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<a href='https://github.com/akkartik/mu/blob/main/baremetal/115write-byte.subx'>https://github.com/akkartik/mu/blob/main/baremetal/115write-byte.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="subxComment"># Write a single byte to a stream.</span>
<span id="L2" class="LineNr"> 2 </span><span class="subxComment">#</span>
<span id="L3" class="LineNr"> 3 </span><span class="subxComment"># We need to do this in machine code because streams need to be opaque types,</span>
<span id="L4" class="LineNr"> 4 </span><span class="subxComment"># and we don't yet support opaque types in Mu.</span>
<span id="L5" class="LineNr"> 5 </span>
<span id="L6" class="LineNr"> 6 </span>== code
<span id="L7" class="LineNr"> 7 </span><span class="subxComment"># instruction effective address register displacement immediate</span>
<span id="L8" class="LineNr"> 8 </span><span class="subxS1Comment"># . op subop mod rm32 base index scale r32</span>
<span id="L9" class="LineNr"> 9 </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="L10" class="LineNr">10 </span>
<span id="L11" class="LineNr">11 </span><span class="subxComment"># Write lower byte of 'n' to 'f'.</span>
<span id="L12" class="LineNr">12 </span><span class="subxFunction">append-byte</span>: <span class="subxComment"># f: (addr stream byte), n: int</span>
<span id="L13" class="LineNr">13 </span> <span class="subxS1Comment"># . prologue</span>
<span id="L14" class="LineNr">14 </span> 55/push-ebp
<span id="L15" class="LineNr">15 </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="L16" class="LineNr">16 </span> <span class="subxS1Comment"># . save registers</span>
<span id="L17" class="LineNr">17 </span> 51/push-ecx
<span id="L18" class="LineNr">18 </span> 57/push-edi
<span id="L19" class="LineNr">19 </span> <span class="subxComment"># edi = f</span>
<span id="L20" class="LineNr">20 </span> 8b/copy 1/mod/*+disp8 5/rm32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 7/r32/edi 8/disp8 <span class="Normal"> . </span> <span class="subxComment"># copy *(ebp+8) to edi</span>
<span id="L21" class="LineNr">21 </span> <span class="subxComment"># ecx = f->write</span>
<span id="L22" class="LineNr">22 </span> 8b/copy 0/mod/indirect 7/rm32/edi <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 *edi to ecx</span>
<span id="L23" class="LineNr">23 </span> <span class="subxComment"># if (f->write >= f->size) abort</span>
<span id="L24" class="LineNr">24 </span> 3b/compare 1/mod/*+disp8 7/rm32/edi <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 1/r32/ecx 8/disp8 <span class="Normal"> . </span> <span class="subxComment"># compare ecx with *(edi+8)</span>
<span id="L25" class="LineNr">25 </span> 7d/jump-if->= $append-byte:<a href='501draw-text.mu.html#L519'>abort</a>/disp8
<span id="L26" class="LineNr">26 </span><span class="Constant">$append-byte:to-stream</span>:
<span id="L27" class="LineNr">27 </span> <span class="subxComment"># write to stream</span>
<span id="L28" class="LineNr">28 </span> <span class="subxComment"># f->data[f->write] = LSB(n)</span>
<span id="L29" class="LineNr">29 </span> 31/xor 3/mod/direct 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"># clear eax</span>
<span id="L30" class="LineNr">30 </span> 8a/copy-byte 1/mod/*+disp8 5/rm32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 0/r32/AL 0xc/disp8 <span class="Normal"> . </span> <span class="subxComment"># copy byte at *(ebp+12) to AL</span>
<span id="L31" class="LineNr">31 </span> 88/copy-byte 1/mod/*+disp8 4/rm32/sib 7/base/edi 1/index/ecx <span class="Normal"> . </span> 0/r32/AL 0xc/disp8 <span class="Normal"> . </span> <span class="subxComment"># copy AL to *(edi+ecx+12)</span>
<span id="L32" class="LineNr">32 </span> <span class="subxComment"># ++f->write</span>
<span id="L33" class="LineNr">33 </span> ff 0/subop/increment 0/mod/indirect 7/rm32/edi <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="subxComment"># increment *edi</span>
<span id="L34" class="LineNr">34 </span><span class="Constant">$append-byte:end</span>:
<span id="L35" class="LineNr">35 </span> <span class="subxS1Comment"># . restore registers</span>
