_request function for the required pieces.

/*BEGIN_LEGAL 
Copyright (c) 2007, Intel Corp.
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.

    * Redistributions in binary form must reproduce the above
      copyright notice, this list of conditions and the following
      disclaimer in the documentation and/or other materials provided
      with the distribution.

    * Neither the name of Intel Corporation nor the names of its
      contributors may be used to endorse or promote products derived
      from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
END_LEGAL */

// This is an example of how to use the encoder from scratch in the context
// of parsing a string from the command line.  


#include <iostream>
#include <iomanip>
#include <vector>
#include <sstream>
#include <cassert>
extern "C" {
#include "xed-interface.h"
#include "xed-portability.h"
#include "xed-examples-util.h"
}
#include "xed-enc-lang.H"




using namespace std;
static char xed_enc_lang_toupper(char c) {
    if (c >= 'a' && c <= 'z')
        return c-'a'+'A';
    return c;
}

static string upcase(string s) {
    string t = "";
    xed_uint_t len = static_cast<xed_uint_t>(s.size());
    xed_uint_t i;
    for(i=0 ; i < len ; i++ ) 
        t = t + xed_enc_lang_toupper(s[i]);
    return t;
}

unsigned int
xed_split_args(const string& sep, 
               const string& input, 
               vector<string>& output_array)
{
    // returns the number of args
    // rip off the separator characters and split the src string based on separators.
    
    // find the string between last_pos and pos. pos is after last_pos
    string::size_type last_pos = input.find_first_not_of(sep, 0);
    string::size_type pos = input.find_first_of(sep, last_pos);  
    if (CLIENT_VERBOSE3)
        printf("input %s\tlast_pos " XED_FMT_U " pos " XED_FMT_U "\n", 
               input.c_str() , STATIC_CAST(xed_uint_t,last_pos), STATIC_CAST(xed_uint_t,pos));
    int i=0;
    while( pos != string::npos && last_pos != string::npos ) 
    {
        string a = input.substr(last_pos, pos-last_pos);
        output_array.push_back(a); 
        if (CLIENT_VERBOSE3)
            printf("\t\tlast_pos " XED_FMT_U " pos " XED_FMT_U " i %d\n", 
                   STATIC_CAST(xed_uint_t,last_pos),
                   STATIC_CAST(xed_uint_t,pos),
                   i);
        last_pos = input.find_first_not_of(sep, pos);
        pos = input.find_first_of(sep, last_pos);  
        i++;
    }
    if (last_pos != string::npos && pos == string::npos)
    {
        if (CLIENT_VERBOSE3)
            printf("\t\tGetting last substring at " XED_FMT_U "\n", STATIC_CAST(xed_uint_t,last_pos));
        string a = input.substr(last_pos); // get the rest of the string
        output_array.push_back(a);
        i++;
    }
    if (CLIENT_VERBOSE3)
        printf("\t returning %d\n",i);
    return i;
}

vector<string> 
tokenize(const string& s,
         const string& delimiter) {
    vector<string> v;
    (void) xed_split_args(delimiter, s, v);
    return v;
}


void slash_split(const string& src,
                 string& first, // output
                 string&  second) //output
{
  string::size_type p = src.find("/");
  if (p == string::npos) {
    first = src;
    second = "";
  }
  else {
    first = src.substr(0,p);
    second = src.substr(p+1);
  }
}

class immed_parser_t {
  public:
    xed_bool_t valid;
    string immed;
    unsigned int width_bits;
    xed_uint64_t immed_val;
    string tok0;

    immed_parser_t(const string& s, 
                   const string& arg_tok0) //CONS
        : valid(false),
          tok0(arg_tok0)
    {
        vector<string> vs = tokenize(s,"(),");
        if (vs.size() == 2) {
            if (vs[0] == tok0) {
                string immed_str = vs[1];
                immed_val = convert_ascii_hex_to_int(immed_str.c_str());
                width_bits = static_cast<unsigned int>(immed_str.size()*4); // nibbles to bits
                valid = true;
            }
        }
    }

    void
    print(ostream& o) const {
        o << tok0 
          << "(" ;
        if (valid) 
            o << hex << immed_val << dec;
        else 
            o << "???";
        o << ")";
    }

};

ostream& operator<<(ostream& o, const immed_parser_t& x) 
{
    x.print(o);
    return o;
}


class seg_parser_t
{
  public:
    xed_bool_t valid;
    xed_reg_enum_t segment_reg;
    string segment;

    seg_parser_t(const string& s) // CONS
        : valid(false),
          segment_reg(XED_REG_INVALID)
    {
        vector<string> vs = tokenize(s,"(),");
        xed_uint_t ntokens = static_cast<xed_uint_t>(vs.size());
        cerr << "ntokens " << ntokens << ": " ;
        for(unsigned int i=0;i<ntokens;i++) cerr  << '[' << vs[i] << "] ";
        cerr << endl;
        if (ntokens == 2 && vs[0] == "SEG") {
            segment = vs[1];
            segment_reg = str2xed_reg_enum_t(segment.c_str());
            if (segment_reg != XED_REG_INVALID && xed_reg_class(segment_reg) == XED_REG_CLASS_SR) {
                valid=true;
            }
        }
    }

