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<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="SalientComment">## String comparison: return 1 iff the two args passed in at the commandline are equal.</span>
<span id="L2" class="LineNr"> 2 </span><span class="Comment">#</span>
<span id="L3" class="LineNr"> 3 </span><span class="Comment"># To run (from the subx directory):</span>
<span id="L4" class="LineNr"> 4 </span><span class="Comment">#   $ subx translate examples/ex10.subx -o examples/ex10</span>
<span id="L5" class="LineNr"> 5 </span><span class="Comment">#   $ subx run examples/ex10 abc abd</span>
<span id="L6" class="LineNr"> 6 </span><span class="Comment"># Expected result:</span>
<span id="L7" class="LineNr"> 7 </span><span class="Comment">#   $ echo $?</span>
<span id="L8" class="LineNr"> 8 </span><span class="Comment">#   0  # false</span>
<span id="L9" class="LineNr"> 9 </span>
<span id="L10" class="LineNr">10 </span>== code
<span id="L11" class="LineNr">11 </span><span class="Comment"># instruction                     effective address                                                   operand     displacement    immediate</span>
<span id="L12" class="LineNr">12 </span><span class="Comment"># op          subop               mod             rm32          base        index         scale       r32</span>
<span id="L13" class="LineNr">13 </span><span class="Comment"># 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes</span>
<span id="L14" class="LineNr">14 </span>
<span id="L15" class="LineNr">15 </span><span class="Comment"># main: return argv_equal(argv[1], argv[2])</span>
<span id="L16" class="LineNr">16 </span><span class="Comment">#       At the start of a SubX program:</span>
<span id="L17" class="LineNr">17 </span><span class="Comment">#         argc: *ESP</span>
<span id="L18" class="LineNr">18 </span><span class="Comment">#         argv[0]: *(ESP+4)</span>
<span id="L19" class="LineNr">19 </span><span class="Comment">#         argv[1]: *(ESP+8)</span>
<span id="L20" class="LineNr">20 </span><span class="Comment">#         ...</span>
<span id="L21" class="LineNr">21 </span>  <span class="Comment"># prolog</span>
<span id="L22" class="LineNr">22 </span>  89/copy                         3/mod/direct    5/rm32/EBP   <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>          4/r32/ESP  <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>                <span class="Comment"># copy ESP to EBP</span>
<span id="L23" class="LineNr">23 </span>  <span class="Comment"># call argv_equal(argv[1], argv[2])</span>
<span id="L24" class="LineNr">24 </span>    <span class="Comment"># push argv[2]</span>
<span id="L25" class="LineNr">25 </span>  ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>          0xc/disp8      <span class="Delimiter"> . </span>                <span class="Comment"># push *(EBP+12)</span>
<span id="L26" class="LineNr">26 </span>    <span class="Comment"># push argv[1]</span>
<span id="L27" class="LineNr">27 </span>  ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>          0x8/disp8      <span class="Delimiter"> . </span>                <span class="Comment"># push *(EBP+8)</span>
<span id="L28" class="LineNr">28 </span>    <span class="Comment"># call</span>
<span id="L29" class="LineNr">29 </span>  e8/call argv_equal/disp32
<span id="L30" class="LineNr">30 </span>  <span class="Comment"># exit(EAX)</span>
<span id="L31" class="LineNr">31 </span>$exit:
<span id="L32" class="LineNr">32 </span>  89/copy                         3/mod/direct    3/rm32/EBX   <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>          0/r32/EAX  <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>                <span class="Comment"># copy EAX to EBX</span>
<span id="L33" class="LineNr">33 </span>  b8/copy                        <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>              1/imm32           <span class="Comment"># copy to EAX</span>
<span id="L34" class="LineNr">34 </span>  cd/syscall  0x80/imm8
<span id="L35" class="LineNr">35 </span>
<span id="L36" class="LineNr">36 </span><span class="Comment"># compare two null-terminated ascii strings</span>
<span id="L37" class="LineNr">37 </span><span class="Comment"># reason for the name: the only place we should have null-terminated ascii strings is from commandline args</span>
