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<a href='https://github.com/akkartik/mu/blob/main/linux/ex9.subx'>https://github.com/akkartik/mu/blob/main/linux/ex9.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="subxComment"># Example showing arg order on the stack.</span>
<span id="L2" class="LineNr"> 2 </span><span class="subxComment">#</span>
<span id="L3" class="LineNr"> 3 </span><span class="subxComment"># Show difference between ascii codes of first letter of first arg and first</span>
<span id="L4" class="LineNr"> 4 </span><span class="subxComment"># letter of second arg.</span>
<span id="L5" class="LineNr"> 5 </span><span class="subxComment">#</span>
<span id="L6" class="LineNr"> 6 </span><span class="subxComment"># To run:</span>
<span id="L7" class="LineNr"> 7 </span><span class="subxComment">#   $ bootstrap/bootstrap translate ex9.subx -o ex9</span>
<span id="L8" class="LineNr"> 8 </span><span class="subxComment">#   $ bootstrap/bootstrap run ex9 z x</span>
<span id="L9" class="LineNr"> 9 </span><span class="subxComment"># Expected result:</span>
<span id="L10" class="LineNr">10 </span><span class="subxComment">#   $ echo $?</span>
<span id="L11" class="LineNr">11 </span><span class="subxComment">#   2</span>
<span id="L12" class="LineNr">12 </span><span class="subxComment">#</span>
<span id="L13" class="LineNr">13 </span><span class="subxComment"># At the start of a SubX program:</span>
<span id="L14" class="LineNr">14 </span><span class="subxComment">#   argc: *esp</span>
<span id="L15" class="LineNr">15 </span><span class="subxComment">#   argv[0]: *(esp+4)</span>
<span id="L16" class="LineNr">16 </span><span class="subxComment">#   argv[1]: *(esp+8)</span>
<span id="L17" class="LineNr">17 </span><span class="subxComment">#   ...</span>
<span id="L18" class="LineNr">18 </span><span class="subxComment"># Locals start from esp-4 downwards.</span>
<span id="L19" class="LineNr">19 </span>
<span id="L20" class="LineNr">20 </span>== code
<span id="L21" class="LineNr">21 </span><span class="subxComment">#   instruction                     effective address                                                   register    displacement    immediate</span>
<span id="L22" class="LineNr">22 </span><span class="subxS1Comment"># . op          subop               mod             rm32          base        index         scale       r32</span>
<span id="L23" class="LineNr">23 </span><span class="subxS1Comment"># . 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="L24" class="LineNr">24 </span>
<span id="L25" class="LineNr">25 </span><span class="SpecialChar">Entry</span>:
<span id="L26" class="LineNr">26 </span>    <span class="subxS1Comment"># . prologue</span>
<span id="L27" class="LineNr">27 </span>    89/copy                         3/mod/direct    5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          4/r32/esp  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy esp to ebp</span>
<span id="L28" class="LineNr">28 </span>    <span class="subxComment"># ascii-difference(argv[1], argv[2])</span>
<span id="L29" class="LineNr">29 </span>    <span class="subxS2Comment"># . . push argv[2]</span>
<span id="L30" class="LineNr">30 </span>    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>          0xc/disp8      <span class="Normal"> . </span>                <span class="subxComment"># push *(ebp+12)</span>
<span id="L31" class="LineNr">31 </span>    <span class="subxS2Comment"># . . push argv[1]</span>
<span id="L32" class="LineNr">32 </span>    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>          8/disp8        <span class="Normal"> . </span>                <span class="subxComment"># push *(ebp+8)</span>
<span id="L33" class="LineNr">33 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L34" class="LineNr">34 </span>    e8/call  <a href='ex9.subx.html#L41'>ascii-difference</a>/disp32
<span id="L35" class="LineNr">35 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L36" class="LineNr">36 </span>    81          0/subop/add         3/mod/direct    4/rm32/esp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>         <span class="Normal"> . </span>              8/imm32           <span class="subxComment"># add to esp</span>
<span id="L37" class="LineNr">37 </span>    <span class="subxComment"># exit(eax)</span>
<span id="L38" class="LineNr">38 </span>    89/copy                         3/mod/direct    3/rm32/ebx   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          0/r32/eax  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy eax to ebx</span>
<span id="L39" class="LineNr">39 </span>    e8/call  <a href='000init.subx.html#L18'>syscall_exit</a>/disp32
<span id="L40" class="LineNr">40 </span>
<span id="L41" class="LineNr">41 </span><span class="subxFunction">ascii-difference</span>:  <span class="subxComment"># (s1, s2): null-terminated ascii strings</span>
<span id="L42" class="LineNr">42 </span>    <span class="subxComment"># a = first letter of s1 (ecx)</span>
<span id="L43" class="LineNr">43 </span>    8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none <span class="Normal"> . </span>          0/r32/eax   4/disp8        <span class="Normal"> . </span>                <span class="subxComment"># copy *(esp+4) to eax</span>
<span id="L44" class="LineNr">44 </span>    8b/copy                         0/mod/indirect  0/rm32/eax   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          0/r32/eax  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy *eax to eax</span>
<span id="L45" class="LineNr">45 </span>    <span class="subxComment"># b = first letter of s2 (edx)</span>
<span id="L46" class="LineNr">46 </span>    8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none <span class="Normal"> . </span>          1/r32/ecx   8/disp8                           <span class="subxComment"># copy *(esp+8) to ecx</span>
<span id="L47" class="LineNr">47 </span>    8b/copy                         0/mod/indirect  1/rm32/ecx   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          1/r32/ecx  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy *ecx to ecx</span>
<span id="L48" class="LineNr">48 </span>    <span class="subxComment"># a-b</span>
<span id="L49" class="LineNr">49 </span>    29/subtract                     3/mod/direct    0/rm32/eax   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          1/r32/ecx  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># subtract ecx from eax</span>
<span id="L50" class="LineNr">50 </span>    c3/return
<span id="L51" class="LineNr">51 </span>
<span id="L52" class="LineNr">52 </span><span class="subxS2Comment"># . . vim&#0058;nowrap:textwidth=0</span>
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