};

class mem_bis_parser_t 
{
    // parse: MEM[length]([segment:]base,index,scale[,displacement])
    // parse: AGEN(base,index,scale[,displacement])
    // The displacement is optional
    // The length of the memop is usually optional 
    //   but required for x87 ops, for example.
  public:
    xed_bool_t valid;
    xed_bool_t mem;

    xed_bool_t agen;
    xed_bool_t disp_valid;
    string segment;
    string base;
    string index;
    string scale;
    string disp; //displacement
    xed_reg_enum_t segment_reg;
    xed_reg_enum_t base_reg;
    xed_reg_enum_t index_reg;
    xed_uint8_t scale_val;


    xed_int64_t disp_val;
    unsigned int disp_width_bits;

    unsigned int mem_len;

    void
    print(ostream& o) const {
        if (agen) 
            o << "AGEN"; 
        if (mem) 
            o << "MEM"; 
        if (mem_len) 
            o << setw(1) << mem_len;
        o << "(";
        if (segment_reg != XED_REG_INVALID)
            o << segment_reg << ":";
        o << base_reg;
        o << "," << index_reg 
          << "," 
          << (unsigned int) scale_val;
        if (disp_valid) 
            o <<  "," << disp;
        o << ")";
     
    }
  
    mem_bis_parser_t(const string& s) //CONS
        : valid(false),
          disp_valid(false),
          base("INVALID"),
          index("INVALID"),
          scale("1"),
          segment_reg(XED_REG_INVALID),
          base_reg(XED_REG_INVALID),
          index_reg(XED_REG_INVALID),
          disp_val(0),
          disp_width_bits(0),
          mem_len(0)
    {

        mem = false;
        agen = false;
        vector<string> vs = tokenize(s,"(),");
        xed_uint_t ntokens = static_cast<xed_uint_t>(vs.size());
        if (ntokens >= 2 && ntokens <= 5) {
            if (vs[0] == "AGEN") {
                agen = true;
            }
            else if (vs[0].substr(0,3) == "MEM") {
                mem = true;
                if (vs[0].size() > 3) {
                    string len = vs[0].substr(3);
                    mem_len = strtol(len.c_str(),0,0);
                    //printf("mem_len  = " XED_FMT_U "\n", mem_len);
                }
            }
            else             {
                return;
            }

            segment = "INVALID";
            string seg_and_base = upcase(vs[1]);
            vector<string> sb = tokenize(seg_and_base,":");
            int seg_and_base_tokens = STATIC_CAST(int,sb.size());
            if (seg_and_base_tokens == 1) {
                segment = "INVALID";
                base = sb[0];
            }
            else if (seg_and_base_tokens == 2) {
                if (agen) {
                    xedex_derror("AGENs cannot have segment overrides");
                }
                segment = sb[0];
                base = sb[1];
            }
            else            {
                printf("seg_and_base_tokens = %d\n",seg_and_base_tokens);
                xedex_derror("Bad segment-and-base specifier.");
            }

            if (base == "-" || base == "NA") {
                base = "INVALID";
            }
            if (ntokens > 2) {
                index = upcase(vs[2]);
                if (index == "-" || index == "NA") {
                    index = "INVALID";
                }
            }

            if (ntokens > 3) {
                scale = vs[3];
                if (scale == "-" || scale == "NA") {
                    scale = "1";
                }
            }
            if (scale == "1" || scale == "2" || scale == "4" || scale == "8") {
                valid=true;
                scale_val = STATIC_CAST(xed_uint8_t,strtol(scale.c_str(), 0, 10));
                segment_reg = str2xed_reg_enum_t(segment.c_str());
                base_reg = str2xed_reg_enum_t(base.c_str());
                index_reg = str2xed_reg_enum_t(index.c_str());

                // look for a displacement
                if (ntokens == 5 && vs[4] != "-") {
                    disp = vs[4];
                    disp_valid = true;
                    unsigned int nibbles = STATIC_CAST(int,disp.size());
                    if (nibbles & 1) {
                        // ensure an even number of nibbles
                        string zero("0");
                        disp = zero + disp;
                        nibbles++;
                    }
                    disp_val = convert_ascii_hex_to_int(disp.c_str());
                    disp_width_bits = nibbles*4; // nibbles to bits
                }
            }

        }
    
    }
};

ostream& operator<<(ostream& o, const mem_bis_parser_t& x) {
  x.print(o);
  return o;
}

xed_encoder_request_t parse_encode_request(ascii_encode_request_t& areq) {
    unsigned int i;
    xed_encoder_request_t req;
    xed_encoder_request_zero_set_mode(&req,&(areq.dstate)); // calls xed_encoder_request_zero()

    /* This is the important function here. This encodes an instruction from scratch.
       
