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<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="SalientComment">## Example showing arg order on the stack.</span>
<span id="L2" class="LineNr"> 2 </span><span class="Comment"># Show difference between ascii codes of first letter of first arg and first</span>
<span id="L3" class="LineNr"> 3 </span><span class="Comment"># letter of second arg.</span>
<span id="L4" class="LineNr"> 4 </span><span class="Comment">#</span>
<span id="L5" class="LineNr"> 5 </span><span class="Comment"># To run (from the subx directory):</span>
<span id="L6" class="LineNr"> 6 </span><span class="Comment"># $ subx translate examples/ex9.subx -o examples/ex9</span>
<span id="L7" class="LineNr"> 7 </span><span class="Comment"># $ subx run examples/ex9 z x</span>
<span id="L8" class="LineNr"> 8 </span><span class="Comment"># Expected result:</span>
<span id="L9" class="LineNr"> 9 </span><span class="Comment"># $ echo $?</span>
<span id="L10" class="LineNr">10 </span><span class="Comment"># 2</span>
<span id="L11" class="LineNr">11 </span><span class="Comment">#</span>
<span id="L12" class="LineNr">12 </span><span class="Comment"># At the start of a SubX program:</span>
<span id="L13" class="LineNr">13 </span><span class="Comment"># argc: *ESP</span>
<span id="L14" class="LineNr">14 </span><span class="Comment"># argv[0]: *(ESP+4)</span>
<span id="L15" class="LineNr">15 </span><span class="Comment"># argv[1]: *(ESP+8)</span>
<span id="L16" class="LineNr">16 </span><span class="Comment"># ...</span>
<span id="L17" class="LineNr">17 </span><span class="Comment"># Locals start from ESP-4 downwards.</span>
<span id="L18" class="LineNr">18 </span>
<span id="L19" class="LineNr">19 </span>== code
<span id="L20" class="LineNr">20 </span><span class="Comment"># instruction effective address operand displacement immediate</span>
<span id="L21" class="LineNr">21 </span><span class="Comment"># op subop mod rm32 base index scale r32</span>
<span id="L22" class="LineNr">22 </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="L23" class="LineNr">23 </span>
<span id="L24" class="LineNr">24 </span> <span class="Comment"># prolog</span>
<span id="L25" class="LineNr">25 </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="L26" class="LineNr">26 </span> <span class="Comment"># call ascii-difference(argv[1], argv[2])</span>
<span id="L27" class="LineNr">27 </span> <span class="Comment"># push argv[2]</span>
<span id="L28" class="LineNr">28 </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="L29" class="LineNr">29 </span> <span class="Comment"># push argv[1]</span>
<span id="L30" class="LineNr">30 </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="L31" class="LineNr">31 </span> <span class="Comment"># call</span>
<span id="L32" class="LineNr">32 </span> e8/call ascii-difference/disp32
<span id="L33" class="LineNr">33 </span> <span class="Comment"># discard args</span>
<span id="L34" class="LineNr">34 </span> 81 0/subop/add 3/mod/direct 4/rm32/ESP <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> 8/imm32 <span class="Comment"># add to ESP</span>
<span id="L35" class="LineNr">35 </span> <span class="Comment"># exit(EAX)</span>
<span id="L36" class="LineNr">36 </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="L37" class="LineNr">37 </span> b8/copy-to-EAX 1/imm32/exit
<span id="L38" class="LineNr">38 </span> cd/syscall 0x80/imm8
<span id="L39" class="LineNr">39 </span>
<span id="L40" class="LineNr">40 </span>ascii-difference: <span class="Comment"># (s1, s2) : null-terminated ascii strings</span>
<span id="L41" class="LineNr">41 </span> <span class="Comment"># a = first letter of s1 (ECX)</span>
<span id="L42" class="LineNr">42 </span> 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 0/r32/EAX 4/disp8 <span class="Delimiter"> . </span> <span class="Comment"># copy *(ESP+4) to EAX</span>
<span id="L43" class="LineNr">43 </span> 8b/copy 0/mod/indirect 0/rm32/EAX <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 EAX</span>
<span id="L44" class="LineNr">44 </span> <span class="Comment"># b = first letter of s2 (EDX)</span>
<span id="L45" class="LineNr">45 </span> 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 1/r32/ECX 8/disp8 <span class="Comment"># copy *(ESP+8) to ECX</span>
<span id="L46" class="LineNr">46 </span> 8b/copy 0/mod/indirect 1/rm32/ECX <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> 1/r32/ECX <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Comment"># copy *ECX to ECX</span>
<span id="L47" class="LineNr">47 </span> <span class="Comment"># a-b</span>
<span id="L48" class="LineNr">48 </span> 29/subtract 3/mod/direct 0/rm32/EAX <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> 1/r32/ECX <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Comment"># subtract ECX from EAX</span>
<span id="L49" class="LineNr">49 </span> c3/return
<span id="L50" class="LineNr">50 </span>
<span id="L51" class="LineNr">51 </span><span class="Comment"># vim:nowrap:textwidth=0</span>
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