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<a href='https://github.com/akkartik/mu/blob/main/linux/factorial.subx'>https://github.com/akkartik/mu/blob/main/linux/factorial.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="subxComment">## compute the factorial of 5, and print the result</span>
<span id="L2" class="LineNr"> 2 </span><span class="subxComment">#</span>
<span id="L3" class="LineNr"> 3 </span><span class="subxComment"># To run:</span>
<span id="L4" class="LineNr"> 4 </span><span class="subxComment"># $ bootstrap/bootstrap translate [01]*.subx factorial.subx -o factorial</span>
<span id="L5" class="LineNr"> 5 </span><span class="subxComment"># $ bootstrap/bootstrap run 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"># $ bootstrap/bootstrap run 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="subxFunction">factorial</span>: <span class="subxComment"># n: int -> _/eax: int</span>
<span id="L22" class="LineNr"> 22 </span> <span class="subxS1Comment"># . prologue</span>
<span id="L23" class="LineNr"> 23 </span> 55/push-ebp
<span id="L24" class="LineNr"> 24 </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="L25" class="LineNr"> 25 </span> 51/push-ecx
<span id="L26" class="LineNr"> 26 </span> <span class="subxComment"># if (n <= 1) return 1</span>
<span id="L27" class="LineNr"> 27 </span> b8/copy-to-eax 1/imm32
<span id="L28" class="LineNr"> 28 </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="L29" class="LineNr"> 29 </span> 7e/jump-if-<= $factorial:end/disp8
<span id="L30" class="LineNr"> 30 </span> <span class="subxComment"># var ecx: int = n-1</span>
<span id="L31" class="LineNr"> 31 </span> 8b/copy 1/mod/*+disp8 5/rm32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> 1/r32/ecx 8/disp8 <span class="Normal"> . </span> <span class="subxComment"># copy *(ebp+8) to ecx</span>
<span id="L32" class="LineNr"> 32 </span> 49/decrement-ecx
<span id="L33" class="LineNr"> 33 </span> <span class="subxComment"># var eax: int = factorial(n-1)</span>
<span id="L34" class="LineNr"> 34 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L35" class="LineNr"> 35 </span> 51/push-ecx
<span id="L36" class="LineNr"> 36 </span> <span class="subxS2Comment"># . . call</span>
<span id="L37" class="LineNr"> 37 </span> e8/call <a href='factorial.subx.html#L21'>factorial</a>/disp32
<span id="L38" class="LineNr"> 38 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L39" class="LineNr"> 39 </span> 81 0/subop/add 3/mod/direct 4/rm32/esp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 4/imm32 <span class="subxComment"># add to esp</span>
<span id="L40" class="LineNr"> 40 </span> <span class="subxComment"># return n * factorial(n-1)</span>
<span id="L41" class="LineNr"> 41 </span> f7 4/subop/multiply 1/mod/*+disp8 5/rm32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> 8/disp8 <span class="Normal"> . </span> <span class="subxComment"># multiply *(ebp+8) into eax</span>
<span id="L42" class="LineNr"> 42 </span> <span class="subxComment"># TODO: check for overflow</span>
<span id="L43" class="LineNr"> 43 </span><span class="Constant">$factorial:end</span>:
<span id="L44" class="LineNr"> 44 </span> <span class="subxComment"># restore registers</span>
<span id="L45" class="LineNr"> 45 </span> 59/pop-to-ecx
<span id="L46" class="LineNr"> 46 </span> <span class="subxS1Comment"># . epilogue</span>
<span id="L47" class="LineNr"> 47 </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="L48" class="LineNr"> 48 </span> 5d/pop-to-ebp
<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="subxTest">test-factorial</span>:
<span id="L52" class="LineNr"> 52 </span> <span class="subxComment"># factorial(5)</span>
<span id="L53" class="LineNr"> 53 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L54" class="LineNr"> 54 </span> 68/push 5/imm32
<span id="L55" class="LineNr"> 55 </span> <span class="subxS2Comment"># . . call</span>
<span id="L56" class="LineNr"> 56 </span> e8/call <a href='factorial.subx.html#L21'>factorial</a>/disp32
<span id="L57" class="LineNr"> 57 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L58" class="LineNr"> 58 </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="L59" class="LineNr"> 59 </span> <span class="subxComment"># check-ints-equal(eax, 120, msg)</span>
<span id="L60" class="LineNr"> 60 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L61" class="LineNr"> 61 </span> 68/push <span class="Constant">"F - test-factorial"</span>/imm32
