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<a href='https://github.com/akkartik/mu/blob/master/subx/apps/factorial.subx'>https://github.com/akkartik/mu/blob/master/subx/apps/factorial.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr">  1 </span><span class="subxComment">## compute the factorial of 5, and return the result in the exit code</span>
<span id="L2" class="LineNr">  2 </span><span class="subxComment">#</span>
<span id="L3" class="LineNr">  3 </span><span class="subxComment"># To run (from the subx directory):</span>
<span id="L4" class="LineNr">  4 </span><span class="subxComment">#   $ ./subx translate apps/factorial.subx -o apps/factorial</span>
<span id="L5" class="LineNr">  5 </span><span class="subxComment">#   $ ./subx run apps/factorial</span>
<span id="L6" class="LineNr">  6 </span><span class="subxComment"># Expected result:</span>
<span id="L7" class="LineNr">  7 </span><span class="subxComment">#   $ echo $?</span>
<span id="L8" class="LineNr">  8 </span><span class="subxComment">#   120</span>
<span id="L9" class="LineNr">  9 </span><span class="subxComment">#</span>
<span id="L10" class="LineNr"> 10 </span><span class="subxComment"># You can also run the automated test suite:</span>
<span id="L11" class="LineNr"> 11 </span><span class="subxComment">#   $ ./subx run apps/factorial test</span>
<span id="L12" class="LineNr"> 12 </span><span class="subxComment"># Expected output:</span>
<span id="L13" class="LineNr"> 13 </span><span class="subxComment">#   ........</span>
<span id="L14" class="LineNr"> 14 </span><span class="subxComment"># Every '.' indicates a passing test. Failing tests get a 'F'.</span>
<span id="L15" class="LineNr"> 15 </span>
<span id="L16" class="LineNr"> 16 </span>== code
<span id="L17" class="LineNr"> 17 </span><span class="subxComment">#   instruction                     effective address                                                   register    displacement    immediate</span>
<span id="L18" class="LineNr"> 18 </span><span class="subxS1Comment"># . op          subop               mod             rm32          base        index         scale       r32</span>
<span id="L19" class="LineNr"> 19 </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="L20" class="LineNr"> 20 </span>
<span id="L21" class="LineNr"> 21 </span><span class="SpecialChar">Entry</span>:  <span class="subxComment"># run tests if necessary, compute `factorial(5)` if not</span>
<span id="L22" class="LineNr"> 22 </span>
<span id="L23" class="LineNr"> 23 </span><span class="CommentedCode">#?     # for debugging: run a single test; don't bother setting status code</span>
<span id="L24" class="LineNr"> 24 </span><span class="CommentedCode">#?     e8/call test-get-num-reads-single-digit/disp32</span>
<span id="L25" class="LineNr"> 25 </span><span class="CommentedCode">#?     eb/jump  $main:end/disp8</span>
<span id="L26" class="LineNr"> 26 </span>
<span id="L27" class="LineNr"> 27 </span>    <span class="subxS1Comment"># . prolog</span>
<span id="L28" class="LineNr"> 28 </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="L29" class="LineNr"> 29 </span>    <span class="subxH1Comment"># - if argc &gt; 1 and argv[1] == &quot;test&quot;, then return run_tests()</span>
<span id="L30" class="LineNr"> 30 </span>    <span class="subxS1Comment"># . argc &gt; 1</span>
<span id="L31" class="LineNr"> 31 </span>    81          7/subop/compare     1/mod/*+disp8   5/rm32/EBP   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>          0/disp8         1/imm32           <span class="subxComment"># compare *EBP</span>
<span id="L32" class="LineNr"> 32 </span>    7e/jump-if-lesser-or-equal  $run-main/disp8
<span id="L33" class="LineNr"> 33 </span>    <span class="subxS1Comment"># . argv[1] == &quot;test&quot;</span>
<span id="L34" class="LineNr"> 34 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L35" class="LineNr"> 35 </span>    68/push  <span class="Constant">&quot;test&quot;</span>/imm32