<span id="L38" class="LineNr">38 </span>argv_equal:  <span class="Comment"># (s1, s2) : null-terminated ascii strings -&gt; EAX : boolean</span>
<span id="L39" class="LineNr">39 </span>  <span class="Comment"># initialize s1 (ECX) and s2 (EDX)</span>
<span id="L40" class="LineNr">40 </span>  8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none <span class="Delimiter"> . </span>          1/r32/ECX   4/disp8        <span class="Delimiter"> . </span>                <span class="Comment"># copy *(ESP+4) to ECX</span>
<span id="L41" class="LineNr">41 </span>  8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none <span class="Delimiter"> . </span>          2/r32/EDX   8/disp8        <span class="Delimiter"> . </span>                <span class="Comment"># copy *(ESP+8) to EDX</span>
<span id="L42" class="LineNr">42 </span>  <span class="Comment"># while (true)</span>
<span id="L43" class="LineNr">43 </span>$argv_loop:
<span id="L44" class="LineNr">44 </span>    <span class="Comment"># c1/EAX, c2/EBX = *s1, *s2</span>
<span id="L45" class="LineNr">45 </span>  b8/copy  0/imm32  <span class="Comment"># clear EAX</span>
<span id="L46" class="LineNr">46 </span>  8a/copy                         0/mod/indirect  1/rm32/ECX   <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>          0/r32/EAX  <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>                <span class="Comment"># copy byte at *ECX to lower byte of EAX</span>
<span id="L47" class="LineNr">47 </span>  bb/copy  0/imm32  <span class="Comment"># clear EBX</span>
<span id="L48" class="LineNr">48 </span>  8a/copy                         0/mod/indirect  2/rm32/EDX   <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>          3/r32/EBX  <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>                <span class="Comment"># copy byte at *EDX to lower byte of EBX</span>
<span id="L49" class="LineNr">49 </span>    <span class="Comment"># if (c1 == 0) break</span>
<span id="L50" class="LineNr">50 </span>  3d/compare-EAX  0/imm32
<span id="L51" class="LineNr">51 </span>  74/jump-if-equal  $argv_break/disp8
<span id="L52" class="LineNr">52 </span>    <span class="Comment"># if (c1 != c2) return false</span>
<span id="L53" class="LineNr">53 </span>  39/compare                      3/mod/direct    0/rm32/EAX   <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>          3/r32/EBX  <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>                <span class="Comment"># compare EAX with EBX</span>
<span id="L54" class="LineNr">54 </span>  75/jump-if-not-equal  $argv_fail/disp8
<span id="L55" class="LineNr">55 </span>    <span class="Comment"># ++s1, ++s2</span>
<span id="L56" class="LineNr">56 </span>  41/inc-ECX
<span id="L57" class="LineNr">57 </span>  42/inc-EDX
<span id="L58" class="LineNr">58 </span>  <span class="Comment"># end while</span>
<span id="L59" class="LineNr">59 </span>  eb/jump  $argv_loop/disp8
<span id="L60" class="LineNr">60 </span>$argv_break:
<span id="L61" class="LineNr">61 </span>  <span class="Comment"># if (c2 == 0) return true</span>
<span id="L62" class="LineNr">62 </span>  81          7/subop/compare     3/mod/direct    3/rm32/EBX   <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>              0/imm32           <span class="Comment"># compare EBX</span>
<span id="L63" class="LineNr">63 </span>  75/jump-if-not-equal  $argv_fail/disp8
<span id="L64" class="LineNr">64 </span>  b8/copy                        <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>              1/imm32           <span class="Comment"># copy to EAX</span>
<span id="L65" class="LineNr">65 </span>  c3/return
<span id="L66" class="LineNr">66 </span>  <span class="Comment"># return false</span>
<span id="L67" class="LineNr">67 </span>$argv_fail:
<span id="L68" class="LineNr">68 </span>  b8/copy                        <span class="Delimiter"> . </span>             <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>           <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>         <span class="Delimiter"> . </span>              0/imm32           <span class="Comment"># copy to EAX</span>
<span id="L69" class="LineNr">69 </span>  c3/return
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