<span id="L62" class="LineNr"> 62 </span> 68/push 0x78/imm32/expected-120
<span id="L63" class="LineNr"> 63 </span> 50/push-eax
<span id="L64" class="LineNr"> 64 </span> <span class="subxS2Comment"># . . call</span>
<span id="L65" class="LineNr"> 65 </span> e8/call <a href='102test.subx.html#L23'>check-ints-equal</a>/disp32
<span id="L66" class="LineNr"> 66 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L67" class="LineNr"> 67 </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="L68" class="LineNr"> 68 </span> <span class="subxComment"># end</span>
<span id="L69" class="LineNr"> 69 </span> c3/return
<span id="L70" class="LineNr"> 70 </span>
<span id="L71" class="LineNr"> 71 </span><span class="SpecialChar">Entry</span>: <span class="subxComment"># run tests if necessary, compute `factorial(5)` if not</span>
<span id="L72" class="LineNr"> 72 </span> <span class="subxS1Comment"># . prologue</span>
<span id="L73" class="LineNr"> 73 </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="L74" class="LineNr"> 74 </span>
<span id="L75" class="LineNr"> 75 </span> <span class="subxComment"># initialize heap (needed by tests elsewhere)</span>
<span id="L76" class="LineNr"> 76 </span> <span class="subxS1Comment"># . Heap = new-segment(Heap-size)</span>
<span id="L77" class="LineNr"> 77 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L78" class="LineNr"> 78 </span> 68/push <span class="SpecialChar"><a href='120allocate.subx.html#L27'>Heap</a></span>/imm32
<span id="L79" class="LineNr"> 79 </span> ff 6/subop/push 0/mod/indirect 5/rm32/.disp32 <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="SpecialChar"><a href='120allocate.subx.html#L34'>Heap-size</a></span>/disp32 <span class="subxComment"># push *Heap-size</span>
<span id="L80" class="LineNr"> 80 </span> <span class="subxS2Comment"># . . call</span>
<span id="L81" class="LineNr"> 81 </span> e8/call <a href='104new-segment.subx.html#L40'>new-segment</a>/disp32
<span id="L82" class="LineNr"> 82 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L83" class="LineNr"> 83 </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="L84" class="LineNr"> 84 </span>
<span id="L85" class="LineNr"> 85 </span> <span class="subxComment"># if (argc <= 1) return factorial(5)</span>
<span id="L86" class="LineNr"> 86 </span><span class="Constant">$run-main</span>:
<span id="L87" class="LineNr"> 87 </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="L88" class="LineNr"> 88 </span> 7f/jump-if-> $main:run-tests/disp8
<span id="L89" class="LineNr"> 89 </span> <span class="subxComment"># eax = factorial(5)</span>
<span id="L90" class="LineNr"> 90 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L91" class="LineNr"> 91 </span> 68/push 5/imm32
<span id="L92" class="LineNr"> 92 </span> <span class="subxS2Comment"># . . call</span>
<span id="L93" class="LineNr"> 93 </span> e8/call <a href='factorial.subx.html#L21'>factorial</a>/disp32
<span id="L94" class="LineNr"> 94 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L95" class="LineNr"> 95 </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="L96" class="LineNr"> 96 </span> <span class="subxComment"># var buffer/ecx: (stream byte 10) # number of decimal digits a 32-bit number can have</span>
<span id="L97" class="LineNr"> 97 </span> 81 5/subop/subtract 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> 0xa/imm32 <span class="subxComment"># subtract from esp</span>
<span id="L98" class="LineNr"> 98 </span> 68/push 0xa/imm32/decimal-digits-in-32bit-number
<span id="L99" class="LineNr"> 99 </span> 68/push 0/imm32/read
<span id="L100" class="LineNr">100 </span> 68/push 0/imm32/write
<span id="L101" class="LineNr">101 </span> 89/copy 3/mod/direct 1/rm32/ecx <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 ecx</span>
<span id="L102" class="LineNr">102 </span> <span class="subxComment"># write-int32-decimal(buffer, eax)</span>
<span id="L103" class="LineNr">103 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L104" class="LineNr">104 </span> 50/push-eax
<span id="L105" class="LineNr">105 </span> 51/push-ecx
<span id="L106" class="LineNr">106 </span> <span class="subxS2Comment"># . . call</span>
<span id="L107" class="LineNr">107 </span> e8/call <a href='126write-int-decimal.subx.html#L8'>write-int32-decimal</a>/disp32