<span id="L36" class="LineNr"> 36 </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="L37" class="LineNr"> 37 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L38" class="LineNr"> 38 </span>    e8/call  <a href='../052kernel-string-equal.subx.html#L31'>kernel-string-equal?</a>/disp32
<span id="L39" class="LineNr"> 39 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L40" class="LineNr"> 40 </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="L41" class="LineNr"> 41 </span>    <span class="subxS1Comment"># . check result</span>
<span id="L42" class="LineNr"> 42 </span>    3d/compare-EAX-and  1/imm32
<span id="L43" class="LineNr"> 43 </span>    75/jump-if-not-equal  $run-main/disp8
<span id="L44" class="LineNr"> 44 </span>    <span class="subxS1Comment"># . run-tests()</span>
<span id="L45" class="LineNr"> 45 </span>    e8/call  run-tests/disp32
<span id="L46" class="LineNr"> 46 </span>    8b/copy                         0/mod/indirect  5/rm32/.disp32           <span class="Normal"> . </span>           <span class="Normal"> . </span>          0/r32/EAX   <span class="SpecialChar"><a href='../051test.subx.html#L90'>Num-test-failures</a></span>/disp32          <span class="subxComment"># copy *Num-test-failures to EAX</span>
<span id="L47" class="LineNr"> 47 </span>    eb/jump  $main:end/disp8  <span class="subxComment"># where EAX will get copied to EBX</span>
<span id="L48" class="LineNr"> 48 </span><span class="Constant">$run-main</span>:
<span id="L49" class="LineNr"> 49 </span>    <span class="subxH1Comment"># - otherwise return factorial(5)</span>
<span id="L50" class="LineNr"> 50 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L51" class="LineNr"> 51 </span>    68/push  5/imm32
<span id="L52" class="LineNr"> 52 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L53" class="LineNr"> 53 </span>    e8/call  <a href='factorial.subx.html#L62'>factorial</a>/disp32
<span id="L54" class="LineNr"> 54 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L55" class="LineNr"> 55 </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>              4/imm32           <span class="subxComment"># add to ESP</span>
<span id="L56" class="LineNr"> 56 </span><span class="Constant">$main:end</span>:
<span id="L57" class="LineNr"> 57 </span>    <span class="subxComment"># syscall(exit, EAX)</span>
<span id="L58" class="LineNr"> 58 </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="L59" class="LineNr"> 59 </span>    b8/copy-to-EAX  1/imm32/exit
<span id="L60" class="LineNr"> 60 </span>    cd/syscall  0x80/imm8
<span id="L61" class="LineNr"> 61 </span>
<span id="L62" class="LineNr"> 62 </span><span class="subxFunction">factorial</span>:  <span class="subxComment"># n : int -&gt; int/EAX</span>
<span id="L63" class="LineNr"> 63 </span>    <span class="subxS1Comment"># . prolog</span>
<span id="L64" class="LineNr"> 64 </span>    55/push-EBP
<span id="L65" class="LineNr"> 65 </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="L66" class="LineNr"> 66 </span>    53/push-EBX
<span id="L67" class="LineNr"> 67 </span>    <span class="subxComment"># EAX = 1 (base case)</span>
<span id="L68" class="LineNr"> 68 </span>    b8/copy-to-EAX  1/imm32
<span id="L69" class="LineNr"> 69 </span>    <span class="subxComment"># if (n &lt;= 1) return</span>
<span id="L70" class="LineNr"> 70 </span>    81          7/subop/compare     1/mod/*+disp8   5/rm32/EBP   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>          8/disp8         1/imm32           <span class="subxComment"># compare *(EBP+8)</span>
<span id="L71" class="LineNr"> 71 </span>    7e/jump-if-&lt;=  $factorial:end/disp8
<span id="L72" class="LineNr"> 72 </span>    <span class="subxComment"># EBX = n-1</span>
<span id="L73" class="LineNr"> 73 </span>    8b/copy                         1/mod/*+disp8   5/rm32/EBP   <span class="Normal"> . </span>         <span class="Normal"> . </span>                        3/r32/EBX   8/disp8        <span class="Normal"> . </span>                <span class="subxComment"># copy *(EBP+8) to EBX</span>
