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<span id="L1" class="LineNr"> 1 </span><span class="SalientComment">## compute the factorial of 5, and return the result in the exit code</span>
<span id="L2" class="LineNr"> 2 </span><span class="Comment">#</span>
<span id="L3" class="LineNr"> 3 </span><span class="Comment"># To run:</span>
<span id="L4" class="LineNr"> 4 </span><span class="Comment"># $ subx translate apps/factorial.subx apps/factorial</span>
<span id="L5" class="LineNr"> 5 </span><span class="Comment"># $ subx run apps/factorial</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"># 120</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:</span>
<span id="L16" class="LineNr">16 </span> <span class="Comment"># prepare to make a call</span>
<span id="L17" class="LineNr">17 </span> 55/push <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> <span class="Delimiter"> . </span> <span class="Comment"># push EBP</span>
<span id="L18" class="LineNr">18 </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="L19" class="LineNr">19 </span> <span class="Comment"># factorial(5)</span>
<span id="L20" class="LineNr">20 </span> 68/push <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> 5/imm32 <span class="Comment"># push 5</span>
<span id="L21" class="LineNr">21 </span> e8/call <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> factorial/disp32
<span id="L22" class="LineNr">22 </span> <span class="Comment"># discard arg</span>
<span id="L23" class="LineNr">23 </span> 5a/pop <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> <span class="Delimiter"> . </span> <span class="Comment"># pop into EDX</span>
<span id="L24" class="LineNr">24 </span> <span class="Comment"># clean up after call</span>
<span id="L25" class="LineNr">25 </span> 89/copy 3/mod/direct 4/rm32/ESP <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> 5/r32/EBP <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Comment"># copy EBP to ESP</span>
<span id="L26" class="LineNr">26 </span> 5d/pop <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> <span class="Delimiter"> . </span> <span class="Comment"># pop to EBP</span>
<span id="L27" class="LineNr">27 </span>
<span id="L28" class="LineNr">28 </span> <span class="Comment"># exit(EAX)</span>
<span id="L29" class="LineNr">29 </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="L30" class="LineNr">30 </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 1 to EAX</span>
<span id="L31" class="LineNr">31 </span> cd/syscall <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> 0x80/imm8 <span class="Comment"># int 80h</span>
<span id="L32" class="LineNr">32 </span>
<span id="L33" class="LineNr">33 </span><span class="Comment"># factorial(n)</span>
<span id="L34" class="LineNr">34 </span>factorial:
<span id="L35" class="LineNr">35 </span> <span class="Comment"># initialize EAX to 1 (base case)</span>
<span id="L36" class="LineNr">36 </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 1 to EAX</span>
<span id="L37" class="LineNr">37 </span> <span class="Comment"># if (n <= 1) jump exit</span>
<span id="L38" class="LineNr">38 </span> 81 7/subop/compare 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> 4/disp8 1/imm32 <span class="Comment"># compare *(ESP+4) with 1</span>
<span id="L39" class="LineNr">39 </span> 7e/jump-if-<= <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> $factorial:exit/disp8 <span class="Comment"># jump if <= to $factorial:exit</span>
<span id="L40" class="LineNr">40 </span> <span class="Comment"># EBX: n-1</span>
<span id="L41" class="LineNr">41 </span> 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 3/r32/EBX 4/disp8 <span class="Delimiter"> . </span> <span class="Comment"># copy *(ESP+4) to EBX</span>
<span id="L42" class="LineNr">42 </span> 81 5/subop/subtract 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> 1/imm32 <span class="Comment"># subtract 1 from EBX</span>
<span id="L43" class="LineNr">43 </span> <span class="Comment"># prepare call</span>
<span id="L44" class="LineNr">44 </span> 55/push <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> <span class="Delimiter"> . </span> <span class="Comment"># push EBP</span>
<span id="L45" class="LineNr">45 </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="L46" class="LineNr">46 </span> <span class="Comment"># EAX: factorial(n-1)</span>
<span id="L47" class="LineNr">47 </span> 53/push <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> <span class="Delimiter"> . </span> <span class="Comment"># push EBX</span>
<span id="L48" class="LineNr">48 </span> e8/call <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> factorial/disp32
<span id="L49" class="LineNr">49 </span> <span class="Comment"># discard arg</span>
<span id="L50" class="LineNr">50 </span> 5e/pop <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> <span class="Delimiter"> . </span> <span class="Comment"># pop into ESI</span>
<span id="L51" class="LineNr">51 </span> <span class="Comment"># clean up after call</span>
<span id="L52" class="LineNr">52 </span> 89/copy 3/mod/direct 4/rm32/ESP <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> 5/r32/EBP <span class="Delimiter"> . </span> <span class="Delimiter"> . </span> <span class="Comment"># copy EBP to ESP</span>
<span id="L53" class="LineNr">53 </span> 5d/pop <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> <span class="Delimiter"> . </span> <span class="Comment"># pop to EBP</span>
<span id="L54" class="LineNr">54 </span> <span class="Comment"># refresh n</span>
<span id="L55" class="LineNr">55 </span> 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 2/r32/EDX 4/disp8 <span class="Delimiter"> . </span> <span class="Comment"># copy *(ESP+4) to EDX</span>
<span id="L56" class="LineNr">56 </span> <span class="Comment"># return n * factorial(n-1)</span>
<span id="L57" class="LineNr">57 </span> f7 4/subop/multiply 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 4/disp8 <span class="Delimiter"> . </span> <span class="Comment"># multiply *(ESP+4) (n) into EAX (factorial(n-1))</span>
<span id="L58" class="LineNr">58 </span> <span class="Comment"># TODO: check for overflow</span>
<span id="L59" class="LineNr">59 </span>$factorial:exit:
<span id="L60" class="LineNr">60 </span> c3/return
<span id="L61" class="LineNr">61 </span>
<span id="L62" class="LineNr">62 </span><span class="Comment"># vim:ft=subx:nowrap:so=0</span>
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