<span id="L108" class="LineNr">108 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L109" class="LineNr">109 </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="L110" class="LineNr">110 </span> <span class="subxComment"># write-stream(stderr, buffer)</span>
<span id="L111" class="LineNr">111 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L112" class="LineNr">112 </span> 51/push-ecx
<span id="L113" class="LineNr">113 </span> 68/push 2/imm32/stderr
<span id="L114" class="LineNr">114 </span> <span class="subxS2Comment"># . . call</span>
<span id="L115" class="LineNr">115 </span> e8/call <a href='113write-stream.subx.html#L17'>write-stream</a>/disp32
<span id="L116" class="LineNr">116 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L117" class="LineNr">117 </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="L118" class="LineNr">118 </span> <span class="subxComment"># write(stderr, Newline)</span>
<span id="L119" class="LineNr">119 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L120" class="LineNr">120 </span> 68/push <span class="SpecialChar"><a href='102test.subx.html#L82'>Newline</a></span>/imm32
<span id="L121" class="LineNr">121 </span> 68/push 2/imm32/stderr
<span id="L122" class="LineNr">122 </span> <span class="subxS2Comment"># . . call</span>
<span id="L123" class="LineNr">123 </span> e8/call <a href='108write.subx.html#L24'>write</a>/disp32
<span id="L124" class="LineNr">124 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L125" class="LineNr">125 </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="L126" class="LineNr">126 </span> <span class="subxComment">#</span>
<span id="L127" class="LineNr">127 </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="L128" class="LineNr">128 </span> eb/jump $main:end/disp8
<span id="L129" class="LineNr">129 </span><span class="Constant">$main:run-tests</span>:
<span id="L130" class="LineNr">130 </span> <span class="subxComment"># otherwise if first arg is "test", then return run_tests()</span>
<span id="L131" class="LineNr">131 </span> <span class="subxComment"># if (!kernel-string-equal?(argv[1], "test")) goto do-nothing</span>
<span id="L132" class="LineNr">132 </span> <span class="subxS1Comment"># . eax = kernel-string-equal?(argv[1], "test")</span>
<span id="L133" class="LineNr">133 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L134" class="LineNr">134 </span> 68/push <span class="Constant">"test"</span>/imm32
<span id="L135" class="LineNr">135 </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="L136" class="LineNr">136 </span> <span class="subxS2Comment"># . . call</span>
<span id="L137" class="LineNr">137 </span> e8/call <a href='103kernel-string-equal.subx.html#L31'>kernel-string-equal?</a>/disp32
<span id="L138" class="LineNr">138 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L139" class="LineNr">139 </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="L140" class="LineNr">140 </span> <span class="subxS1Comment"># . if (eax == false) goto do-nothing</span>
<span id="L141" class="LineNr">141 </span> 3d/compare-eax-and 0/imm32/false
<span id="L142" class="LineNr">142 </span> 74/jump-if-= $main:do-nothing/disp8
<span id="L143" class="LineNr">143 </span> <span class="subxComment"># run-tests()</span>
<span id="L144" class="LineNr">144 </span> e8/call run-tests/disp32
<span id="L145" class="LineNr">145 </span> <span class="subxComment"># exit(*Num-test-failures)</span>
<span id="L146" class="LineNr">146 </span> 8b/copy 0/mod/indirect 5/rm32/.disp32 <span class="Normal"> . </span> <span class="Normal"> . </span> 3/r32/ebx <span class="SpecialChar"><a href='102test.subx.html#L89'>Num-test-failures</a></span>/disp32 <span class="subxComment"># copy *Num-test-failures to ebx</span>
<span id="L147" class="LineNr">147 </span> eb/jump $main:end/disp8
<span id="L148" class="LineNr">148 </span><span class="Constant">$main:do-nothing</span>:
<span id="L149" class="LineNr">149 </span> bb/copy-to-ebx 0/imm32
<span id="L150" class="LineNr">150 </span><span class="Constant">$main:end</span>:
<span id="L151" class="LineNr">151 </span> e8/call <a href='000init.subx.html#L18'>syscall_exit</a>/disp32
<span id="L152" class="LineNr">152 </span>
<span id="L153" class="LineNr">153 </span><span class="subxS2Comment"># . . vim:nowrap:textwidth=0</span>
</pre>
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