<span id="L36" class="LineNr">36 </span> 5f/pop-to-edi
<span id="L37" class="LineNr">37 </span> 59/pop-to-ecx
<span id="L38" class="LineNr">38 </span> <span class="subxS1Comment"># . epilogue</span>
<span id="L39" class="LineNr">39 </span> 89/copy 3/mod/direct 4/rm32/esp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 5/r32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="subxComment"># copy ebp to esp</span>
<span id="L40" class="LineNr">40 </span> 5d/pop-to-ebp
<span id="L41" class="LineNr">41 </span> c3/return
<span id="L42" class="LineNr">42 </span>
<span id="L43" class="LineNr">43 </span><span class="Constant">$append-byte:<a href='501draw-text.mu.html#L519'>abort</a></span>:
<span id="L44" class="LineNr">44 </span> (<a href='501draw-text.mu.html#L222'>draw-text-wrapping-right-then-down-from-cursor-over-full-screen</a> 0 <span class="Constant">"append-byte: out of space\n"</span> 3 0) <span class="subxComment"># 3=cyan</span>
<span id="L45" class="LineNr">45 </span> {
<span id="L46" class="LineNr">46 </span> eb/jump <span class="Constant">loop</span>/disp8
<span id="L47" class="LineNr">47 </span> }
<span id="L48" class="LineNr">48 </span> <span class="subxComment"># never gets here</span>
<span id="L49" class="LineNr">49 </span>
<span id="L50" class="LineNr">50 </span><span class="subxTest">test-append-byte-single</span>:
<span id="L51" class="LineNr">51 </span> <span class="subxH1Comment"># - check that append-byte writes to first byte of 'file'</span>
<span id="L52" class="LineNr">52 </span> <span class="subxComment"># setup</span>
<span id="L53" class="LineNr">53 </span> <span class="subxS1Comment"># . clear-stream(_test-stream)</span>
<span id="L54" class="LineNr">54 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L55" class="LineNr">55 </span> 68/push <a href='108write.subx.html#L125'>_test-stream</a>/imm32
<span id="L56" class="LineNr">56 </span> <span class="subxS2Comment"># . . call</span>
<span id="L57" class="LineNr">57 </span> e8/call <a href='106stream.subx.html#L20'>clear-stream</a>/disp32
<span id="L58" class="LineNr">58 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L59" class="LineNr">59 </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="L60" class="LineNr">60 </span> <span class="subxComment"># append-byte(_test-stream, 'A')</span>
<span id="L61" class="LineNr">61 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L62" class="LineNr">62 </span> 68/push 0x41/imm32
<span id="L63" class="LineNr">63 </span> 68/push <a href='108write.subx.html#L125'>_test-stream</a>/imm32
<span id="L64" class="LineNr">64 </span> <span class="subxS2Comment"># . . call</span>
<span id="L65" class="LineNr">65 </span> e8/call <a href='115write-byte.subx.html#L12'>append-byte</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> 8/imm32 <span class="subxComment"># add to esp</span>
<span id="L68" class="LineNr">68 </span> <span class="subxComment"># check-stream-equal(_test-stream, "A", msg)</span>
<span id="L69" class="LineNr">69 </span> <span class="subxS2Comment"># . . push args</span>
<span id="L70" class="LineNr">70 </span> 68/push <span class="Constant">"F - test-append-byte-single"</span>/imm32
<span id="L71" class="LineNr">71 </span> 68/push <span class="Constant">"A"</span>/imm32
<span id="L72" class="LineNr">72 </span> 68/push <a href='108write.subx.html#L125'>_test-stream</a>/imm32
<span id="L73" class="LineNr">73 </span> <span class="subxS2Comment"># . . call</span>
<span id="L74" class="LineNr">74 </span> e8/call <a href='109stream-equal.subx.html#L194'>check-stream-equal</a>/disp32
<span id="L75" class="LineNr">75 </span> <span class="subxS2Comment"># . . discard args</span>
<span id="L76" class="LineNr">76 </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="L77" class="LineNr">77 </span> <span class="subxS1Comment"># . end</span>
<span id="L78" class="LineNr">78 </span> c3/return
<span id="L79" class="LineNr">79 </span>
<span id="L80" class="LineNr">80 </span><span class="subxS2Comment"># . . vim:nowrap:textwidth=0</span>
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