    You must set:
    the machine mode (machine width, addressing widths)
    the effective operand width
    the iclass
    for some instructions you need to specify prefixes (like REP or LOCK).
    the operands:
           operand kind (XED_OPERAND_{AGEN,MEM0,MEM1,IMM0,IMM1,RELBR,PTR,REG0...REG15}
           operand order 
                    xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_*);
                    where the operand_index is a sequential index starting at zero.

           operand details 
                     FOR MEMOPS: base,segment,index,scale,displacement for memops, 
                  FOR REGISTERS: register name
                 FOR IMMEDIATES: immediate values
       
     */
    

    switch(xed_state_get_machine_mode(&(areq.dstate))) {
        // set the default width.
      case XED_MACHINE_MODE_LONG_64:
        xed_encoder_request_set_effective_operand_width(&req, 32);
        xed_encoder_request_set_effective_address_size(&req, 64);
        break;

      case XED_MACHINE_MODE_LEGACY_32:
      case XED_MACHINE_MODE_LONG_COMPAT_32:
        xed_encoder_request_set_effective_operand_width(&req, 32);
        xed_encoder_request_set_effective_address_size(&req, 32);
        break;

      case XED_MACHINE_MODE_LEGACY_16:
      case XED_MACHINE_MODE_LONG_COMPAT_16:
        xed_encoder_request_set_effective_operand_width(&req, 16);
        xed_encoder_request_set_effective_address_size(&req, 16);
        break;

      default:
        assert(0);
    }

    //FIXME: allow changing the effective address size from the above defaults.

    vector<string> tokens = tokenize(areq.command," ");
    // first token has the operand and our temporary hack for the immediate

    string first, second;
    unsigned int token_index = 0;

    while(token_index < tokens.size()) {
        slash_split(tokens[token_index], first, second);
        if (CLIENT_VERBOSE3)
            printf( "[%s][%s][%s]\n", tokens[0].c_str(), first.c_str(), second.c_str());

        if (token_index == 0 && first == "REP") {
            xed_encoder_request_set_rep(&req);
            token_index++;
            continue;
        }
        else if (token_index == 0 && first == "REPNE") {
            xed_encoder_request_set_repne(&req);
            token_index++;
            continue;
        }
  
        token_index++;
        break;
    }

    // we can attempt to override the mode 
    if (second == "8") 
        xed_encoder_request_set_effective_operand_width(&req, 8);
    else if (second == "16") 
        xed_encoder_request_set_effective_operand_width(&req, 16);
    else if (second == "32") 
        xed_encoder_request_set_effective_operand_width(&req, 32);
    else if (second == "64") 
        xed_encoder_request_set_effective_operand_width(&req, 64);

    first = upcase(first);
    xed_iclass_enum_t iclass =  str2xed_iclass_enum_t(first.c_str());
    if (iclass == XED_ICLASS_INVALID) {
        ostringstream os;
        os << "Bad instruction name: " << first;
        xedex_derror(os.str().c_str());
    }
    xed_encoder_request_set_iclass(&req, iclass );

    xed_uint_t memop = 0;
    xed_uint_t regnum = 0;
    // put the operands in the request. Loop through tokens 
    // (skip the opcode iclass, handled above)
    xed_uint_t operand_index = 0;
    for( i=token_index; i < tokens.size(); i++, operand_index++ ) {
        string str_res_reg, second_x;
        slash_split(tokens[i], str_res_reg, second_x);
        str_res_reg = upcase(str_res_reg);
        // prune the AGEN or MEM(base,index,scale[,displacement]) text from str_res_reg
        // FIXME: add MEM(immed) for the OC1_A and OC1_O types????
        mem_bis_parser_t mem_bis(str_res_reg);
        if (mem_bis.valid) {
            if (mem_bis.mem) {
                if (memop == 0) {
                    // Tell XED that we have a memory operand
                    xed_encoder_request_set_mem0(&req);
                    // Tell XED that the mem0 operand is the next operand:
                    xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_MEM0);
                }
                else {
                    xed_encoder_request_set_mem1(&req);
                    // Tell XED that the mem1 operand is the next operand:
                    xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_MEM1);
                }
                memop++;
            }
            else if (mem_bis.agen) {
                // Tell XED we have an AGEN
                xed_encoder_request_set_agen(&req);
                // The AGEN is the next operand
                xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_AGEN);
            }
            else 
                assert(mem_bis.agen || mem_bis.mem);

            xed_reg_class_enum_t rc = xed_gpr_reg_class(mem_bis.base_reg);
            xed_reg_class_enum_t rci = xed_gpr_reg_class(mem_bis.index_reg);
            if (mem_bis.base_reg != XED_REG_INVALID && mem_bis.index_reg != XED_REG_INVALID) 
                if (rc != rci) {
                    ostringstream os;
                    os << "The base and index regs do not agree on the address size" << endl;
                    xedex_derror(os.str().c_str()); // dies
                }
            
            if (rc == XED_REG_CLASS_GPR32 || rci == XED_REG_CLASS_GPR32) 
                xed_encoder_request_set_effective_address_size(&req, 32);