<span id="L74" class="LineNr"> 74 </span>    81          5/subop/subtract    3/mod/direct    3/rm32/EBX   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>         <span class="Normal"> . </span>              1/imm32           <span class="subxComment"># subtract from EBX</span>
<span id="L75" class="LineNr"> 75 </span>    <span class="subxComment"># EAX = factorial(n-1)</span>
<span id="L76" class="LineNr"> 76 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L77" class="LineNr"> 77 </span>    53/push-EBX
<span id="L78" class="LineNr"> 78 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L79" class="LineNr"> 79 </span>    e8/call  <a href='factorial.subx.html#L62'>factorial</a>/disp32
<span id="L80" class="LineNr"> 80 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L81" class="LineNr"> 81 </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>              4/imm32           <span class="subxComment"># add to ESP</span>
<span id="L82" class="LineNr"> 82 </span>    <span class="subxComment"># return n * factorial(n-1)</span>
<span id="L83" class="LineNr"> 83 </span>    f7          4/subop/multiply    1/mod/*+disp8   5/rm32/EBP   <span class="Normal"> . </span>         <span class="Normal"> . </span>                                    8/disp8        <span class="Normal"> . </span>                <span class="subxComment"># multiply *(EBP+8) into EAX</span>
<span id="L84" class="LineNr"> 84 </span>    <span class="subxComment"># TODO: check for overflow</span>
<span id="L85" class="LineNr"> 85 </span><span class="Constant">$factorial:end</span>:
<span id="L86" class="LineNr"> 86 </span>    <span class="subxS1Comment"># . epilog</span>
<span id="L87" class="LineNr"> 87 </span>    5b/pop-to-EBX
<span id="L88" class="LineNr"> 88 </span>    89/copy                         3/mod/direct    4/rm32/ESP   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          5/r32/EBP  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy EBP to ESP</span>
<span id="L89" class="LineNr"> 89 </span>    5d/pop-to-EBP
<span id="L90" class="LineNr"> 90 </span>    c3/return
<span id="L91" class="LineNr"> 91 </span>
<span id="L92" class="LineNr"> 92 </span><span class="subxTest">test-factorial</span>:
<span id="L93" class="LineNr"> 93 </span>    <span class="subxComment"># factorial(5)</span>
<span id="L94" class="LineNr"> 94 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L95" class="LineNr"> 95 </span>    68/push  5/imm32
<span id="L96" class="LineNr"> 96 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L97" class="LineNr"> 97 </span>    e8/call  <a href='factorial.subx.html#L62'>factorial</a>/disp32
<span id="L98" class="LineNr"> 98 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L99" class="LineNr"> 99 </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>              4/imm32           <span class="subxComment"># add to ESP</span>
<span id="L100" class="LineNr">100 </span>    <span class="subxComment"># check-ints-equal(EAX, 120, msg)</span>
<span id="L101" class="LineNr">101 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L102" class="LineNr">102 </span>    68/push  <span class="Constant">&quot;F - test-factorial&quot;</span>/imm32
<span id="L103" class="LineNr">103 </span>    68/push  0x78/imm32/expected-120
<span id="L104" class="LineNr">104 </span>    50/push-EAX
<span id="L105" class="LineNr">105 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L106" class="LineNr">106 </span>    e8/call  <a href='../051test.subx.html#L24'>check-ints-equal</a>/disp32
<span id="L107" class="LineNr">107 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L108" class="LineNr">108 </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>              0xc/imm32         <span class="subxComment"># add to ESP</span>
<span id="L109" class="LineNr">109 </span>    <span class="subxComment"># end</span>
<span id="L110" class="LineNr">110 </span>    c3/return
<span id="L111" class="LineNr">111 </span>
<span id="L112" class="LineNr">112 </span><span class="subxS2Comment"># . . vim&#0058;nowrap:textwidth=0</span>
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