            // fill in the memory fields
            xed_encoder_request_set_base0(&req, mem_bis.base_reg);
            xed_encoder_request_set_index(&req, mem_bis.index_reg);
            xed_encoder_request_set_scale(&req, mem_bis.scale_val);
            xed_encoder_request_set_seg0(&req, mem_bis.segment_reg);

            if (mem_bis.mem_len) 
                xed_encoder_request_set_memory_operand_length(&req, mem_bis.mem_len ); // BYTES
            if (mem_bis.disp_valid)
                xed_encoder_request_set_memory_displacement(&req,
                                                            mem_bis.disp_val,
                                                            mem_bis.disp_width_bits/8);
            continue;
        }

        seg_parser_t seg_parser(str_res_reg);
        if (seg_parser.valid) {
            printf("Setting segment to %s\n", xed_reg_enum_t2str(seg_parser.segment_reg));
            xed_encoder_request_set_seg0(&req, seg_parser.segment_reg);
            xed_encoder_request_set_operand_order(&req, operand_index, XED_OPERAND_SEG0);
            continue;
        }

        immed_parser_t imm(str_res_reg, "IMM");
        if (imm.valid) {
            if (CLIENT_VERBOSE3) 
                printf("Setting immediate value to " XED_FMT_LX "\n", imm.immed_val);
            xed_encoder_request_set_uimm0_bits(&req, 
                                               imm.immed_val,
                                               imm.width_bits);
            xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_IMM0);
            continue;
        }
        immed_parser_t simm(str_res_reg, "SIMM");
        if (simm.valid) {
            if (CLIENT_VERBOSE3) 
                printf("Setting immediate value to " XED_FMT_LX "\n", simm.immed_val);
            xed_encoder_request_set_simm(&req, 
                                         STATIC_CAST(xed_int32_t,simm.immed_val),
                                         simm.width_bits/8); //FIXME
            xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_IMM0);
            continue;
        }
        immed_parser_t imm2(str_res_reg, "IMM2");
        if (imm2.valid) {
            if (imm2.width_bits != 8)
                xedex_derror("2nd immediate must be just 1 byte long");
            xed_encoder_request_set_uimm1(&req, imm2.immed_val);
            xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_IMM1);
            continue;
        }

        immed_parser_t disp(str_res_reg, "BRDISP");
        if (disp.valid) {
            if (CLIENT_VERBOSE3) 
                printf("Setting  displacement value to " XED_FMT_LX "\n", disp.immed_val);
            xed_encoder_request_set_branch_displacement(&req,
                                                        STATIC_CAST(xed_uint32_t,disp.immed_val),
                                                        disp.width_bits/8); //FIXME
            xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_RELBR);
            xed_encoder_request_set_relbr(&req);
            continue;
        }

        immed_parser_t ptr_disp(str_res_reg, "PTR");
        if (ptr_disp.valid) {
            if (CLIENT_VERBOSE3) 
                printf("Setting pointer displacement value to " XED_FMT_LX "\n", ptr_disp.immed_val);
            xed_encoder_request_set_branch_displacement(&req,
                                                        STATIC_CAST(xed_uint32_t,ptr_disp.immed_val),
                                                        ptr_disp.width_bits/8); //FIXME
            xed_encoder_request_set_operand_order(&req,operand_index, XED_OPERAND_PTR);
            xed_encoder_request_set_ptr(&req);
            continue;
        }

        xed_reg_enum_t reg = str2xed_reg_enum_t(str_res_reg.c_str());
        if (reg == XED_REG_INVALID) {
            ostringstream os;
            os << "Bad register name: " << str_res_reg << " on operand " << i;
            xedex_derror(os.str().c_str()); // dies
        }
        // The registers operands aer numbered starting from the first one
        // as XED_OPERAND_REG0. We incremenet regnum (below) every time we add a
        // register operands.
        xed_operand_enum_t r = STATIC_CAST(xed_operand_enum_t,XED_OPERAND_REG0 + regnum);
        // store the register identifer in the operand storage field
        xed_encoder_request_set_reg(&req, r, reg);
        // store the operand storage field name in the encode-order array
        xed_encoder_request_set_operand_order(&req, operand_index, r);
        regnum++;
    } // for loop

    return req;
}

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